The Evolution of Man

LIST OF ILLUSTRATIONS

Fig. 1. The human ovum
Fig. 2. Stem-cell of an echinoderm
Fig. 3. Three epithelial cells
Fig. 4. Five spiny or grooved cells
Fig. 5. Ten liver-cells
Fig. 6. Nine star-shaped bone-cells
Fig. 7. Eleven star-shaped cells
Fig. 8. Unfertilised ovum of an echinoderm
Fig. 9. A large branching nerve-cell
Fig. 10. Blood-cells
Fig. 11. Indirect or mitotic cell-division
Fig. 12. Mobile cells
Fig. 13. Ova of various animals
Fig. 14. The human ovum
Fig. 15. Fertilised ovum of hen
Fig. 16. A creeping amœba
Fig. 17. Division of an amœba
Fig. 18. Ovum of a sponge
Fig. 19. Blood-cells, or phagocytes
Fig. 20. Spermia or spermatozoa
Fig. 21. Spermatozoa of various animals
Fig. 22. A single human spermatozoon
Fig. 23. Fertilisation of the ovum
Fig. 24. Impregnated echinoderm ovum
Fig. 25. Impregnation of the star-fish ovum
Figs. 26–27. Impregnation of sea-urchin ovum
Fig. 28. Stem-cell of a rabbit
Fig. 29. Gastrulation of a coral
Fig. 30. Gastrula of a gastræad
Fig. 31. Gastrula of a worm
Fig. 32. Gastrula of an echinoderm
Fig. 33. Gastrula of an arthropod
Fig. 34. Gastrula of a mollusc
Fig. 35. Gastrula of a vertebrate
Fig. 36. Gastrula of a lower sponge
Fig. 37. Cells from the primary germinal layers
Fig. 38. Gastrulation of the amphioxus
Fig. 39. Gastrula of the amphioxus
Fig. 40. Cleavage of the frog’s ovum
Figs. 41–44. Sections of fertilised toad ovum
Figs. 45–48. Gastrulation of the salamander
Fig. 49. Segmentation of the lamprey
Fig. 50. Gastrulation of the lamprey
Fig. 51. Gastrulation of ceratodus
Fig. 52. Ovum of a deep-sea bony fish
Fig. 53. Segmentation of a bony fish
Fig. 54. Discoid gastrula of a bony fish
Figs. 55–56. Sections of blastula of shark
Fig. 57. Discoid segmentation of bird’s ovum
Figs. 58–61. Gastrulation of the bird
Fig. 62. Germinal disk of the lizard
Figs. 63–64. Gastrulation of the opossum
Figs. 65–67. Gastrulation of the opossum
Figs. 68–71. Gastrulation of the rabbit
Fig. 72. Gastrula of the placental mammal
Fig. 73. Gastrula of the rabbit
Figs. 74–75. Diagram of the four secondary germinal layers
Figs. 76–77. Cœlomula of sagitta
Fig. 78. Section of young sagitta
Figs. 79–80. Section of amphioxus-larvæ
Figs. 81–82. Section of amphioxus-larvæ
Figs. 83–84. Chordula of the amphioxus
Figs. 85–86. Chordula of the amphibia
Figs. 87–88. Section of cœlomula-embryos of vertebrates
Figs. 89–90. Section of cœlomula-embryo of triton
Fig. 91. Dorsal part of three triton-embryos
Fig. 92. Chordula-embryo of a bird
Fig. 93. Vertebrate-embryo of a bird
Figs. 94–95. Section of the primitive streak of a chick
Fig. 96. Section of the primitive groove of a rabbit
Fig. 97. Section of primitive mouth of a human embryo
Figs. 98–102. The ideal primitive vertebrate
Fig. 103. Redundant mammary glands
Fig. 104. A Greek gynecomast
Fig. 105. Severance of the discoid mammal embryo
Figs. 106–107. The visceral embryonic vesicle
Fig. 108. Four entodermic cells
Fig. 109. Two entodermic cells
Figs. 110–114. Ovum of a rabbit
Figs. 115–118. Embryonic vesicle of a rabbit
Fig. 119. Section of the gastrula of four vertebrates
Figs 120. Embryonic shield of a rabbit
Figs. 121–123. Dorsal shield and embryonic shield of a rabbit.
Fig. 124. Cœlomula of the amphioxus
Fig. 125. Chordula of a frog
Fig. 126. Section of frog-embryo
Figs. 127–128. Dorsal shield of a chick
Fig. 129. Section of hind end of a chick
Fig. 130. Germinal area of the rabbit
Fig. 131. Embryo of the opossum
Fig. 132. Embryonic shield of the rabbit
Fig. 133. Human embryo at the sandal-stage
Fig. 134. Embryonic shield of rabbit
Fig. 135. Embryonic shield of opossum
Fig. 136. Embryonic disk of a chick
Fig. 137. Embryonic disk of a higher vertebrate
Figs. 138–142. Sections of maturing mammal embryo
Figs. 143–146. Sections of embryonic chicks
Fig. 147. Section of embryonic chick
Fig. 148. Section of fore-half of chick-embryo
Figs. 149–150. Sections of human embryos
Fig. 151. Section of a shark-embryo
Fig. 152. Section of a duck-embryo
Figs. 153–155. Sole-shaped embryonic disk of chick
Figs. 156–157. Embryo of the amphioxus
Figs. 158–160. Embryo of the amphioxus
Figs. 161–162. Sections of shark-embryos
Fig. 163. Section of a Triton-embryo
Figs. 164–166. Vertebræ
Fig. 167. Head of a shark-embryo
Figs. 168–169. Head of a chick-embryo
Fig. 170. Head of a dog-embryo
Fig. 171. Human embryo of the fourth week
Fig. 172. Section of shoulder of chick-embryo
Fig. 173. Section of pelvic region of chick-embryo
Fig. 174. Development of the lizard’s legs
Fig. 175. Human-embryo five weeks old
Figs. 176–178. Embryos of the bat
Fig. 179. Human embryos
Fig. 180. Human embryo of the fourth week
Fig. 181. Human embryo of the fifth week
Fig. 182. Section of tail of human embryo
Figs. 183–184. Human embryo dissected
Fig. 185. Miss Julia Pastrana
Figs. 186–190. Human embryos
Fig. 191. Human embryos of sixteen to eighteen ays
Figs. 192–193. Human embryo of fourth week
Fig. 194. Human embryo with its membranes
Fig. 195. Diagram of the embryonic organs
Fig. 196. Section of the pregnant womb
Fig. 197. Embryo of siamang-gibbon
Fig. 198. Section of pregnant womb
Figs. 199–200. Human fœtus–placenta
Fig. 201. Vitelline vessels in germinative area
Fig. 202. Boat-shaped embryo of the dog
Fig. 203. Lar or white-handed gibbon
Fig. 204. Young orang
Fig. 205. Wild orang
Fig. 206. Bald-headed chimpanzee
Fig. 207. Female chimpanzee
Fig. 208. Female gorilla
Fig. 209. Male giant-gorilla
Fig. 210. The lancelet
Fig. 211. Section of the head of the lancelet
Fig. 212. Section of an amphioxus-larva
Fig. 213. Diagram of preceding
Fig. 214. Section of a young amphioxus
Fig. 215. Diagram of a young amphioxus
Fig. 216. Transverse section of lancelet
Fig. 217. Section through the middle of the lancelet
Fig. 218. Section of a primitive-fish embryo
Fig. 219. Section of the head of the lancelet
Figs. 220. Organisation of an ascidia
Figs. 221. Organisation of an ascidia
Figs. 222–224. Sections of young amphioxus-larvæ
Fig. 225. An appendicaria
Fig. 226. Chroococcus minor
Fig. 227. Aphanocapsa primordialis
Fig. 228. Protamœba
Fig. 229. Original ovum-cleavage
Fig. 230. Morula
Figs. 231–232. Magosphæra planula
Fig. 233. Modern gastræads
Figs. 234–235. Prophysema primordiale
Figs. 236–237. Ascula of gastrophysema
Fig. 238. Olynthus
Fig. 239. Aphanostomum langii
Figs. 240–241. A turbellarian
Figs. 242–243. Chætonotus
Fig. 244. A nemertine worm
Fig. 245. An enteropneust
Fig. 246. Section of the branchial gut
Fig. 247. The marine lamprey
Fig. 248. Fossil primitive fish
Fig. 249. Embryo of a shark
Fig. 250. Man-eating shark
Fig. 251. Fossil angel-shark
Fig. 252. Tooth of a gigantic shark
Figs. 253–255. Crossopterygii
Fig. 256. Fossil dipneust
Fig. 257. The Australian dipneust
Figs. 258–259. Young ceratodus
Fig. 260. Fossil amphibian
Fig. 261. Larva of the spotted salamander
Fig. 262. Larva of common frog
Fig. 263. Fossil mailed amphibian
Fig. 264. The new zealand lizard
Fig. 265. Homœosaurus pulchellus
Fig. 266. Skull of a permian lizard
Fig. 267. Skull of a theromorphum
Fig. 268. Lower jaw of a primitive mammal
Figs. 269–270. The ornithorhyncus
Fig. 271. Lower jaw of a promammal
Fig. 272. The crab-eating opossum
Fig. 273. Fœtal membranes of the human embryo
Fig. 274. Skull of a fossil lemur
Fig. 275. The slender lori
Fig. 276. The white-nosed ape
Fig. 277. The drill-baboon
Figs. 278–282. Skeletons of man and the anthropoid apes
Fig. 283. Skull of the java ape-man
Fig. 284. Section of the human skin
Fig. 285. Epidermic cells
Fig. 286. Rudimentary lachrymal glands
Fig. 287. The female breast
Fig. 288. Mammary gland of a new-born infant
Fig. 289. Embryo of a bear
Fig. 290. Human embryo
Fig. 291. Central marrow of a human embryo
Figs. 292–293. The human brain
Figs. 294–296. Central marrow of human embryo
Fig. 297. Head of a chick embryo
Fig. 298. Brain of three craniote embryos
Fig. 299. Brain of a shark
Fig. 300. Brain and spinal cord of a frog
Fig. 301. Brain of an ox-embryo
Fig. 302. Brain of a human embryo
Fig. 303. Brain of a human embryo
Fig. 304. Brain of the rabbit
Fig. 305. Bead of a shark
Figs. 306–310. Heads of chick-embryos
Fig. 311. Section of mouth of human embryo
Fig. 312. Diagram of mouth-nose cavity
Figs. 313–314. Heads of human embryo
Figs. 315–316. Face of human embryo
Fig. 317. The human eye
Fig. 318. Eye of the chick embryo
Fig. 319. Section of eye of a human embryo
Fig. 320. The human ear
Fig. 321. The bony labyrinth
Fig. 322. Development of the labyrinth
Fig. 323. Primitive skull of human embryo
Fig. 324. Rudimentary muscles of the ear
Figs. 325–326. The human skeleton
Fig. 327. The human vertebral column
Fig. 328. Piece of the dorsal cord
Figs. 329–330. Dorsal vertebræ
Fig. 331. Intervertebral disk
Fig. 332. Human skull
Fig. 333. Skull of new-born child
Fig. 334. Head-skeleton of a primitive fish
Fig. 335. Skulls of nine primates
Figs. 336–338. Evolution of the fin
Fig. 339. Skeleton of the fore-leg of an amphibian
Fig. 340. Skeleton of gorilla’s hand
Fig. 341. Skeleton of human hand
Fig. 342. Skeleton of hand of six mammals
Figs. 343–345. Arm and hand of three anthropoids
Fig. 346. Section of fish’s tail
Fig. 347. Human skeleton
Fig. 348. Skeleton of the giant gorilla
Fig. 349. The human stomach
Fig. 350. Section of the head of a rabbit-embryo
Fig. 351. Shark’s teeth
Fig. 352. Gut of a human embryo
Figs. 353–354. Gut of a dog embryo
Figs. 355–356. Sections of head of lamprey
Fig. 357. Viscera of a human embryo
Fig. 358. Red blood-cells
Fig. 359. Vascular tissue
Fig. 360. Section of trunk of a chick-embryo
Fig. 361. Merocytes
Fig. 362. Vascular system of an annelid
Fig. 363. Head of a fish-embryo
Figs. 364–366. The five arterial arches
Figs. 367–370. The five arterial arches
Figs. 371–372. Heart of a rabbit-embryo
Figs. 373–374. Heart of a dog-embryo
Figs. 375–377. Heart of a human embryo
Fig. 378. Heart of adult man
Fig. 379. Section of head of a chick-embryo
Fig. 380. Section of a human embryo
Figs. 381–382. Sections of a chick-embryo
Fig. 383. Embryos of sagitta
Fig. 384. Kidneys of bdellostoma
Fig. 385. Section of embryonic shield
Figs. 386–387. Primitive kidneys
Fig. 388. Pig-embryo
Fig. 389. Human embryo
Figs. 390–392. Rudimentary kidneys and sexual organs
Figs. 393–394. Urinary and sexual organs of salamander
Fig. 395. Primitive kidneys of human embryo
Figs. 396–398. Urinary organs of ox-embryos
Fig. 399. Sexual organs of water-mole
Figs. 400–401. Original position of sexual glands
Fig. 402. Urogenital system of human embryo
Fig. 403. Section of ovary
Figs. 404–406. Graafian follicles
Fig. 407. A ripe graafian follicle
Fig. 408. The human ovum

GLOSSARY

Acrania: animals without skull (cranium).
Anthropogeny: the evolution (genesis) of man (anthropos).
Anthropology: the science of man.
Archi-: (in compounds) the first or typical—as, archi-cytula,
archi-gastrula, etc.
Biogeny: the science of the genesis of life (bios).
Blast-: (in compounds) pertaining to the early embryo (blastos = a
bud); hence:—
    Blastoderm: skin (derma) or enclosing layer of the embryo.
    Blastosphere: the embryo in the hollow sphere stage.
    Blastula: same as preceding.
    Epiblast: the outer layer of the embryo (ectoderm).
    Hypoblast: the inner layer of the embryo (endoderm).
Branchial: pertaining to the gills (branchia).
Caryo-: (in compounds) pertaining to the nucleus (caryon); hence:—
    Caryokineses: the movement of the nucleus.
    Caryolysis: dissolution of the nucleus.
    Caryoplasm: the matter of the nucleus.
Centrolecithal: see under Lecith-.
Chordaria and Chordonia: animals with a dorsal chord or back-bone.
Cœlom or Cœloma: the body-cavity in the embryo; hence:—
    Cœlenterata: animals without a body-cavity.
    Cœlomaria: animals with a body-cavity.
    Cœlomation: formation of the body-cavity.
Cyto-: (in compounds) pertaining to the cell (cytos); hence:—
    Cytoblast: the nucleus of the cell.
    Cytodes: cell-like bodies, imperfect cells.
    Cytoplasm: the matter of the body of the cell.
    Cytosoma: the body (soma) of the cell.
Cryptorchism: abnormal retention of the testicles in the body.
Deutoplasm: see Plasm.
Dualism: the belief in the existence of two entirely distinct
principles (such as matter and spirit).
Dysteleology: the science of those features in organisms which refute
the “design-argument”.
Ectoderm: the outer (ekto) layer of the embryo.
Entoderm: the inner (ento) layer of the embryo.
Epiderm: the outer layer of the skin.
Epigenesis: the theory of gradual development of organs in the embryo.
Epiphysis: the third or central eye in the early vertebrates.
Episoma: see Soma.
Epithelia: tissues covering the surface of parts of the body (such as
the mouth, etc.)
Gonads: the sexual glands.
Gonochorism: separation of the male and female sexes.
Gonotomes: sections of the sexual glands.
Gynecomast: a male with the breasts (masta) of a woman (gyne).
Hepatic: pertaining to the liver (hepar).
Holoblastic: embryos in which the animal and vegetal cells divide
equally (holon = whole).
Hypermastism: the possession of more than the normal breasts (masta).
Hypobranchial: underneath (hypo) the gills.
Hypophysis: sensitive-offshoot from the brain in the vertebrate.
Hyposoma: see Soma.
Lecith-: pertaining to the yelk (lecithus); hence:—
    Centrolecithal: eggs with the yelk in the centre.
    Lecithoma: the yelk-sac.
    Telolecithal: eggs with the yelk at one end.
Meroblastic: cleaving in part (meron) only.
Meta-: (in compounds) the “after” or secondary stage; hence:—
    Metagaster: the secondary or permanent gut (gaster).
    Metaplasm: secondary or differentiated plasm.
    Metastoma: the secondary or permanent mouth (stoma).
    Metazoa: the higher or later animals, made up of many cells.
    Metovum: the mature or advanced ovum.
Metamera: the segments into which the embryo breaks up.
Metamerism: the segmentation of the embryo.
Monera: the most primitive of the unicellular organisms.
Monism: belief in the fundamental unity of all things.
Morphology: the science of organic forms (generally equivalent to
anatomy).
Myotomes: segments into which the muscles break up.
Nephra: the kidneys; hence:—
    Nephridia: the rudimentary kidney-organs.
    Nephrotomes: the segments of the developing kidneys.
Ontogeny: the science of the development of the individual (generally
equivalent to embryology).
Perigenesis: the genesis of the movements in the vital particles.
Phagocytes: cells that absorb food (phagein = to eat).
Phylogeny: the science of the evolution of species (phyla).
Planocytes: cells that move about (planein).
Plasm: the colloid or jelly-like matter of which organisms are
composed; hence:—
    Caryoplasm: the matter of the nucleus (caryon).
    Cytoplasm: the matter of the body of the cell.
    Deutoplasm: secondary or differentiated plasm.
    Metaplasm: secondary or differentiated plasm.
    Protoplasm: primitive or undifferentiated plasm.
Plasson: the simplest form of plasm.
Plastidules: small particles of plasm.
Polyspermism: the penetration of more than one sperm-cell into the ovum.
Pro- or Prot: (in compounds) the earlier form (opposed to Meta); hence:—
    Prochorion: the first form of the chorion.
    Progaster: the first or primitive stomach.
    Pronephridia: the earlier form of the kidneys.
    Prorenal: the earlier form of the kidneys.
    Prostoma: the first or primitive mouth.
    Protists: the earliest or unicellular organisms.
    Provertebræ: the earliest phase of the vertebræ.
    Protophyta: the primitive or unicellular plants.
    Protoplasm: undifferentiated plasm.
    Protozoa: the primitive or unicellular animals.
Renal: pertaining to the kidneys (renes).
Scatulation: packing or boxing-up (scatula = a box).
Sclerotomes: segments into which the primitive skeleton falls.
Soma: the body; hence:—
    Cytosoma: the body of the cell (cytos).
    Episoma: the upper or back-half of the embryonic body.
    Somites: segments of the embryonic body.
    Hyposoma: the under or belly-half of the embryonic body.
Teleology: the belief in design and purpose (telos) in nature.
Telolecithal: see Lecith-.
Umbilical: pertaining to the navel (umbilicus).
Vitelline: pertaining to the yelk (vitellus).

PREFACE

[BY JOSEPH MCCABE]

The work which we now place within the reach of every reader of the English
tongue is one of the finest productions of its distinguished author. The first
edition appeared in 1874. At that time the conviction of man’s natural
evolution was even less advanced in Germany than in England, and the work
raised a storm of controversy. Theologians—forgetting the commonest facts
of our individual development—spoke with the most profound disdain of the
theory that a Luther or a Goethe could be the outcome of development from a
tiny speck of protoplasm. The work, one of the most distinguished of them said,
was “a fleck of shame on the escutcheon of Germany.” To-day its
conclusion is accepted by influential clerics, such as the Dean of Westminster,
and by almost every biologist and anthropologist of distinction in Europe.
Evolution is not a laboriously reached conclusion, but a guiding truth, in
biological literature to-day.

There was ample evidence to substantiate the conclusion even in the first
edition of the book. But fresh facts have come to light in each decade, always
enforcing the general truth of man’s evolution, and at times making
clearer the line of development. Professor Haeckel embodied these in successive
editions of his work. In the fifth edition, of which this is a translation,
reference will be found to the very latest facts bearing on the evolution of
man, such as the discovery of the remarkable effect of mixing human blood with
that of the anthropoid ape. Moreover, the ample series of illustrations has
been considerably improved and enlarged; there is no scientific work published,
at a price remotely approaching that of the present edition, with so abundant
and excellent a supply of illustrations. When it was issued in Germany, a few
years ago, a distinguished biologist wrote in the Frankfurter Zeitung
that it would secure immortality for its author, the most notable critic of the
idea of immortality. And the Daily Telegraph reviewer described the
English version as a “handsome edition of Haeckel’s monumental
work,” and “an issue worthy of the subject and the author.”

The influence of such a work, one of the most constructive that Haeckel has
ever written, should extend to more than the few hundred readers who are able
to purchase the expensive volumes of the original issue. Few pages in the story
of science are more arresting and generally instructive than this great picture
of “mankind in the making.” The horizon of the mind is healthily
expanded as we follow the search-light of science down the vast avenues of past
time, and gaze on the uncouth forms that enter into, or illustrate, the
line of our ancestry. And if the imagination recoils from the strange and
remote figures that are lit up by our search-light, and hesitates to accept
them as ancestral forms, science draws aside another veil and reveals another
picture to us. It shows us that each of us passes, in our embryonic
development, through a series of forms hardly less uncouth and unfamiliar. Nay,
it traces a parallel between the two series of forms. It shows us man beginning
his existence, in the ovary of the female infant, as a minute and simple speck
of jelly-like plasm. It shows us (from analogy) the fertilised ovum breaking
into a cluster of cohering cells, and folding and curving, until the limb-less,
head-less, long-tailed fœtus looks like a worm-shaped body. It then
points out how gill-slits and corresponding blood-vessels appear, as in a lowly
fish, and the fin-like extremities bud out and grow into limbs, and so on;
until, after a very clear ape-stage, the definite human form emerges from the
series of transformations.

It is with this embryological evidence for our evolution that the present
volume is concerned. There are illustrations in the work that will make the
point clear at a glance. Possibly too clear; for the simplicity of the
idea and the eagerness to apply it at every point have carried many, who borrow
hastily from Haeckel, out of their scientific depth. Haeckel has never shared
their errors, nor encouraged their superficiality. He insists from the outset
that a complete parallel could not possibly be expected. Embryonic life itself
is subject to evolution. Though there is a general and substantial law—as
most of our English and American authorities admit—that the embryonic
series of forms recalls the ancestral series of forms, the parallel is blurred
throughout and often distorted. It is not the obvious resemblance of the
embryos of different animals, and their general similarity to our extinct
ancestors in this or that organ, on which we must rest our case. A careful
study must be made of the various stages through which all embryos pass, and an
effort made to prove their real identity and therefore genealogical relation.

This is a task of great subtlety and delicacy. Many scientists have worked at
it together with Professor Haeckel—I need only name our own Professor
Balfour and Professor Ray Lankester—and the scheme is fairly complete.
But the general reader must not expect that even so clear a writer as Haeckel
can describe these intricate processes without demanding his very careful
attention. Most of the chapters in the present volume (and the second volume
will be less difficult) are easily intelligible to all; but there are points at
which the line of argument is necessarily subtle and complex. In the hope that
most readers will be induced to master even these more difficult chapters, I
will give an outline of the characteristic argument of the work.
Haeckel’s distinctive services in regard to man’s evolution have
been: (1) The construction of a complete ancestral tree, though, of course,
some of the stages in it are purely conjectural, and not final; (2) The tracing
of the remarkable reproduction of ancestral forms in the embryonic
development of the individual. Naturally, he has not worked alone in either
department. The second volume of this work will embody the first of these two
achievements; the present one is mainly concerned with the latter. It will be
useful for the reader to have a synopsis of the argument and an explanation of
some of the chief terms invented or employed by the author.

The main theme of the work is that, in the course of their embryonic
development, all animals, including man, pass roughly and rapidly through a
series of forms which represents the succession of their ancestors in the past.
After a severe and extensive study of embryonic phenomena, Haeckel has drawn up
a “law” (in the ordinary scientific sense) to this effect, and has
called it “the biogenetic law,” or the chief law relating to the
evolution (genesis) of life (bios). This law is widely and
increasingly accepted by embryologists and zoologists. It is enough to quote a
recent declaration of the great American zoologist, President D. Starr Jordan:
“It is, of course, true that the life-history of the individual is an
epitome of the life-history of the race”; while a distinguished German
zoologist (Sarasin) has described it as being of the same use to the biologist
as spectrum analysis is to the astronomer.

But the reproduction of ancestral forms in the course of the embryonic
development is by no means always clear, or even always present. Many of the
embryonic phases do not recall ancestral stages at all. They may have done so
originally, but we must remember that the embryonic life itself has been
subject to adaptive changes for millions of years. All this is clearly
explained by Professor Haeckel. For the moment, I would impress on the reader
the vital importance of fixing the distinction from the start. He must
thoroughly familiarise himself with the meaning of five terms. Biogeny
is the development of life in general (both in the individual and the species),
or the sciences describing it. Ontogeny is the development (embryonic
and post-embryonic) of the individual (on), or the science describing
it. Phylogeny is the development of the race or stem (phulon), or
the science describing it. Roughly, ontogeny may be taken to mean
embryology, and phylogeny what we generally call evolution. Further, the
embryonic phenomena sometimes reproduce ancestral forms, and they are then
called palingenetic (from palin = again): sometimes they do not
recall ancestral forms, but are later modifications due to adaptation, and they
are then called cenogenetic (from kenos = new or foreign). These
terms are now widely used, but the reader of Haeckel must understand them
thoroughly.

The first five chapters are an easy account of the history of embryology and
evolution. The sixth and seventh give an equally clear account of the sexual
elements and the process of conception. But some of the succeeding chapters
must deal with embryonic processes so unfamiliar, and pursue them through so
wide a range of animals in a brief space, that, in spite of the 200
illustrations, they will offer difficulty to many a reader. As our aim is to
secure, not a superficial acquiescence in conclusions, but a fair comprehension
of the truths of science, we have retained these chapters. However, I will give
a brief and clear outline of the argument, so that the reader with little
leisure may realise their value.

When the animal ovum (egg-cell) has been fertilised, it divides and subdivides
until we have a cluster of cohering cells, externally not unlike a raspberry or
mulberry. This is the morula (= mulberry) stage. The cluster becomes
hollow, or filled with fluid in the centre, all the cells rising to the
surface. This is the blastula (hollow ball) stage. One half of the
cluster then bends or folds in upon the other, as one might do with a thin
indiarubber ball, and we get a vase-shaped body with hollow interior (the first
stomach, or “primitive gut”), an open mouth (the first or
“primitive mouth”), and a wall composed of two layers of cells (two
“germinal layers”). This is the gastrula (stomach) stage,
and the process of its formation is called gastrulation. A glance at the
illustration (Fig. 29) will make this perfectly clear.

So much for the embryonic process in itself. The application to evolution has
been a long and laborious task. Briefly, it was necessary to show that
all the multicellular animals passed through these three stages, so that
our biogenetic law would enable us to recognise them as reminiscences of
ancestral forms. This is the work of Chapters VIII and IX. The difficulty can
be realised in this way: As we reach the higher animals the ovum has to take up
a large quantity of yelk, on which it may feed in developing. Think of the
bird’s “egg.” The effect of this was to flatten the germ (the
morula and blastula) from the first, and so give, at first sight,
a totally different complexion to what it has in the lowest animals. When we
pass the reptile and bird stage, the large yelk almost disappears (the germ now
being supplied with blood by the mother), but the germ has been permanently
altered in shape, and there are now a number of new embryonic processes
(membranes, blood-vessel connections, etc.). Thus it was no light task to trace
the identity of this process of gastrulation in all the animals. It has
been done, however; and with this introduction the reader will be able to
follow the proof. The conclusion is important. If all animals pass through the
curious gastrula stage, it must be because they all had a common ancestor of
that nature. To this conjectural ancestor (it lived before the period of
fossilisation begins) Haeckel gives the name of the Gastræa, and
in the second volume we shall see a number of living animals of this type
(“gastræads”).

The line of argument is the same in the next chapter. After laborious and
careful research (though this stage is not generally admitted in the same sense
as the previous one), a fourth common stage was discovered, and given the name
of the Cœlomula. The blastula had one layer of cells, the
blastoderm (derma = skin): the gastrula two layers, the
ectoderm (“outer skin”) and entoderm (“inner
skin”). Now a third layer (mesoderm = middle skin) is formed,
by the growth inwards of two pouches or folds of the skin. The pouches blend
together, and form a single cavity (the body cavity, or cœlom), and its
two walls are two fresh “germinal layers.” Again, the identity of
the process has to be proved in all the higher classes of animals, and when
this is done we have another ancestral stage, the Cœlomæa.

The remaining task is to build up the complex frame of the higher
animals—always showing the identity of the process (on which the
evolutionary argument depends) in enormously different conditions of embryonic
life—out of the four “germinal layers.” Chapter IX prepares
us for the work by giving us a very clear account of the essential structure of
the back-boned (vertebrate) animal, and the probable common ancestor of all the
vertebrates (a small fish of the lancelet type). Chapters XI–XIV then
carry out the construction step by step. The work is now simpler, in the sense
that we leave all the invertebrate animals out of account; but there are so
many organs to be fashioned out of the four simple layers that the reader must
proceed carefully. In the second volume each of these organs will be dealt with
separately, and the parallel will be worked out between its embryonic and its
phylogenetic (evolutionary) development. The general reader may wait for this
for a full understanding. But in the meantime the wonderful story of the
construction of all our organs in the course of a few weeks (the human frame is
perfectly formed, though less than two inches in length, by the twelfth week)
from so simple a material is full of interest. It would be useless to attempt
to summarise the process. The four chapters are themselves but a summary of it,
and the eighty fine illustrations of the process will make it sufficiently
clear. The last chapter carries the story on to the point where man at last
parts company with the anthropoid ape, and gives a full account of the
membranes or wrappers that enfold him in the womb, and the connection with the
mother.

In conclusion, I would urge the reader to consult, at his free library perhaps,
the complete edition of this work, when he has read the present abbreviated
edition. Much of the text has had to be condensed in order to bring out the
work at our popular price, and the beautiful plates of the complete edition
have had to be omitted. The reader will find it an immense assistance if he can
consult the library edition.

JOSEPH MCCABE

Cricklewood, March, 1906.

HAECKEL’S CLASSIFICATION OF THE ANIMAL WORLD

(This classification is given for the purpose of explaining Haeckel’s
use of terms in this volume. The general reader should bear in mind
that it differs very considerably from more recent schemes of
classification. He should compare the scheme framed by Professor E.
Ray Lankester.)

THE EVOLUTION OF MAN

Chapter I.
THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION

The field of natural phenomena into which I would
introduce my readers in the following chapters has a quite peculiar
place in the broad realm of scientific inquiry. There is no object
of investigation that touches man more closely, and the knowledge
of which should be more acceptable to him, than his own frame. But
among all the various branches of the natural history of mankind,
or anthropology, the story of his development by natural
means must excite the most lively interest. It gives us the key of
the great world-riddles at which the human mind has been working
for thousands of years. The problem of the nature of man, or the
question of man’s place in nature, and the cognate inquiries
as to the past, the earliest history, the present situation, and
the future of humanity—all these most important questions are
directly and intimately connected with that branch of study which
we call the science of the evolution of man, or, in one word,
“Anthropogeny” (the genesis of man). Yet it is an
astonishing fact that the science of the evolution of man does not
even yet form part of the scheme of general education. In fact,
educated people even in our day are for the most part quite
ignorant of the important truths and remarkable phenomena which
anthropogeny teaches us.

As an illustration of this curious state of things, it may be
pointed out that most of what are considered to be
“educated” people do not know that every human being is
developed from an egg, or ovum, and that this egg is one simple
cell, like any other plant or animal egg. They are equally ignorant
that in the course of the development of this tiny, round egg-cell
there is first formed a body that is totally different from the
human frame, and has not the remotest resemblance to it. Most of
them have never seen such a human embryo in the earlier period of
its development, and do not know that it is quite indistinguishable
from other animal embryos. At first the embryo is no more than a
round cluster of cells, then it becomes a simple hollow sphere, the
wall of which is composed of a layer of cells. Later it approaches
very closely, at one period, to the anatomic structure of the
lancelet, afterwards to that of a fish, and again to the typical
build of the amphibia and mammals. As it continues to develop, a
form appears which is like those we find at the lowest stage of
mammal-life (such as the duck-bills), then a form that resembles
the marsupials, and only at a late stage a form that has a
resemblance to the ape; until at last the definite human form
emerges and closes the series of transformations. These suggestive
facts are, as I said, still almost unknown to the general
public—so completely unknown that, if one casually mentions
them, they are called in question or denied outright as
fairy-tales. Everybody knows that the butterfly emerges from the
pupa, and the pupa from a quite different thing called a larva, and
the larva from the butterfly’s egg. But few besides medical
men are aware that man, in the course of his individual
formation, passes through a series of transformations which are not
less surprising and wonderful than the familiar metamorphoses of
the butterfly.

The mere description of these remarkable changes through which
man passes during his embryonic life should arouse considerable
interest. But the mind will experience a far keener satisfaction
when we trace these curious facts to their causes, and
when we learn to behold in them natural phenomena which are of the
highest importance throughout the whole field of human knowledge.
They throw light first of all on the “natural history of
creation,” then on psychology, or “the science of the
soul,” and through this on the whole of philosophy. And as
the general results of every branch of inquiry are summed up in
philosophy, all the sciences come in turn to be touched and
influenced more or less by the study of the evolution of man.

But when I say that I propose to present here the most important
features of these phenomena and trace them to their causes, I take
the term, and I interpret my task, in a very much wider sense than
is usual. The lectures which have been delivered on this subject in
the universities during the last half-century are almost
exclusively adapted to medical men. Certainly, the medical man has
the greatest interest in studying the origin of the human body,
with which he is daily occupied. But I must not give here this
special description of the embryonic processes such as it has
hitherto been given, as most of my readers have not studied
anatomy, and are not likely to be entrusted with the care of the
adult organism. I must content myself with giving some parts of the
subject only in general outline, and must not enter upon all the
marvellous, but very intricate and not easily described, details
that are found in the story of the development of the human frame.
To understand these fully a knowledge of anatomy is needed. I will
endeavour to be as plain as possible in dealing with this branch of
science. Indeed, a sufficient general idea of the course of the
embryonic development of man can be obtained without going too
closely into the anatomic details. I trust we may be able to arouse
the same interest in this delicate field of inquiry as has been
excited already in other branches of science; though we shall meet
more obstacles here than elsewhere.

The story of the evolution of man, as it has hitherto been
expounded to medical students, has usually been confined to
embryology—more correctly, ontogeny—or the
science of the development of the individual human organism. But
this is really only the first part of our task, the first half of
the story of the evolution of man in that wider sense in which we
understand it here. We must add as the second half—as another
and not less important and interesting branch of the science of the
evolution of the human stem—phylogeny: this may be
described as the science of the evolution of the various animal
forms from which the human organism has been developed in the
course of countless ages. Everybody now knows of the great
scientific activity that was occasioned by the publication of
Darwin’s Origin of Species in 1859. The chief direct
consequence of this publication was to provoke a fresh inquiry into
the origin of the human race, and this has proved beyond question
our gradual evolution from the lower species. We give the name of
“Phylogeny” to the science which describes this ascent
of man from the lower ranks of the animal world. The chief source
that it draws upon for facts is “Ontogeny,” or
embryology, the science of the development of the individual
organism. Moreover, it derives a good deal of support from
paleontology, or the science of fossil remains, and even more
from comparative anatomy, or morphology.

These two branches of our science—on the one side ontogeny
or embryology, and on the other phylogeny, or the science of
race-evolution—are most vitally connected. The one cannot be
understood without the other. It is only when the two branches
fully co-operate and supplement each other that
“Biogeny” (or the science of the genesis of life in the
widest sense) attains to the rank of a philosophic science. The
connection between them is not external and superficial, but
profound, intrinsic, and causal. This is a discovery made by recent
research, and it is most clearly and correctly expressed in the
comprehensive law which I have called “the fundamental law of
organic evolution,” or “the fundamental law of
biogeny.” This general law, to which we shall find ourselves
constantly recurring, and on the recognition of which depends
one’s whole insight into the story of evolution, may be
briefly expressed in the phrase: “The history of the
fœtus is a recapitulation of the history of the race”;
or, in other words, “Ontogeny is a recapitulation of
phylogeny.” It may be more fully stated as follows: The
series of forms through which the individual organism passes during
its development from the ovum to the complete bodily structure is a
brief, condensed repetition of the long series of forms which the animal
ancestors of the said organism, or the ancestral forms of the
species, have passed through from the earliest period of organic
life down to the present day.

The causal character of the relation which connects embryology
with stem-history is due to the action of heredity and adaptation.
When we have rightly understood these, and recognised their great
importance in the formation of organisms, we can go a step further
and say: Phylogenesis is the mechanical cause of
ontogenesis.^[1] In other words, the development of the
stem, or race, is, in accordance with the laws of heredity and
adaptation, the cause of all the changes which appear in a
condensed form in the evolution of the fœtus.

[1]
The term “genesis,” which occurs
throughout, means, of course, “birth” or origin. From
this we get: Biogeny = the origin of life (bios);
Anthropogeny = the origin of man (anthropos); Ontogeny = the
origin of the individual (on); Phylogeny = the origin of the
species (phulon); and so on. In each case the term may refer
to the process itself, or to the science describing the
process.—Translator.

The chain of manifold animal forms which represent the ancestry
of each higher organism, or even of man, according to the theory of
descent, always form a connected whole. We may designate this
uninterrupted series of forms with the letters of the alphabet: A,
B, C, D, E, etc., to Z. In apparent contradiction to what I have
said, the story of the development of the individual, or the
ontogeny of most organisms, only offers to the observer a part of
these forms; so that the defective series of embryonic forms would
run: A, B, D, F, H, K, M, etc.; or, in other cases, B, D, H, L, M,
N, etc. Here, then, as a rule, several of the evolutionary forms of
the original series have fallen out. Moreover, we often
find—to continue with our illustration from the
alphabet—one or other of the original letters of the
ancestral series represented by corresponding letters from a
different alphabet. Thus, instead of the Roman B and D, we often
have the Greek Β and Δ. In this case the text of the
biogenetic law has been corrupted, just as it had been abbreviated
in the preceding case. But, in spite of all this, the series of
ancestral forms remains the same, and we are in a position to
discover its original complexion.

In reality, there is always a certain parallel between the two
evolutionary series. But it is obscured from the fact that in the
embryonic succession much is wanting that certainly existed in the
earlier ancestral succession. If the parallel of the two series
were complete, and if this great fundamental law affirming the
causal connection between ontogeny and phylogeny in the proper
sense of the word were directly demonstrable, we should only have
to determine, by means of the microscope and the dissecting knife,
the series of forms through which the fertilised ovum passes in its
development; we should then have before us a complete picture of
the remarkable series of forms which our animal ancestors have
successively assumed from the dawn of organic life down to the
appearance of man. But such a repetition of the ancestral history
by the individual in its embryonic life is very rarely complete. We
do not often find our full alphabet. In most cases the
correspondence is very imperfect, being greatly distorted and
falsified by causes which we will consider later. We are thus, for
the most part, unable to determine in detail, from the study of its
embryology, all the different shapes which an organism’s
ancestors have assumed; we usually—and especially in the case
of the human fœtus—encounter many gaps. It is true that
we can fill up most of these gaps satisfactorily with the help of
comparative anatomy, but we cannot do so from direct embryological
observation. Hence it is important that we find a large number of
lower animal forms to be still represented in the course of
man’s embryonic development. In these cases we may draw our
conclusions with the utmost security as to the nature of the
ancestral form from the features of the form which the embryo
momentarily assumes.

To give a few examples, we can infer from the fact that the
human ovum is a simple cell that the first ancestor of our species
was a tiny unicellular being, something like the amœba. In
the same way, we know, from the fact that the human fœtus
consists, at the first, of two simple cell-layers (the
gastrula), that the gastræa, a form with two such
layers, was certainly in the line of our ancestry. A later human
embryonic form (the chordula) points just as clearly to a
worm-like ancestor (the prochordonia), the nearest living
relation of which is found among the actual ascidiæ. To this
succeeds a most important embryonic stage (acrania), in
which our headless fœtus presents, in the main, the structure of the
lancelet. But we can only indirectly and approximately, with the
aid of comparative anatomy and ontogeny, conjecture what lower
forms enter into the chain of our ancestry between the
gastræa and the chordula, and between this and the lancelet.
In the course of the historical development many intermediate
structures have gradually fallen out, which must certainly have
been represented in our ancestry. But, in spite of these many, and
sometimes very appreciable, gaps, there is no contradiction between
the two successions. In fact, it is the chief purpose of this work
to prove the real harmony and the original parallelism of the two.
I hope to show, on a substantial basis of facts, that we can draw
most important conclusions as to our genealogical tree from the
actual and easily-demonstrable series of embryonic changes. We
shall then be in a position to form a general idea of the wealth of
animal forms which have figured in the direct line of our ancestry
in the lengthy history of organic life.

In this evolutionary appreciation of the facts of embryology we
must, of course, take particular care to distinguish sharply and
clearly between the primitive, palingenetic (or ancestral)
evolutionary processes and those due to cenogenesis.^[2] By
palingenetic processes, or embryonic recapitulations,
we understand all those phenomena in the development of the
individual which are transmitted from one generation to another by
heredity, and which, on that account, allow us to draw direct
inferences as to corresponding structures in the development of the
species. On the other hand, we give the name of cenogenetic
processes, or embryonic variations, to all those phenomena
in the fœtal development that cannot be traced to inheritance
from earlier species, but are due to the adaptation of the
fœtus, or the infant-form, to certain conditions of its
embryonic development. These cenogenetic phenomena are foreign or
later additions; they allow us to draw no direct inference whatever
as to corresponding processes in our ancestral history, but rather
hinder us from doing so.

[2]
Palingenesis = new birth, or re-incarnation (palin = again,
genesis or genea = development); hence its application to the
phenomena which are recapitulated by heredity from earlier ancestral forms.
Cenogenesis = foreign or negligible development (kenos and
genea); hence, those phenomena which come later in the story of life to
disturb the inherited structure, by a fresh adaptation to
environment.—Translator.

This careful discrimination between the primary or palingenetic
processes and the secondary or cenogenetic is of great importance
for the purposes of the scientific history of a species, which has
to draw conclusions from the available facts of embryology,
comparative anatomy, and paleontology, as to the processes in the
formation of the species in the remote past. It is of the same
importance to the student of evolution as the careful distinction
between genuine and spurious texts in the works of an ancient
writer, or the purging of the real text from interpolations and
alterations, is for the student of philology. It is true that this
distinction has not yet been fully appreciated by many scientists.
For my part, I regard it as the first condition for forming any
just idea of the evolutionary process, and I believe that we must,
in accordance with it, divide embryology into two
sections—palingenesis, or the science of recapitulated forms;
and cenogenesis, or the science of supervening structures.

To give at once a few examples from the science of man’s
origin in illustration of this important distinction, I may
instance the following processes in the embryology of man, and of
all the higher vertebrates, as palingenetic: the formation
of the two primary germinal layers and of the primitive gut, the
undivided structure of the dorsal nerve-tube, the appearance of a
simple axial rod between the medullary tube and the gut, the
temporary formation of the gill-clefts and arches, the primitive
kidneys, and so on.^[3] All these, and many other important
structures, have clearly been transmitted by a steady heredity from
the early ancestors of the mammal, and are, therefore, direct
indications of the presence of similar structures in the history of
the stem. On the other hand, this is certainly not the case with
the following embryonic forms, which we must describe as
cenogenetic processes: the formation of the yelk-sac, the
allantois, the placenta, the amnion, the serolemma, and the
chorion—or, generally speaking, the various fœtal
membranes and the corresponding changes in the blood vessels.
Further instances are: the dual structure of the heart cavity, the
temporary division of the plates of the primitive vertebræ
and lateral plates, the secondary closing of the ventral
and intestinal walls, the formation of the navel, and so on. All
these and many other phenomena are certainly not traceable to
similar structures in any earlier and completely-developed
ancestral form, but have arisen simply by adaptation to the
peculiar conditions of embryonic life (within the fœtal
membranes). In view of these facts, we may now give the following
more precise expression to our chief law of biogeny: The evolution
of the fœtus (or ontogenesis) is a condensed and
abbreviated recapitulation of the evolution of the stem (or
phylogenesis); and this recapitulation is the more complete in
proportion as the original development (or palingenesis) is
preserved by a constant heredity; on the other hand, it becomes
less complete in proportion as a varying adaptation to new
conditions increases the disturbing factors in the development (or
cenogenesis).

[3]
All these, and the following structures, will be fully described in later
chapters.—Translator.

The cenogenetic alterations or distortions of the original
palingenetic course of development take the form, as a rule, of a
gradual displacement of the phenomena, which is slowly effected by
adaptation to the changed conditions of embryonic existence during
the course of thousands of years. This displacement may take place
as regards either the position or the time of a phenomenon.

The great importance and strict regularity of the
time-variations in embryology have been carefully studied recently
by Ernest Mehnert, in his Biomechanik (Jena, 1898). He
contends that our biogenetic law has not been impaired by the
attacks of its opponents, and goes on to say: “Scarcely any
piece of knowledge has contributed so much to the advance of
embryology as this; its formulation is one of the most signal
services to general biology. It was not until this law passed into
the flesh and blood of investigators, and they had accustomed
themselves to see a reminiscence of ancestral history in embryonic
structures, that we witnessed the great progress which
embryological research has made in the last two decades.” The
best proof of the correctness of this opinion is that now the most
fruitful work is done in all branches of embryology with the aid of
this biogenetic law, and that it enables students to attain every
year thousands of brilliant results that they would never have
reached without it.

It is only when one appreciates the cenogenetic processes in
relation to the palingenetic, and when one takes careful account of
the changes which the latter may suffer from the former, that the
radical importance of the biogenetic law is recognised, and it is
felt to be the most illuminating principle in the science of
evolution. In this task of discrimination it is the silver thread
in relation to which we can arrange all the phenomena of this realm
of marvels—the “Ariadne thread,” which alone
enables us to find our way through this labyrinth of forms. Hence
the brothers Sarasin, the zoologists, could say with perfect
justice, in their study of the evolution of the Ichthyophis,
that “the great biogenetic law is just as important for the
zoologist in tracing long-extinct processes as spectrum analyses is
for the astronomer.”

Even at an earlier period, when a correct acquaintance with the
evolution of the human and animal frame was only just being
obtained—and that is scarcely eighty years ago!—the
greatest astonishment was felt at the remarkable similarity
observed between the embryonic forms, or stages of fœtal
development, in very different animals; attention was called even
then to their close resemblance to certain fully-developed animal
forms belonging to some of the lower groups. The older scientists
(Oken, Treviranus, and others) knew perfectly well that these lower
forms in a sense illustrated and fixed, in the hierarchy of the
animal world, a temporary stage in the evolution of higher forms.
The famous anatomist Meckel spoke in 1821 of a “similarity
between the development of the embryo and the series of
animals.” Baer raised the question in 1828 how far, within
the vertebrate type, the embryonic forms of the higher animals
assume the permanent shapes of members of lower groups. But it was
impossible fully to understand and appreciate this remarkable
resemblance at that time. We owe our capacity to do this to the
theory of descent; it is this that puts in their true light the
action of heredity on the one hand and adaptation on
the other. It explains to us the vital importance of their constant
reciprocal action in the production of organic forms. Darwin was
the first to teach us the great part that was played in this by the
ceaseless struggle for existence between living things, and to show
how, under the influence of this (by natural selection), new
species were produced and maintained solely by the interaction of
heredity and adaptation. It was thus Darwinism that first opened
our eyes to a true comprehension of the supremely important
relations between the two parts of the science of organic
evolution—Ontogeny and Phylogeny.

Heredity and adaptation are, in fact, the two constructive
physiological functions of living things; unless we understand
these properly we can make no headway in the study of evolution.
Hence, until the time of Darwin no one had a clear idea of the real
nature and causes of embryonic development. It was impossible to
explain the curious series of forms through which the human embryo
passed; it was quite unintelligible why this strange succession of
animal-like forms appeared in the series at all. It had previously
been generally assumed that the man was found complete in all his
parts in the ovum, and that the development consisted only in an
unfolding of the various parts, a simple process of growth. This is
by no means the case. On the contrary, the whole process of the
development of the individual presents to the observer a connected
succession of different animal-forms; and these forms display a
great variety of external and internal structure. But why
each individual human being should pass through this series of
forms in the course of his embryonic development it was quite
impossible to say until Lamarck and Darwin established the theory
of descent. Through this theory we have at last detected the real
causes, the efficient causes, of the individual development;
we have learned that these mechanical causes suffice of
themselves to effect the formation of the organism, and that there
is no need of the final causes which were formerly assumed.
It is true that in the academic philosophies of our time these
final causes still figure very prominently; in the new philosophy
of nature we can entirely replace them by efficient causes. We
shall see, in the course of our inquiry, how the most wonderful and
hitherto insoluble enigmas in the human and animal frame have
proved amenable to a mechanical explanation, by causes acting
without prevision, through Darwin’s reform of the science of
evolution. We have everywhere been able to substitute unconscious
causes, acting from necessity, for conscious, purposive
causes.^[4]

[4]
The monistic or mechanical philosophy of nature holds that only unconscious,
necessary, efficient causes are at work in the whole field of nature, in
organic life as well as in inorganic changes. On the other hand, the dualist or
vitalist philosophy of nature affirms that unconscious forces are only at work
in the inorganic world, and that we find conscious, purposive, or final causes
in organic nature.

If the new science of evolution had done no more than this,
every thoughtful man would have to admit that it had accomplished
an immense advance in knowledge. It means that in the whole of
philosophy that tendency which we call monistic, in opposition to
the dualistic, which has hitherto prevailed, must be
accepted.^[5] At this point the science of human evolution
has a direct and profound bearing on the foundations of philosophy.
Modern anthropology has, by its astounding discoveries during the
second half of the nineteenth century, compelled us to take a
completely monistic view of life. Our bodily structure and its
life, our embryonic development and our evolution as a species,
teach us that the same laws of nature rule in the life of man as in
the rest of the universe. For this reason, if for no others, it is
desirable, nay, indispensable, that every man who wishes to form a
serious and philosophic view of life, and, above all, the expert
philosopher, should acquaint himself with the chief facts of this
branch of science.

[5]
Monism is neither purely materialistic nor purely spiritualistic, but a
reconciliation of these two principles, since it regards the whole of nature as
one, and sees only efficient causes at work in it. Dualism, on the contrary,
holds that nature and spirit, matter and force, the world and God, inorganic
and organic nature, are separate and independent existences. Cf. The Riddle
of the Universe, chap. xii.

The facts of embryology have so great and obvious a significance in this
connection that even in recent years dualist and teleological philosophers have
tried to rid themselves of them by simply denying them. This was done, for
instance, as regards the fact that man is developed from an egg, and that this
egg or ovum is a simple cell, as in the case of other animals. When I had
explained this pregnant fact and its significance in my History of
Creation, it was described in many of the theological journals as a
dishonest invention of my own. The fact that the embryos of man and the dog
are, at a certain stage of their development, almost indistinguishable was also
denied. When we examine the human embryo in the third or fourth week of its
development, we find it to be quite different in shape and structure from the
full-grown human being, but almost identical with that of the ape, the dog, the
rabbit, and

other mammals, at the same stage of ontogeny. We find a bean-shaped body of
very simple construction, with a tail below and a pair of fins at the sides,
something like those of a fish, but very different from the limbs of man and
the mammals. Nearly the whole front half of the body is taken up by a shapeless
head without face, at the sides of which we find gill-clefts and arches as in
the fish. At this stage of its development the human embryo does not differ in
any essential detail from that of the ape, dog, horse, ox, etc., at a
corresponding period. This important fact can easily be verified at any moment
by a comparison of the embryos of man, the dog, rabbit, etc. Nevertheless, the
theologians and dualist philosophers pronounced it to be a materialistic
invention; even scientists, to whom the facts should be known, have sought to
deny them.

There could not be a clearer proof of the profound importance of
these embryological facts in favour of the monistic philosophy than
is afforded by these efforts of its opponents to get rid of them by
silence or denial. The truth is that these facts are most
inconvenient for them, and are quite irreconcilable with their
views. We must be all the more pressing on our side to put them in
their proper light. I fully agree with Huxley when he says, in his
Man’s Place in Nature: “Though these facts are
ignored by several well-known popular leaders, they are easy to
prove, and are accepted by all scientific men; on the other hand,
their importance is so great that those who have once mastered them
will, in my opinion, find few other biological discoveries to
astonish them.”

We shall make it our chief task to study the evolution of
man’s bodily frame and its various organs in their external
form and internal structures. But I may observe at once that this
is accompanied step by step with a study of the evolution of their
functions. These two branches of inquiry are inseparably united in
the whole of anthropology, just as in zoology (of which the former
is only a section) or general biology. Everywhere the peculiar form
of the organism and its structures, internal and external, is
directly related to the special physiological functions which the
organism or organ has to execute. This intimate connection of
structure and function, or of the instrument and the work done by
it, is seen in the science of evolution and all its parts. Hence
the story of the evolution of structures, which is our immediate
concern, is also the history of the development of functions; and
this holds good of the human organism as of any other.

At the same time, I must admit that our knowledge of the
evolution of functions is very far from being as complete as our
acquaintance with the evolution of structures. One might say, in
fact, that the whole science of evolution has almost confined
itself to the study of structures; the evolution of
functions hardly exists even in name. That is the fault of the
physiologists, who have as yet concerned themselves very little
about evolution. It is only in recent times that physiologists like
W. Engelmann, W. Preyer, M. Verworn, and a few others, have
attacked the evolution of functions.

It will be the task of some future physiologist to engage in the
study of the evolution of functions with the same zeal and success
as has been done for the evolution of structures in morphogeny (the
science of the genesis of forms). Let me illustrate the close
connection of the two by a couple of examples. The heart in the
human embryo has at first a very simple construction, such as we
find in permanent form among the ascidiæ and other low
organisms; with this is associated a very simple system of
circulation of the blood. Now, when we find that with the
full-grown heart there comes a totally different and much more
intricate circulation, our inquiry into the development of the
heart becomes at once, not only an anatomical, but also a
physiological, study. Thus it is clear that the ontogeny of the
heart can only be understood in the light of its phylogeny (or
development in the past), both as regards function and structure.
The same holds true of all the other organs and their functions.
For instance, the science of the evolution of the alimentary canal,
the lungs, or the sexual organs, gives us at the same time, through
the exact comparative investigation of structure-development, most
important information with regard to the evolution of the functions
of these organs.

This significant connection is very clearly seen in the
evolution of the nervous system. This system is in the economy of
the human body the medium of sensation, will, and even thought, the
highest of the psychic functions; in a word, of all the various functions which constitute the
proper object of psychology. Modern anatomy and physiology have
proved that these psychic functions are immediately dependent on
the fine structure and the composition of the central nervous
system, or the internal texture of the brain and spinal cord. In
these we find the elaborate cell-machinery, of which the psychic or
soul-life is the physiological function. It is so intricate that
most men still look upon the mind as something supernatural that
cannot be explained on mechanical principles.

But embryological research into the gradual appearance and the
formation of this important system of organs yields the most
astounding and significant results. The first sketch of a central
nervous system in the human embryo presents the same very simple
type as in the other vertebrates. A spinal tube is formed in the
external skin of the back, and from this first comes a simple
spinal cord without brain, such as we find to be the permanent
psychic organ in the lowest type of vertebrate, the amphioxus. Not
until a later stage is a brain formed at the anterior end of this
cord, and then it is a brain of the most rudimentary kind, such as
we find permanently among the lower fishes. This simple brain
develops step by step, successively assuming forms which correspond
to those of the amphibia, the reptiles, the duck-bills, and the
lemurs. Only in the last stage does it reach the highly organised
form which distinguishes the apes from the other vertebrates, and
which attains its full development in man.

Comparative physiology discovers a precisely similar growth. The
function of the brain, the psychic activity, rises step by step
with the advancing development of its structure.

Thus we are enabled, by this story of the evolution of the
nervous system, to understand at length the natural development
of the human mind and its gradual unfolding. It is only with
the aid of embryology that we can grasp how these highest and most
striking faculties of the animal organism have been historically
evolved. In other words, a knowledge of the evolution of the spinal
cord and brain in the human embryo leads us directly to a
comprehension of the historic development (or phylogeny) of the
human mind, that highest of all faculties, which we regard as
something so marvellous and supernatural in the adult man. This is
certainly one of the greatest and most pregnant results of
evolutionary science. Happily our embryological knowledge of
man’s central nervous system is now so adequate, and agrees
so thoroughly with the complementary results of comparative anatomy
and physiology, that we are thus enabled to obtain a clear insight
into one of the highest problems of philosophy, the phylogeny of
the soul, or the ancestral history of the mind of man. Our chief
support in this comes from the embryological study of it, or the
ontogeny of the soul. This important section of psychology owes its
origin especially to W. Preyer, in his interesting works, such as
The Mind of the Child. The Biography of a Baby (1900), of
Milicent Washburn Shinn, also deserves mention. [See also
Preyer’s Mental Development in the Child
(translation), and Sully’s Studies of Childhood and
Children’s Ways.]

In this way we follow the only path along which we may hope to
reach the solution of this difficult problem.

Thirty-six years have now elapsed since, in my General
Morphology, I established phylogeny as an independent science
and showed its intimate causal connection with ontogeny; thirty
years have passed since I gave in my gastræa-theory the proof
of the justice of this, and completed it with the theory of
germinal layers. When we look back on this period we may ask, What
has been accomplished during it by the fundamental law of biogeny?
If we are impartial, we must reply that it has proved its fertility
in hundreds of sound results, and that by its aid we have acquired
a vast fund of knowledge which we should never have obtained
without it.

There has been no dearth of attacks—often violent
attacks—on my conception of an intimate causal connection
between ontogenesis and phylogenesis; but no other satisfactory
explanation of these important phenomena has yet been offered to
us. I say this especially with regard to Wilhelm His’s theory
of a “mechanical evolution,” which questions the truth
of phylogeny generally, and would explain the complicated embryonic
processes without going beyond by simple physical
changes—such as the bending and folding of leaves by
electricity, the origin of cavities through unequal strain of the
tissues, the formation of processes by uneven growth, and so on.
But the fact is that these embryological phenomena themselves
demand explanation in turn, and this can only be found, as a rule,
in the corresponding changes in the long ancestral series, or in
the physiological functions of heredity and adaptation.

Chapter II.
THE OLDER EMBRYOLOGY

It is in many ways useful, on entering upon the study of any science, to cast a
glance at its historical development. The saying that “everything is best
understood in its growth” has a distinct application to science. While we
follow its gradual development we get a clearer insight into its aims and
objects. Moreover, we shall see that the present condition of the science of
human evolution, with all its characteristics, can only be rightly understood
when we examine its historical growth. This task will, however, not detain us
long. The study of man’s evolution is one of the latest branches of
natural science, whether you consider the embryological or the phylogenetic
section of it.

Apart from the few germs of our science which we find in classical antiquity,
and which we shall notice presently, we may say that it takes its definite
rise, as a science, in the year 1759, when one of the greatest German
scientists, Caspar Friedrich Wolff, published his Theoria generationis.
That was the foundation-stone of the science of animal embryology. It was not
until fifty years later, in 1809, that Jean Lamarck published his
Philosophie Zoologique—the first effort to provide a base for the
theory of evolution; and it was another half-century before Darwin’s work
appeared (in 1859), which we may regard as the first scientific attainment of
this aim. But before we go further into this solid establishment of evolution,
we must cast a brief glance at that famous philosopher and scientist of
antiquity, who stood alone in this, as in many other branches of science, for
more than 2000 years: the “father of Natural History,” Aristotle.

The extant scientific works of Aristotle deal with many different sides of
biological research; the most comprehensive of them is his famous History of
Animals. But not less interesting is the smaller work, On the Generation
of Animals (Peri zoon geneseos). This work treats especially of embryonic
development, and it is of great interest as being the earliest of its kind and
the only one that has come down to us in any completeness from classical
antiquity.

Aristotle studied embryological questions in various classes of animals, and
among the lower groups he learned many most remarkable facts which we only
rediscovered between 1830 and 1860. It is certain, for instance, that he was
acquainted with the very peculiar mode of propagation of the cuttlefishes, or
cephalopods, in which a yelk-sac hangs out of the mouth of the fœtus. He
knew, also, that embryos come from the eggs of the bee even when they have not
been fertilised. This “parthenogenesis” (or virgin-birth) of the
bees has only been established in our time by the distinguished zoologist of
Munich, Siebold. He discovered that male bees come from the unfertilised, and
female bees only from the fertilised, eggs. Aristotle further states that some
kinds of fishes (of the genus serranus) are hermaphrodites, each
individual having both male and female organs and being able to fertilise
itself; this, also, has been recently confirmed. He knew that the embryo of
many fishes of the shark family is attached to the mother’s body by a
sort of placenta, or nutritive organ very rich in blood; apart from these, such
an arrangement is only found among the higher mammals and man. This placenta of
the shark was looked upon as legendary for a long time, until Johannes
Müller proved it to be a fact in 1839. Thus a number of remarkable
discoveries were found in Aristotle’s embryological work, proving a very
good acquaintance of the great scientist—possibly helped by his
predecessors—with the facts of ontogeny, and a great advance upon
succeeding generations in this respect.

In the case of most of these discoveries he did not merely describe the fact,
but added a number of observations on its significance. Some of these
theoretical remarks are of particular interest, because they show a correct
appreciation of the nature of the embryonic processes. He conceives the
development of the individual as a new formation, in the course of which the
various parts of the body take shape successively. When the human or animal
frame is developed in the mother’s body, or separately in an egg, the
heart—which he regards as the starting-point and centre of the
organism—must appear first. Once the heart is formed the other organs
arise, the internal ones before the external, the upper (those above the
diaphragm) before the lower (or those beneath the diaphragm). The brain is
formed at an early stage, and the eyes grow out of it. These observations are
quite correct. And, if we try to form some idea from these data of
Aristotle’s general conception of the embryonic process, we find a dim
prevision of the theory which Wolff showed 2000 years afterwards to be the
correct view. It is significant, for instance, that Aristotle denied the
eternity of the individual in any respect. He said that the species or genus,
the group of similar individuals, might be eternal, but the individual itself
is temporary. It comes into being in the act of procreation, and passes away at
death.

During the 2000 years after Aristotle no progress whatever was made in general
zoology, or in embryology in particular. People were content to read, copy,
translate, and comment on Aristotle. Scarcely a single independent effort at
research was made in the whole of the period. During the Middle Ages the spread
of strong religious beliefs put formidable obstacles in the way of independent
scientific investigation. There was no question of resuming the advance of
biology. Even when human anatomy began to stir itself once more in the
sixteenth century, and independent research was resumed into the structure of
the developed body, anatomists did not dare to extend their inquiries to the
unformed body, the embryo, and its development. There were many reasons for the
prevailing horror of such studies. It is natural enough, when we remember that
a Bull of Boniface VIII excommunicated every man who ventured to dissect a
human corpse. If the dissection of a developed body were a crime to be thus
punished, how much more dreadful must it have seemed to deal with the embryonic
body still enclosed in the womb, which the Creator himself had decently veiled
from the curiosity of the scientist! The Christian Church, then putting many
thousands to death for unbelief, had a shrewd presentiment of the menace that
science contained against its authority. It was powerful enough to see that its
rival did not grow too quickly.

It was not until the Reformation broke the power of the Church, and a
refreshing breath of the spirit dissolved the icy chains that bound science,
that anatomy and embryology, and all the other branches of research, could
begin to advance once more. However, embryology lagged far behind anatomy. The
first works on embryology appear at the beginning of the sixteenth century. The
Italian anatomist, Fabricius ab Aquapendente, a professor at Padua, opened the
advance. In his two books (De formato fœtu, 1600, and De
formatione fœtus, 1604) he published the older illustrations and
descriptions of the embryos of man and other mammals, and of the hen. Similar
imperfect illustrations were given by Spigelius (De formato fœtu,
1631), and by Needham (1667) and his more famous compatriot, Harvey (1652), who
discovered the circulation of the blood in the animal body and formulated the
important principle, Omne vivum ex vivo (all life comes from
pre-existing life). The Dutch scientist, Swammerdam, published in his Bible
of Nature the earliest observations on the embryology of the frog and the
division of its egg-yelk. But the most important embryological studies in the
sixteenth century were those of the famous Italian, Marcello Malpighi, of
Bologna, who led the way both in zoology and botany. His treatises, De
formatione pulli and De ovo incubato (1687), contain the first
consistent description of the development of the chick in the fertilised egg.

Here I ought to say a word about the important part played by the chick in the
growth of our science. The development of the chick, like that of the young of
all other birds, agrees in all its main features with that of the other chief
vertebrates, and even of man. The three highest classes of
vertebrates—mammals, birds, and reptiles (lizards, serpents, tortoises,
etc.)—have from the beginning of their embryonic development so striking
a resemblance in all the chief points of structure, and especially in their
first forms, that for a long time it is impossible to distinguish between them.
We have known now for some time that we need only examine the embryo of a bird,
which is the easiest to get at, in order to learn the typical mode of
development of a mammal (and therefore of man). As soon as scientists began to
study the human embryo, or the mammal-embryo generally, in its earlier stages
about the middle and end of the seventeenth century, this important fact was
very quickly discovered. It is both theoretically and practically of great
value. As regards the theory of evolution, we can draw the most weighty
inferences from this similarity between the embryos of widely different classes
of animals. But for the practical purposes of embryological research the
discovery is invaluable, because we can fill up the gaps in our imperfect
knowledge of the embryology of the mammals from the more thoroughly studied
embryology of the bird. Hens’ eggs are easily to be had in any quantity,
and the development of the chick may be followed step by step in artificial
incubation. The development of the mammal is much more difficult to follow,
because here the embryo is not detached and enclosed in a large egg, but the
tiny ovum remains in the womb until the growth is completed. Hence, it is very
difficult to keep up sustained observation of the various stages in any great
extent, quite apart from such extrinsic considerations as the cost, the
technical difficulties, and many other obstacles which we encounter when we
would make an extensive study of the fertilised mammal. The chicken has,
therefore, always been the chief object of study in this connection. The
excellent incubators we now have enable us to observe it in any quantity and at
any stage of development, and so follow the whole course of its formation step
by step.

By the end of the seventeenth century Malpighi had advanced as far as it was
possible to do with the imperfect microscope of his time in the embryological
study of the chick. Further progress was arrested until the instrument and the
technical methods should be improved. The vertebrate embryos are so small and
delicate in their earlier stages that you cannot go very far into the study of
them without a good microscope and other technical aid. But this substantial
improvement of the microscope and the other apparatus did not take place until
the beginning of the nineteenth century.

Embryology made scarcely any advance in the first half of the eighteenth
century, when the systematic natural history of plants and animals received so
great an impulse through the publication of Linné’s famous
Systema Naturæ. Not until 1759 did the genius arise who was to
give it an entirely new character, Caspar Friedrich Wolff. Until then
embryology had been occupied almost exclusively in unfortunate and misleading
efforts to build up theories on the imperfect empirical material then
available.

The theory which then prevailed, and remained in favour throughout nearly the
whole of the eighteenth century, was commonly called at that time “the
evolution theory”; it is better to describe it as “the preformation
theory.”^[6] Its chief point is this: There is no new formation
of structures in the embryonic development of any organism, animal or plant, or
even of man; there is only a growth, or unfolding, of parts which have been
constructed or pre-formed from all eternity, though on a very small
scale and closely packed together. Hence, every living germ contains all the
organs and parts of the body, in the form and arrangement they will present
later, already within it, and thus the whole embryological process is merely an
evolution in the literal sense of the word, or an unfolding, of
parts that were pre-formed and folded up in it. So, for instance, we find in
the hen’s egg not merely a simple cell, that divides and subdivides and
forms germinal layers, and at last, after all kinds of variation and cleavage
and reconstruction, brings forth the body of the chick; but there is in every
egg from the first a complete chicken, with all its parts made and neatly
packed. These parts are so small or so transparent that the microscope cannot
detect them. In the hatching, these parts merely grow larger, and spread out in
the normal way.

[6]
This theory is usually known as the “evolution theory” in Germany,
in contradistinction to the “epigenesis theory.” But as it is the
latter that is called the “evolution theory” in England, France,
and Italy, and “evolution” and “epigenesis” are taken
to be synonymous, it seems better to call the first the “pre-formation
theory.”

When this theory is consistently developed it becomes a “scatulation
theory.”^[7] According to its teaching, there was made in the
beginning one couple or one individual of each species of animal or plant; but
this one individual contained the germs of all the other individuals of the
same species who should ever come to life. As the age of the earth was
generally believed at that time to be fixed by the Bible at 5000 or 6000 years,
it seemed possible to calculate how many individuals of each species had lived
in the period, and so had been packed inside the first being that was created.
The theory was consistently extended to man, and it was affirmed that our
common parent Eve had had stored in her ovary the germs of all the children of
men.

[7]
“Packing theory” would be the literal translation. Scatula is the
Latin for a case or box.—Translator.

The theory at first took the form of a belief that it was the females
who were thus encased in the first being. One couple of each species was
created, but the female contained in her ovary all the future individuals of
the species, of either sex. However, this had to be altered when the Dutch
microscopist, Leeuwenhoek, discovered the male spermatozoa in 1690, and showed
that an immense number of these extremely fine and mobile thread-like beings
exist in the male sperm (this will be explained in Chapter VII). This
astonishing discovery was further advanced when it was proved that these living
bodies, swimming about in the seminal fluid, were real animalcules, and, in
fact, were the pre-formed germs of the future generation. When the male and
female procreative elements came together at conception, these thread-like
spermatozoa (“seed-animals”) were supposed to penetrate into the
fertile body of the ovum and begin to develop there, as the plant seed does in
the fruitful earth. Hence, every spermatozoon was regarded as a
homunculus, a tiny complete man; all the parts were believed to be
pre-formed in it, and merely grew larger when it reached its proper medium in
the female ovum. This theory, also, was consistently developed in the sense
that in each of these thread-like bodies the whole of its posterity was
supposed to be present in the minutest form. Adam’s sexual glands were
thought to have contained the germs of the whole of humanity.

This “theory of male scatulation” found itself at once in keen
opposition to the prevailing “female” theory. The two rival
theories at once opened a very lively campaign, and the physiologists of the
eighteenth century were divided into two great camps—the Animalculists
and the Ovulists—which fought vigorously. The animalculists held that the
spermatozoa were the true germs, and appealed to the lively movements and the
structure of these bodies. The opposing party of the Ovulists, who clung to the
older “evolution theory,” affirmed that the ovum is the real germ,
and that the spermatozoa merely stimulate it at conception to begin its growth;
all the future generations are stored in the ovum. This view was held by the
great majority of the biologists of the eighteenth century, in spite of the
fact that Wolff proved it in 1759 to be without foundation. It owed its
prestige chiefly to the circumstance that the most weighty authorities in the
biology and philosophy of the day decided in favour of it, especially Haller,
Bonnet, and Leibnitz.

Albrecht Haller, professor at Göttingen, who is often called the father of
physiology, was a man of wide and varied learning, but he does not occupy a
very high position in regard to insight into natural phenomena. He made a
vigorous defence of the “evolutionary theory” in his famous work,
Elementa physiologiae, affirming: “There is no such thing as
formation (nulla est epigenesis). No part of the animal frame is made
before another; all were made together.” He thus denied that there was
any evolution in the proper sense of the word, and even went so far as to say
that the beard existed in the new-born child and the antlers in the hornless
fawn; all the parts were there in advance, and were merely hidden from the eye
of man for the time being. Haller even calculated the number of human beings
that God must have created on the sixth day and stored away in Eve’s
ovary. He put the number at 200,000 millions, assuming the age of the world to
be 6000 years, the average age of a human being to be thirty years, and the
population of the world at that time to be 1000 millions. And the famous Haller
maintained all this nonsense, in spite of its ridiculous consequences, even
after Wolff had discovered the real course of embryonic development and
established it by direct observation!

Among the philosophers of the time the distinguished Leibnitz was the chief
defender of the “preformation theory,” and by his authority and
literary prestige won many adherents to it. Supported by his system of monads,
according to which body and soul are united in inseparable association and by
their union form the individual, or the “monad,” Leibnitz
consistently extended the “scatulation theory” to the soul, and
held that this was no more evolved than the body. He says, for instance, in his
Théodicée: “I mean that these souls, which one day
are to be the souls of men, are present in the seed, like those of other
species; in such wise that they existed in our ancestors as far back as Adam,
or from the beginning of the world, in the forms of organised bodies.”

The theory seemed to receive considerable support from the observations of one
of its most zealous supporters, Bonnet. In 1745 he discovered, in the
plant-louse, a case of parthenogenesis, or virgin-birth, an interesting form of
reproduction that has lately been found by Siebold and others among various
classes of the articulata, especially crustacea and insects. Among these and
other animals of certain lower species the female may reproduce for several
generations without having been fertilised by the male. These ova that do not
need fertilisation are called “false ova,” pseudova or spores.
Bonnet saw that a female plant-louse, which he had kept in cloistral isolation,
and rigidly removed from contact with males, had on the eleventh day (after
forming a new skin for the fourth time) a living daughter, and during the next
twenty days ninety-four other daughters; and that all of them went on to
reproduce in the same way without any contact with males. It seemed as if this
furnished an irrefutable proof of the truth of the scatulation theory, as it
was held by the Ovulists; it is not surprising to find that the theory then
secured general acceptance.

This was the condition of things when suddenly, in 1759, Caspar Friedrich Wolff
appeared, and dealt a fatal blow at the whole preformation theory with his new
theory of epigenesis. Wolff, the son of a Berlin tailor, was born in 1733, and
went through his scientific and medical studies, first at Berlin under the
famous anatomist Meckel, and afterwards at Halle. Here he secured his doctorate
in his twenty-sixth year, and in his academic dissertation (November 28th,
1759), the Theoria generationis, expounded the new theory of a real
development on a basis of epigenesis. This treatise is, in spite of its
smallness and its obscure phraseology, one of the most valuable in the whole
range of biological literature. It is equally distinguished for the mass of new
and careful observations it contains, and the far-reaching and pregnant ideas
which the author everywhere extracts from his observations and builds into a
luminous and accurate theory of generation. Nevertheless, it met with no
success at the time. Although scientific studies were then assiduously
cultivated owing to the impulse given by Linné—although botanists
and zoologists were no longer counted by dozens, but by hundreds, hardly any
notice was taken of Wolff’s theory. Even when he established the truth of
epigenesis by the most rigorous observations, and demolished the airy structure
of the preformation theory, the “exact” scientist Haller proved one
of the most strenuous supporters of the old theory, and rejected Wolff’s
correct view with a dictatorial “There is no such thing as
evolution.” He even went on to say that religion was menaced by the new
theory! It is not surprising that the whole of the physiologists of the second
half of the eighteenth century submitted to the ruling of this physiological
pontiff, and attacked the theory of epigenesis as a dangerous innovation. It
was not until more than fifty years afterwards that Wolff’s work was
appreciated. Only when Meckel translated into German in 1812 another valuable
work of Wolff’s on The Formation of the Alimentary Canal (written
in 1768), and called attention to its great importance, did people begin to
think of him once more; yet this obscure writer had evinced a profounder
insight into the nature of the living organism than any other scientist of the
eighteenth century.

Wolff’s idea led to an appreciable advance over the whole field of
biology. There is such a vast number of new and important observations and
pregnant thoughts in his writings that we have only gradually learned to
appreciate them rightly in the course of the nineteenth century. He opened up
the true path for research in many directions. In the first place, his theory
of epigenesis gave us our first real insight into the nature of embryonic
development. He showed convincingly that the development of every organism
consists of a series of new formations, and that there is no trace
whatever of the complete form either in the ovum or the spermatozoon. On the
contrary, these are quite simple bodies, with a very different purport. The
embryo which is developed from them is also quite different, in its internal
arrangement and outer configuration, from the complete organism. There is no
trace whatever of preformation or in-folding of organs. To-day we can scarcely
call epigenesis a theory, because we are convinced it is a fact, and can
demonstrate it at any moment with the aid of the microscope.

Wolff furnished the conclusive empirical proof of his theory in his classic
dissertation on The Formation of the Alimentary Canal (1768). In its
complete state the alimentary canal of the hen is a long and complex tube, with
which the lungs, liver, salivary glands, and many other small glands, are
connected. Wolff showed that in the early stages of the embryonic chick there
is no trace whatever of this complicated tube with all its dependencies, but
instead of it only a flat, leaf-shaped body; that, in fact, the whole embryo
has at first the appearance of a flat, oval-shaped leaf. When we remember how
difficult the exact observation of so fine and delicate a structure as the
early leaf-shaped body of the chick must have been with the poor microscopes
then in use, we must admire the rare faculty for observation which enabled
Wolff to make the most important discoveries in this most difficult part of
embryology. By this laborious research he reached the correct opinion that the
embryonic body of all the higher animals, such as the birds, is for some time
merely a flat, thin, leaf-shaped disk—consisting at first of one layer,
but afterwards of several. The lowest of these layers is the alimentary canal,
and Wolff followed its development from its commencement to its completion. He
showed how this leaf-shaped structure first turns into a groove, then the
margins of this groove fold together and form a closed canal, and at length the
two external openings of the tube (the mouth and anus) appear.

Moreover, the important fact that the other systems of organs are developed in
the same way, from tubes formed out of simple layers, did not escape Wolff. The
nerveless system, muscular system, and vascular (blood-vessel) system, with all
the organs appertaining thereto, are, like the alimentary system, developed out
of simple leaf-shaped structures. Hence, Wolff came to the view by 1768 which
Pander developed in the Theory of Germinal Layers fifty years
afterwards. His principles are not literally correct; but he comes as near to
the truth in them as was possible at that time, and could be expected of him.

Our admiration of this gifted genius increases when we find that he was also
the precursor of Goethe in regard to the metamorphosis of plants and of the
famous cellular theory. Wolff had, as Huxley showed, a clear presentiment of
this cardinal theory, since he recognised small microscopic globules as the
elementary parts out of which the germinal layers arose.

Finally, I must invite special attention to the mechanical character of
the profound philosophic reflections which Wolff always added to his remarkable
observations. He was a great monistic philosopher, in the best meaning of the
word. It is unfortunate that his philosophic discoveries were ignored as
completely as his observations for more than half a century. We must be all the
more careful to emphasise the fact of their clear monistic tendency.

Chapter III.
MODERN EMBRYOLOGY

We may distinguish three chief periods in the growth of our science of human
embryology. The first has been considered in the preceding chapter; it embraces
the whole of the preparatory period of research, and extends from Aristotle to
Caspar Friedrich Wolff, or to the year 1759, in which the epoch-making
Theoria generationis was published. The second period, with which we
have now to deal, lasts about a century—that is to say, until the
appearance of Darwin’s Origin of Species, which brought about a
change in the very foundations of biology, and, in particular, of embryology.
The third period begins with Darwin. When we say that the second period lasted
a full century, we must remember that Wolff’s work had remained almost
unnoticed during half the time—namely, until the year 1812. During the
whole of these fifty-three years not a single book that appeared followed up
the path that Wolff had opened, or extended his theory of embryonic
development. We merely find his views—perfectly correct views, based on
extensive observations of fact—mentioned here and there as erroneous;
their opponents, who adhered to the dominant theory of preformation, did not
even deign to reply to them. This unjust treatment was chiefly due to the
extraordinary authority of Albrecht von Haller; it is one of the most
astonishing instances of a great authority, as such, preventing for a long time
the recognition of established facts.

The general ignorance of Wolff’s work was so great that at the beginning
of the nineteenth century two scientists of Jena, Oken (1806) and Kieser
(1810), began independent research into the development of the alimentary canal
of the chick, and hit upon the right clue to the embryonic puzzle, without
knowing a word about Wolff’s important treatise on the same subject. They
were treading in his very footsteps without suspecting it. This can be easily
proved from the fact that they did not travel as far as Wolff. It was not until
Meckel translated into German Wolff’s book on the alimentary system, and
pointed out its great importance, that the eyes of anatomists and physiologists
were suddenly opened. At once a number of biologists instituted fresh
embryological inquiries, and began to confirm Wolff’s theory of
epigenesis.

This resuscitation of embryology and development of the epigenesis-theory was
chiefly connected with the university of Würtzburg. One of the professors there
at that time was Döllinger, an eminent biologist, and father of the famous
Catholic historian who later distinguished himself by his opposition to the new
dogma of papal infallibility. Döllinger was both a profound thinker and an
accurate observer. He took the keenest interest in embryology, and worked at it
a good deal. However, he is not himself responsible for any important result in
this field. In 1816 a young medical doctor, whom we may at once designate as
Wolff’s chief successor, Karl Ernst von Baer, came to Würtzburg.
Baer’s conversations with Döllinger on embryology led to a fresh series
of most extensive investigations. Döllinger had expressed a wish that some
young scientist should begin again under his guidance an independent inquiry
into the development of the chick during the hatching of the egg. As neither he
nor Baer had money enough to pay for an incubator and the proper control of the
experiments, and for a competent artist to illustrate the various stages
observed, the lead of the enterprise was given to Christian Pander, a wealthy
friend of Baer’s who had been induced by Baer to come to Würtzburg. An
able engraver, Dalton, was engaged to do the copper-plates. In a short time the
embryology of the chick, in which Baer was taking the greatest indirect
interest, was so far advanced that Pander was able to sketch the main features
of it on the ground of Wolff’s theory in the dissertation he published in
1817. He clearly enunciated the theory of germinal layers which Wolff

had anticipated, and established the truth of Wolff’s idea of a
development of the complicated systems of organs out of simple leaf-shaped
primitive structures. According to Pander, the leaf-shaped object in the
hen’s egg divides, before the incubation has proceeded twelve hours, into
two different layers, an external serous layer and an internal
mucous layer; between the two there develops later a third layer, the
vascular (blood-vessel) layer.^[8]

[8]
The technical terms which are bound to creep into this chapter will be fully
understood later on.—Translator.

Karl Ernst von Baer, who had set afoot Pander’s investigation, and had
shown the liveliest interest in it after Pander’s departure from
Würtzburg, began his own much more comprehensive research in 1819. He published
the mature result nine years afterwards in his famous work, Animal
Embryology: Observation and Reflection (not translated). This classic work
still remains a model of careful observation united to profound philosophic
speculation. The first part appeared in 1828, the second in 1837. The book
proved to be the foundation on which the whole science of embryology has built
down to our own day. It so far surpassed its predecessors, and Pander in
particular, that it has become, after Wolff’s work, the chief base of
modern embryology.

Baer was one of the greatest scientists of the nineteenth century, and
exercised considerable influence on other branches of biology as well. He built
up the theory of germinal layers, as a whole and in detail, so clearly and
solidly that it has been the starting-point of embryological research ever
since. He taught that in all the vertebrates first two and then four of these
germinal layers are formed; and that the earliest rudimentary organs of the
body arise by the conversion of these layers into tubes. He described the first
appearance of the vertebrate embryo, as it may be seen in the globular yelk of
the fertilised egg, as an oval disk which first divides into two layers. From
the upper or animal layer are developed all the organs which accomplish
the phenomena of animal life—the functions of sensation and motion, and
the covering of the body. From the lower or vegetative layer come the
organs which effect the vegetative life of the organism—nutrition,
digestion, blood-formation, respiration, secretion, reproduction, etc.

Each of these original layers divides, according to Baer, into two thinner and
superimposed layers or plates. He calls the two plates of the animal layer, the
skin-stratum and muscle-stratum. From the upper of these plates, the
skin-stratum, the external skin, or outer covering of the body, the
central nervous system, and the sense-organs, are formed. From the lower, or
muscle-stratum, the muscles, or fleshy parts and the bony
skeleton—in a word, the motor organs—are evolved. In the same way,
Baer said, the lower or vegetative layer splits into two plates, which he calls
the vascular-stratum and the mucous-stratum. From the outer of the two (the
vascular) the heart, blood-vessels, spleen, and the other vascular
glands, the kidneys, and sexual glands, are formed. From the fourth or
mucous layer, in fine, we get the internal and digestive lining of the
alimentary canal and all its dependencies, the liver, lungs, salivary glands,
etc. Baer had, in the main, correctly judged the significance of these four
secondary embryonic layers, and he followed the conversion of them into the
tube-shaped primitive organs with great perspicacity. He first solved the
difficult problem of the transformation of this four-fold, flat, leaf-shaped,
embryonic disk into the complete vertebrate body, through the conversion of the
layers or plates into tubes. The flat leaves bend themselves in obedience to
certain laws of growth; the borders of the curling plates approach nearer and
nearer; until at last they come into actual contact. Thus out of the flat
gut-plate is formed a hollow gut-tube, out of the flat spinal plate a hollow
nerve-tube, from the skin-plate a skin-tube, and so on.

Among the many great services which Baer rendered to embryology, especially
vertebrate embryology, we must not forget his discovery of the human ovum.
Earlier scientists had, as a rule, of course, assumed that man developed out of
an egg, like the other animals. In fact, the preformation theory held that the
germs of the whole of humanity were stored already in Eve’s ova. But the
real ovum escaped detection until the year 1827. This ovum is extremely small,
being a tiny round vesicle about the 1/120 of an inch in diameter; it can be
seen under very favourable circumstances with the naked eye as a tiny particle,
but is otherwise quite invisible. This particle is formed in the ovary inside a
much larger

globule, which takes the name of the Graafian follicle, from its discoverer,
Graaf, and had previously been regarded as the true ovum. However, in 1827 Baer
proved that it was not the real ovum, which is much smaller, and is contained
within the follicle. (Compare the end of Chapter XXIX.)

Baer was also the first to observe what is known as the segmentation
sphere of the vertebrate; that is to say, the round vesicle which first
develops out of the impregnated ovum, and the thin wall of which is made up of
a single layer of regular, polygonal (many-cornered) cells (see the
illustration in Chapter XII). Another discovery of his that was of great
importance in constructing the vertebrate stem and the characteristic
organisation of this extensive group (to which man belongs) was the detection
of the axial rod, or the chorda dorsalis. There is a long, round,
cylindrical rod of cartilage which runs down the longer axis of the vertebrate
embryo; it appears at an early stage, and is the first sketch of the spinal
column, the solid skeletal axis of the vertebrate. In the lowest of the
vertebrates, the amphioxus, the internal skeleton consists only of this cord
throughout life. But even in the case of man and all the higher vertebrates it
is round this cord that the spinal column and the brain are afterwards formed.

However, important as these and many other discoveries of Baer’s were in
vertebrate embryology, his researches were even more influential, from the
circumstance that he was the first to employ the comparative method in
studying the development of the animal frame. Baer occupied himself chiefly
with the embryology of vertebrates (especially the birds and fishes). But he by
no means confined his attention to these, gradually taking the various groups
of the invertebrates into his sphere of study. As the general result of his
comparative embryological research, Baer distinguished four different modes of
development and four corresponding groups in the animal world. These chief
groups or types are: 1, the vertebrata; 2, the articulata; 3, the mollusca; and
4, all the lower groups which were then wrongly comprehended under the general
name of the radiata. Georges Cuvier had been the first to formulate this
distinction, in 1812. He showed that these groups present specific differences
in their whole internal structure, and the connection and disposal of their
systems of organs; and that, on the other hand, all the animals of the same
type—say, the vertebrates—essentially agreed in their inner
structure, in spite of the greatest superficial differences. But Baer proved
that these four groups are also quite differently developed from the ovum; and
that the series of embryonic forms is the same throughout for animals of the
same type, but different in the case of other animals. Up to that time the
chief aim in the classification of the animal kingdom was to arrange all the
animals from lowest to highest, from the infusorium to man, in one long and
continuous series. The erroneous idea prevailed nearly everywhere that there
was one uninterrupted chain of evolution from the lowest animal to the highest.
Cuvier and Baer proved that this view was false, and that we must distinguish
four totally different types of animals, on the ground of anatomic structure
and embryonic development.

Baer’s epoch-making works aroused an extraordinary and widespread
interest in embryological research. Immediately afterwards we find a great
number of observers at work in the newly opened field, enlarging it in a very
short time with great energy by their various discoveries in detail. Next to
Baer’s comes the admirable work of Heinrich Rathke, of Königsberg (died
1860); he made an extensive study of the embryology, not only of the
invertebrates (crustaceans, insects, molluscs), but also, and particularly, of
the vertebrates (fishes, tortoises, serpents, crocodiles, etc.). We owe the
first comprehensive studies of mammal embryology to the careful research of
Wilhelm Bischoff, of Munich; his embryology of the rabbit (1840), the dog
(1842), the guinea-pig (1852), and the doe (1854), still form classical
studies. About the same time a great impetus was given to the embryology of the
invertebrates. The way was opened through this obscure province by the studies
of the famous Berlin zoologist, Johannes Müller, on the echinoderms. He was
followed by Albert Kölliker, of Würtzburg, writing on the cuttlefish (or the
cephalopods), Siebold and Huxley on worms and zoophytes, Fritz Muller
(Desterro) on the crustacea, Weismann on insects, and so on. The number of
workers in this field has greatly increased of late, and a quantity of new and
astonishing discoveries have been made. One notices, in several of these recent
works on

embryology, that their authors are too little acquainted with comparative
anatomy and classification. Paleontology is, unfortunately, altogether
neglected by many of these new workers, although this interesting science
furnishes most important facts for phylogeny, and thus often proves of very
great service in ontogeny.

A very important advance was made in our science in 1839, when the cellular
theory was established, and a new field of inquiry bearing on embryology was
suddenly opened. When the famous botanist, M. Schleiden, of Jena, showed in
1838, with the aid of the microscope, that every plant was made up of
innumerable elementary parts, which we call cells, a pupil of Johannes
Müller at Berlin, Theodor Schwann, applied the discovery at once to the animal
organism. He showed that in the animal body as well, when we examine its
tissues in the microscope, we find these cells everywhere to be the elementary
units. All the different tissues of the organism, especially the very
dissimilar tissues of the nerves, muscles, bones, external skin, mucous lining,
etc., are originally formed out of cells; and this is also true of all the
tissues of the plant. These cells are separate living beings; they are the
citizens of the State which the entire multicellular organism seems to be. This
important discovery was bound to be of service to embryology, as it raised a
number of new questions. What is the relation of the cells to the germinal
layers? Are the germinal layers composed of cells, and what is their relation
to the cells of the tissues that form later? How does the ovum stand in the
cellular theory? Is the ovum itself a cell, or is it composed of cells? These
important questions were now imposed on the embryologist by the cellular
theory.

The most notable effort to answer these questions—which were attacked on
all sides by different students—is contained in the famous work,
Inquiries into the Development of the Vertebrates (not translated) of
Robert Remak, of Berlin (1851). This gifted scientist succeeded in mastering,
by a complete reform of the science, the great difficulties which the cellular
theory had at first put in the way of embryology. A Berlin anatomist, Carl
Boguslaus Reichert, had already attempted to explain the origin of the tissues.
But this attempt was bound to miscarry, since its not very clear-headed author
lacked a sound acquaintance with embryology and the cell theory, and even with
the structure and development of the tissue in particular. Remak at length
brought order into the dreadful confusion that Reichert had caused; he gave a
perfectly simple explanation of the origin of the tissues. In his opinion the
animal ovum is always a simple cell : the germinal layers which develop
out of it are always composed of cells; and these cells that constitute the
germinal layers arise simply from the continuous and repeated cleaving
(segmentation) of the original solitary cell. It first divides into two and
then into four cells; out of these four cells are born eight, then sixteen,
thirty-two, and so on. Thus, in the embryonic development of every animal and
plant there is formed first of all out of the simple egg cell, by a repeated
subdivision, a cluster of cells, as Kölliker had already stated in connection
with the cephalopods in 1844. The cells of this group spread themselves out
flat and form leaves or plates; each of these leaves is formed exclusively out
of cells. The cells of different layers assume different shapes, increase, and
differentiate; and in the end there is a further cleavage (differentiation) and
division of work of the cells within the layers, and from these all the
different tissues of the body proceed.

These are the simple foundations of histogeny, or the science that
treats of the development of the tissues ( hista), as it was established
by Remak and Kölliker. Remak, in determining more closely the part which the
different germinal layers play in the formation of the various tissues and
organs, and in applying the theory of evolution to the cells and the tissues
they compose, raised the theory of germinal layers, at least as far as it
regards the vertebrates, to a high degree of perfection.

Remak showed that three layers are formed out of the two germinal layers which
compose the first simple leaf-shaped structure of the vertebrate body (or the
“germinal disk”), as the lower layer splits into two plates. These
three layers have a very definite relation to the various tissues. First of
all, the cells which form the outer skin of the body (the epidermis), with its
various dependencies (hairs, nails, etc.)—that is to say, the entire
outer envelope of the body—are developed out of the outer or upper layer;
but there are also developed in a curious way out of the same layer the cells
which form the central nervous system, the
brain and the spinal cord. In the second place, the inner or lower germinal
layer gives rise only to the cells which form the epithelium (the whole inner
lining) of the alimentary canal and all that depends on it (the lungs, liver,
pancreas, etc.), or the tissues that receive and prepare the nourishment of the
body. Finally, the middle layer gives rise to all the other tissues of the
body, the muscles, blood, bones, cartilage, etc. Remak further proved that this
middle layer, which he calls “the motor-germinative layer,”
proceeds to subdivide into two secondary layers. Thus we find once more the
four layers which Baer had indicated. Remak calls the outer secondary leaf of
the middle layer (Baer’s “muscular layer”) the “skin
layer” (it would be better to say, skin-fibre layer); it forms the outer
wall of the body (the true skin, the muscles, etc.). To the inner secondary
leaf (Baer’s “vascular layer”) he gave the name of the
“alimentary-fibre layer”; this forms the outer envelope of the
alimentary canal, with the mesentery, the heart, the blood-vessels, etc.

On this firm foundation provided by Remak for histogeny, or the science
of the formation of the tissues, our knowledge has been gradually built up and
enlarged in detail. There have been several attempts to restrict and even
destroy Remak’s principles. The two anatomists, Reichert (of Berlin) and
Wilhelm His (of Leipzic), especially, have endeavoured in their works to
introduce a new conception of the embryonic development of the vertebrate,
according to which the two primary germinal layers would not be the sole
sources of formation. But these efforts were so seriously marred by ignorance
of comparative anatomy, an imperfect acquaintance with ontogenesis, and a
complete neglect of phylogenesis, that they could not have more than a passing
success. We can only explain how these curious attacks of Reichert and His came
to be regarded for a time as advances by the general lack of discrimination and
of grasp of the true object of embryology.

Wilhelm His published, in 1868, his extensive Researches into the Earliest
Form of the Vertebrate Body,^[9] one of the curiosities of
embryological literature. The author imagines that he can build a
“mechanical theory of embryonic development” by merely giving an
exact description of the embryology of the chick, without any regard to
comparative anatomy and phylogeny, and thus falls into an error that is almost
without parallel in the history of biological literature. As the final result
of his laborious investigations, His tells us “that a comparatively
simple law of growth is the one essential thing in the first development. Every
formation, whether it consist in cleavage of layers, or folding, or complete
division, is a consequence of this fundamental law.” Unfortunately, he
does not explain what this “law of growth” is; just as other
opponents of the theory of selection, who would put in its place a great
“law of evolution,” omit to tell us anything about the nature of
this. Nevertheless, it is quite clear from His’s works that he imagines
constructive Nature to be a sort of skilful tailor. The ingenious operator
succeeds in bringing into existence, by “evolution,” all the
various forms of living things by cutting up in different ways the germinal
layers, bending and folding, tugging and splitting, and so on.

[9]
None of His’s works have been translated into English.

His’s embryological theories excited a good deal of interest at the time
of publication, and have evoked a fair amount of literature in the last few
decades. He professed to explain the most complicated parts of organic
construction (such as the development of the brain) in the simplest way on
mechanical principles, and to derive them immediately from simple physical
processes (such as unequal distribution of strain in an elastic plate). It is
quite true that a mechanical or monistic explanation (or a reduction of natural
processes) is the ideal of modern science, and this ideal would be realised if
we could succeed in expressing these formative processes in mathematical
formulæ. His has, therefore, inserted plenty of numbers and measurements in his
embryological works, and given them an air of “exact” scholarship
by putting in a quantity of mathematical tables. Unfortunately, they are of no
value, and do not help us in the least in forming an “exact”
acquaintance with the embryonic phenomena. Indeed, they wander from the true
path altogether by neglecting the phylogenetic method; this, he thinks, is
“a mere by-path,” and is “not necessary at all for the
explanation of the facts of
embryology,” which are the direct consequence of physiological
principles. What His takes to be a simple physical process—for instance,
the folding of the germinal layers (in the formation of the medullary tube,
alimentary tube, etc.)—is, as a matter of fact, the direct result of the
growth of the various cells which form those organic structures; but these
growth-motions have themselves been transmitted by heredity from parents and
ancestors, and are only the hereditary repetition of countless phylogenetic
changes which have taken place for thousands of years in the race-history of
the said ancestors. Each of these historical changes was, of course, originally
due to adaptation; it was, in other words, physiological, and reducible to
mechanical causes. But we have, naturally, no means of observing them now. It
is only by the hypotheses of the science of evolution that we can form an
approximate idea of the organic links in this historic chain.

All the best recent research in animal embryology has led to the confirmation
and development of Baer and Remak’s theory of the germinal layers. One of
the most important advances in this direction of late was the discovery that
the two primary layers out of which is built the body of all vertebrates
(including man) are also present in all the invertebrates, with the sole
exception of the lowest group, the unicellular protozoa. Huxley had detected
them in the medusa in 1849. He showed that the two layers of cells from which
the body of this zoophyte is developed correspond, both morphologically and
physiologically, to the two original germinal layers of the vertebrate. The
outer layer, from which come the external skin and the muscles, was then called
by Allman (1853) the “ectoderm” (outer layer, or skin); the inner
layer, which forms the alimentary and reproductory organs, was called the
“entoderm” (= inner layer). In 1867 and the following years the
discovery of the germinal layers was extended to other groups of the
invertebrates. In particular, the indefatigable Russian zoologist, Kowalevsky,
found them in all the most diverse sections of the invertebrates—the
worms, tunicates, echinoderms, molluscs, articulates, etc.

In my monograph on the sponges (1872) I proved that these two primary germinal
layers are also found in that group, and that they may be traced from it right
up to man, through all the various classes, in identical form. This
“homology of the two primary germinal layers” extends through the
whole of the metazoa, or tissue-forming animals; that is to say, through the
whole animal kingdom, with the one exception of its lowest section, the
unicellular beings, or protozoa. These lowly organised animals do not form
germinal layers, and therefore do not succeed in forming true tissue. Their
whole body consists of a single cell (as is the case with the amœbæ and
infusoria), or of a loose aggregation of only slightly differentiated cells,
though it may not even reach the full structure of a single cell (as with the
monera). But in all other animals the ovum first grows into two primary layers,
the outer or animal layer (the ectoderm, epiblast, or ectoblast), and
the inner or vegetal layer (the entoderm, hypoblast, or endoblast); and
from these the tissues and organs are formed. The first and oldest organ of all
these metazoa is the primitive gut (or progaster) and its opening, the
primitive mouth (prostoma). The typical embryonic form of the metazoa, as it is
presented for a time by this simple structure of the two-layered body, is
called the gastrula ; it is to be conceived as the hereditary
reproduction of some primitive common ancestor of the metazoa, which we call
the gastræa. This applies to the sponges and other zoophyta, and to the
worms, the mollusca, echinoderma, articulata, and vertebrata. All these animals
may be comprised under the general heading of “gut animals,” or
metazoa, in contradistinction to the gutless protozoa.

I have pointed out in my Study of the Gastræa Theory [not translated]
(1873) the important consequences of this conception in the morphology and
classification of the animal world. I also divided the realm of metazoa into
two great groups, the lower and higher metazoa. In the first are comprised the
cœlenterata (also called zoophytes, or plant-animals). In the lower
forms of this group the body consists throughout life merely of the primary
germinal layers, with the cells sometimes more and sometimes less
differentiated. But with the higher forms of the cœlentarata (the corals,
higher medusæ, ctenophoræ, and platodes) a middle layer, or mesoderm,
often of considerable size, is developed between the
other two layers; but blood and an internal cavity are still lacking.

To the second great group of the metazoa I gave the name of the
cœlomaria, or bilaterata (or the bilateral higher forms). They
all have a cavity within the body (cœloma), and most of them have blood and
blood-vessels. In this are comprised the six higher stems of the animal
kingdom, the annulata and their descendants, the mollusca, echinoderma,
articulata, tunicata, and vertebrata. In all these bilateral organisms the
two-sided body is formed out of four secondary germinal layers, of which the
inner two construct the wall of the alimentary canal, and the outer two the
wall of the body. Between the two pairs of layers lies the cavity (cœloma).

Although I laid special stress on the great morphological importance of this
cavity in my Study of the Gastræa Theory, and endeavoured to prove the
significance of the four secondary germinal layers in the organisation of the
cœlomaria, I was unable to deal satisfactorily with the difficult question of
the mode of their origin. This was done eight years afterwards by the brothers
Oscar and Richard Hertwig in their careful and extensive comparative studies.
In their masterly Cœlum Theory: An Attempt to Explain the Middle Germinal
Layer [not translated] (1881) they showed that in most of the metazoa,
especially in all the vertebrates, the body-cavity arises in the same way, by
the outgrowth of two sacs from the inner layer. These two cœlom-pouches proceed
from the rudimentary mouth of the gastrula, between the two primary layers. The
inner plate of the two-layered cœlom-pouch (the visceral layer) joins itself to
the entoderm; the outer plate (parietal layer) unites with the ectoderm. Thus
are formed the double-layered gut-wall within and the double-layered body-wall
without; and between the two is formed the cavity of the cœlom, by the blending
of the right and left cœlom-sacs. We shall see this more fully in Chapter X.

The many new points of view and fresh ideas suggested by my gastræa theory and
Hertwig’s cœlom theory led to the publication of a number of writings on
the theory of germinal layers. Most of them set out to oppose it at first, but
in the end the majority supported it. Of late years both theories are accepted
in their essential features by nearly every competent man of science, and light
and order have been introduced into this once dark and contradictory field of
research. A further cause of congratulation for this solution of the great
embryological controversy is that it brought with it a recognition of the need
for phylogenetic study and explanation.

Interest and practice in embryological research have been remarkably stimulated
during the past thirty years by this appreciation of phylogenetic methods.
Hundreds of assiduous and able observers are now engaged in the development of
comparative embryology and its establishment on a basis of evolution, whereas
they numbered only a few dozen not many decades ago. It would take too long to
enumerate even the most important of the countless valuable works which have
enriched embryological literature since that time. References to them will be
found in the latest manuals of embryology of Kölliker, Balfour, Hertwig,
Kollman, Korschelt, and Heider.

Kölliker’s Entwickelungsgeschichte des Menschen und der höherer
Thiere, the first edition of which appeared forty-two years ago, had the
rare merit at that time of gathering into presentable form the scattered
attainments of the science, and expounding them in some sort of unity on the
basis of the cellular theory and the theory of germinal layers. Unfortunately,
the distinguished Würtzburg anatomist, to whom comparative anatomy, histology,
and ontogeny owe so much, is opposed to the theory of descent generally and to
Darwinism in particular. All the other manuals I have mentioned take a decided
stand on evolution. Francis Balfour has carefully collected and presented with
discrimination, in his Manual of Comparative Embryology (1880), the very
scattered and extensive literature of the subject; he has also widened the
basis of the gastræa theory by a comparative description of the rise of the
organs from the germinal layers in all the chief groups of the animal kingdom,
and has given a most thorough empirical support to the principles I have
formulated. A comparison of his work with the excellent Text-book of the
Embryology of the Vertebrates (1890) [translation 1895] of Korschelt and
Heider shows what astonishing progress has been made in the science in the
course of ten years. I would especially recommend the manuals of Julius
Kollmann and Oscar Hertwig to those readers who are stimulated to further study
by these chapters on human
embryology. Kollmann’s work is commendable for its clear treatment of the
subject and very fine original illustrations; its author adheres firmly to the
biogenetic law, and uses it throughout with considerable profit. That is not
the case in Oscar Hertwig’s recent Text-book of the Embryology of Man
and the Mammals [translations 1892 and 1899] (seventh edition 1902). This
able anatomist has of late often been quoted as an opponent of the biogenetic
law, although he himself had demonstrated its great value thirty years ago. His
recent vacillation is partly due to the timidity which our “exact”
scientists have with regard to hypotheses; though it is impossible to make any
headway in the explanation of facts without them. However, the purely
descriptive part of embryology in Hertwig’s Text-book is very
thorough and reliable.

A new branch of embryological research has been studied very assiduously in the
last decade of the nineteenth century—namely, “experimental
embryology.” The great importance which has been attached to the
application of physical experiments to the living organism for the last hundred
years, and the valuable results that it has given to physiology in the study of
the vital phenomena, have led to its extension to embryology. I was the first
to make experiments of this kind during a stay of four months on the Canary
Island, Lanzerote, in 1866. I there made a thorough investigation of the almost
unknown embryology of the siphonophoræ. I cut a number of the embryos of these
animals (which develop freely in the water, and pass through a very curious
transformation), at an early stage, into several pieces, and found that a fresh
organism (more or less complete, according to the size of the piece) was
developed from each particle. More recently some of my pupils have made similar
experiments with the embryos of vertebrates (especially the frog) and some of
the invertebrates. Wilhelm Roux, in particular, has made extensive experiments,
and based on them a special “mechanical embryology,” which has
given rise to a good deal of discussion and controversy. Roux has published a
special journal for these subjects since 1895, the Archiv für
Entwickelungsmechanik. The contributions to it are very varied in value.
Many of them are valuable papers on the physiology and pathology of the embryo.
Pathological experiments—the placing of the embryo in abnormal
conditions—have yielded many interesting results; just as the physiology
of the normal body has for a long time derived assistance from the pathology of
the diseased organism. Other of these mechanical-embryological articles return
to the erroneous methods of His, and are only misleading. This must be said of
the many contributions of mechanical embryology which take up a position of
hostility to the theory of descent and its chief embryological
foundation—the biogenetic law. This law, however, when rightly
understood, is not opposed to, but is the best and most solid support of, a
sound mechanical embryology. Impartial reflection and a due attention to
paleontology and comparative anatomy should convince these one-sided
mechanicists that the facts they have discovered—and, indeed, the whole
embryological process—cannot be fully understood without the theory of
descent and the biogenetic law.

Chapter IV.
THE OLDER PHYLOGENY

The embryology of man and the animals, the history of which we have reviewed in
the last two chapters, was mainly a descriptive science forty years ago. The
earlier investigations in this province were chiefly directed to the discovery,
by careful observation, of the wonderful facts of the embryonic development of
the animal body from the ovum. Forty years ago no one dared attack the question
of the causes of these phenomena. For fully a century, from the year
1759, when Wolff’s solid Theoria generationis appeared, until
1859, when Darwin published his famous Origin of Species, the real causes of
the embryonic processes were quite unknown. No one thought of seeking the
agencies that effected this marvellous succession of structures. The task was
thought to be so difficult as almost to pass beyond the limits of human
thought. It was reserved for Charles Darwin to initiate us into the knowledge
of these causes. This compels us to recognise in this great genius, who wrought
a complete revolution in the whole field of biology, a founder at the same time
of a new period in embryology. It is true that Darwin occupied himself very
little with direct embryological research, and even in his chief work he only
touches incidentally on the embryonic phenomena; but by his reform of the
theory of descent and the founding of the theory of selection he has given us
the means of attaining to a real knowledge of the causes of embryonic
formation. That is, in my opinion, the chief feature in Darwin’s
incalculable influence on the whole science of evolution.

When we turn our attention to this latest period of embryological research, we
pass into the second division of organic evolution—stem-evolution, or
phylogeny. I have already indicated in Chapter I the important and intimate
causal connection between these two sections of the science of
evolution—between the evolution of the individual and that of his
ancestors. We have formulated this connection in the biogenetic law; the
shorter evolution, that of the individual, or ontogenesis, is a rapid
and summary repetition, a condensed recapitulation, of the larger evolution, or
that of the species. In this principle we express all the essential points
relating to the causes of evolution; and we shall seek throughout this work to
confirm this principle and lend it the support of facts. When we look to its
causal significance, perhaps it would be better to formulate the
biogenetic law thus: “The evolution of the species and the stem (
phylon) shows us, in the physiological functions of heredity and
adaptation, the conditioning causes on which the evolution of the individual
depends”; or, more briefly: “Phylogenesis is the mechanical cause
of ontogenesis.”

But before we examine the great achievement by which Darwin revealed the causes
of evolution to us, we must glance at the efforts of earlier scientists to
attain this object. Our historical inquiry into these will be even shorter than
that into the work done in the field of ontogeny. We have very few names to
consider here. At the head of them we find the great French naturalist, Jean
Lamarck, who first established evolution as a scientific theory in 1809. Even
before his time, however, the chief philosopher, Kant, and the chief poet,
Goethe, of Germany had occupied themselves with the subject. But their efforts
passed almost without recognition in the eighteenth century. A
“philosophy of nature” did not arise until the beginning of the
nineteenth century. In the whole of the time before this no one had ventured to
raise seriously the question of the origin of species, which is the culminating
point of phylogeny. On all sides it was regarded as an insoluble enigma.

The whole science of the evolution of man and the other animals is intimately
connected with the question of the nature of species, or with the problem of
the origin of the various animals which we group together under the name of
species. Thus the definition of the species becomes important. It is well known
that this definition was given by Linné, who, in his famous Systema
Naturæ (1735), was the first to classify and name the various groups of
animals and plants, and drew up an orderly scheme of the species then known.
Since that time “species” has been the most important and
indispensable idea in descriptive natural history, in zoological and botanical
classification; although there have been endless controversies as to its real
meaning.

What, then, is this “organic species”? Linné himself appealed
directly to the Mosaic narrative; he believed that, as it is stated in
Genesis, one pair of each species of animals and plants was created in
the beginning, and that all the individuals of each species are the descendants
of these created couples. As for the hermaphrodites (organisms that have male
and female organs in one being), he thought it sufficed to assume the creation
of one sole individual, since this would be fully competent to propagate its
species. Further developing these mystic ideas, Linné went on to borrow from
Genesis the account of the deluge and of Noah’s ark as a ground
for a science of the geographical
and topographical distribution of organisms. He accepted the story that all the
plants, animals, and men on the earth were swept away in a universal deluge,
except the couples preserved with Noah in the ark, and ultimately landed on
Mount Ararat. This mountain seemed to Linné particularly suitable for the
landing, as it reaches a height of more than 16,000 feet, and thus provides in
its higher zones the several climates demanded by the various species of
animals and plants: the animals that were accustomed to a cold climate could
remain at the summit; those used to a warm climate could descend to the foot;
and those requiring a temperate climate could remain half-way down. From this
point the re-population of the earth with animals and plants could proceed.

It was impossible to have any scientific notion of the method of evolution in
Linné’s time, as one of the chief sources of information, paleontology,
was still wholly unknown. This science of the fossil remains of extinct animals
and plants is very closely bound up with the whole question of evolution. It is
impossible to explain the origin of living organisms without appealing to it.
But this science did not rise until a much later date. The real founder of
scientific paleontology was Georges Cuvier, the most distinguished zoologist
who, after Linné, worked at the classification of the animal world, and
effected a complete revolution in systematic zoology at the beginning of the
nineteenth century. In regard to the nature of the species he associated
himself with Linné and the Mosaic story of creation, though this was more
difficult for him with his acquaintance with fossil remains. He clearly showed
that a number of quite different animal populations have lived on the earth;
and he claimed that we must distinguish a number of stages in the history of
our planet, each of which was characterised by a special population of animals
and plants. These successive populations were, he said, quite independent of
each other, and therefore the supernatural creative act, which was demanded as
the origin of the animals and plants by the dominant creed, must have been
repeated several times. In this way a whole series of different creative
periods must have succeeded each other; and in connection with these he had to
assume that stupendous revolutions or cataclysms—something like the
legendary deluge—must have taken place repeatedly. Cuvier was all the
more interested in these catastrophes or cataclysms as geology was just
beginning to assert itself, and great progress was being made in our knowledge
of the structure and formation of the earth’s crust. The various strata
of the crust were being carefully examined, especially by the famous geologist
Werner and his school, and the fossils found in them were being classified; and
these researches also seemed to point to a variety of creative periods. In each
period the earth’s crust, composed of the various strata, seemed to be
differently constituted, just like the population of animals and plants that
then lived on it. Cuvier combined this notion with the results of his own
paleontological and zoological research; and in his effort to get a consistent
view of the whole process of the earth’s history he came to form the
theory which is known as “the catastrophic theory,” or the theory
of terrestrial revolutions. According to this theory, there have been a series
of mighty cataclysms on the earth, and these have suddenly destroyed the whole
animal and plant population then living on it; after each cataclysm there was a
fresh creation of living things throughout the earth. As this creation could
not be explained by natural laws, it was necessary to appeal to an intervention
on the part of the Creator. This catastrophic theory, which Cuvier described in
a special work, was soon generally accepted, and retained its position in
biology for half a century.

However, Cuvier’s theory was completely overthrown sixty years ago by the
geologists, led by Charles Lyell, the most distinguished worker in this field
of science. Lyell proved in his famous Principles of Geology (1830) that
the theory was false, in so far as it concerned the crust of the earth; that it
was totally unnecessary to bring in supernatural agencies or general
catastrophes in order to explain the structure and formation of the mountains;
and that we can explain them by the familiar agencies which are at work to-day
in altering and reconstructing the surface of the earth. These causes
are—the action of the atmosphere and water in its various forms (snow,
ice, fog, rain, the wear of the river, and the stormy ocean), and the volcanic
action which is exerted by the molten central

mass. Lyell convincingly proved that these natural causes are quite adequate to
explain every feature in the build and formation of the crust. Hence
Cuvier’s theory of cataclysms was very soon driven out of the province of
geology, though it remained for another thirty years in undisputed authority in
biology. All the zoologists and botanists who gave any thought to the question
of the origin of organisms adhered to Cuvier’s erroneous idea of
revolutions and new creations.

In order to illustrate the complete stagnancy of biology from 1830 to 1859 on
the question of the origin of the various species of animals and plants, I may
say, from my own experience, that during the whole of my university studies I
never heard a single word said about this most important problem of the
science. I was fortunate enough at that time (1852–1857) to have the most
distinguished masters for every branch of biological science. Not one of them
ever mentioned this question of the origin of species. Not a word was ever said
about the earlier efforts to understand the formation of living things, nor
about Lamarck’s Philosophie Zoologique which had made a fresh
attack on the problem in 1809. Hence it is easy to understand the enormous
opposition that Darwin encountered when he took up the question for the first
time. His views seemed to float in the air, without a single previous effort to
support them. The whole question of the formation of living things was
considered by biologists, until 1859, as pertaining to the province of religion
and transcendentalism; even in speculative philosophy, in which the question
had been approached from various sides, no one had ventured to give it serious
treatment. This was due to the dualistic system of Immanuel Kant, who taught a
natural system of evolution as far as the inorganic world was concerned; but,
on the whole, adopted a supernaturalist system as regards the origin of living
things. He even went so far as to say: “It is quite certain that we
cannot even satisfactorily understand, much less explain, the nature of an
organism and its internal forces on purely mechanical principles; it is so
certain, indeed, that we may confidently say: ‘It is absurd for a man to
imagine even that some day a Newton will arise who will explain the origin of a
single blade of grass by natural laws not controlled by
design’—such a hope is entirely forbidden us.” In these words
Kant definitely adopts the dualistic and teleological point of view for
biological science.

Nevertheless, Kant deserted this point of view at times, particularly in
several remarkable passages which I have dealt with at length in my Natural
History of Creation (chap. v), where he expresses himself in the opposite,
or monistic, sense. In fact, these passages would justify one, as I showed, in
claiming his support for the theory of evolution. However, these monistic
passages are only stray gleams of light; as a rule, Kant adheres in biology to
the obscure dualistic ideas, according to which the forces at work in inorganic
nature are quite different from those of the organic world. This dualistic
system prevails in academic philosophy to-day—most of our philosophers
still regarding these two provinces as totally distinct. They put, on the one
side, the inorganic or “lifeless” world, in which there are at work
only mechanical laws, acting necessarily and without design; and, on the other,
the province of organic nature, in which none of the phenomena can be properly
understood, either as regards their inner nature or their origin, except in the
light of preconceived design, carried out by final or purposive causes.

The prevalence of this unfortunate dualistic prejudice prevented the problem of
the origin of species, and the connected question of the origin of man, from
being regarded by the bulk of people as a scientific question at all until
1859. Nevertheless, a few distinguished students, free from the current
prejudice, began, at the commencement of the nineteenth century, to make a
serious attack on the problem. The merit of this attaches particularly to what
is known as “the older school of natural philosophy,” which has
been so much misrepresented, and which included Jean Lamarck, Buffon, Geoffroy
St. Hilaire, and Blainville in France; Wolfgang Goethe, Reinhold Treviranus,
Schelling, and Lorentz Oken in Germany [and Erasmus Darwin in England].

The gifted natural philosopher who treated this difficult question with the
greatest sagacity and comprehensiveness was Jean Lamarck. He was born at
Bazentin, in Picardy, on August 1st, 1744; he was the son of a clergyman, and
was destined for the Church. But he turned to seek glory in the army, and
eventually devoted himself to science.

His Philosophie Zoologique was the

first scientific attempt to sketch the real course of the origin of species,
the first “natural history of creation” of plants, animals, and
men. But, as in the case of Wolff’s book, this remarkably able work had
no influence whatever; neither one nor the other could obtain any recognition
from their prejudiced contemporaries. No man of science was stimulated to take
an interest in the work, and to develop the germs it contained of the most
important biological truths. The most distinguished botanists and zoologists
entirely rejected it, and did not even deign to reply to it. Cuvier, who lived
and worked in the same city, has not thought fit to devote a single syllable to
this great achievement in his memoir on progress in the sciences, in which the
pettiest observations found a place. In short, Lamarck’s Philosophie
Zoologique shared the fate of Wolff’s theory of development, and was
for half a century ignored and neglected. The German scientists, especially
Oken and Goethe, who were occupied with similar speculations at the same time,
seem to have known nothing about Lamarck’s work. If they had known it,
they would have been greatly helped by it, and might have carried the theory of
evolution much farther than they found it possible to do.

To give an idea of the great importance of the Philosophie Zoologique, I
will briefly explain Lamarck’s leading thought. He held that there was no
essential difference between living and lifeless beings. Nature is one united
and connected system of phenomena; and the forces which fashion the lifeless
bodies are the only ones at work in the kingdom of living things. We have,
therefore, to use the same method of investigation and explanation in both
provinces. Life is only a physical phenomenon. All the plants and animals, with
man at their head, are to be explained, in structure and life, by mechanical or
efficient causes, without any appeal to final causes, just as in the case of
minerals and other inorganic bodies. This applies equally to the origin of the
various species. We must not assume any original creation, or repeated
creations (as in Cuvier’s theory), to explain this, but a natural,
continuous, and necessary evolution. The whole evolutionary process has been
uninterrupted. All the different kinds of animals and plants which we see
to-day, or that have ever lived, have descended in a natural way from earlier
and different species; all come from one common stock, or from a few common
ancestors. These remote ancestors must have been quite simple organisms of the
lowest type, arising by spontaneous generation from inorganic matter. The
succeeding species have been constantly modified by adaptation to their varying
environment (especially by use and habit), and have transmitted their
modifications to their successors by heredity.

Lamarck was the first to formulate as a scientific theory the natural origin of
living things, including man, and to push the theory to its extreme
conclusions—the rise of the earliest organisms by spontaneous generation
(or abiogenesis) and the descent of man from the nearest related mammal, the
ape. He sought to explain this last point, which is of especial interest to us
here, by the same agencies which he found at work in the natural origin of the
plant and animal species. He considered use and habit (adaptation) on the one
hand, and heredity on the other, to be the chief of these agencies. The most
important modifications of the organs of plants and animals are due, in his
opinion, to the function of these very organs, or to the use or disuse of them.
To give a few examples, the woodpecker and the humming-bird have got their
peculiarly long tongues from the habit of extracting their food with their
tongues from deep and narrow folds or canals; the frog has developed the web
between his toes by his own swimming; the giraffe has lengthened his neck by
stretching up to the higher branches of trees, and so on. It is quite certain
that this use or disuse of organs is a most important factor in organic
development, but it is not sufficient to explain the origin of species.

To adaptation we must add heredity as the second and not less important agency,
as Lamarck perfectly recognised. He said that the modification of the organs in
any one individual by use or disuse was slight, but that it was increased by
accumulation in passing by heredity from generation to generation. But he
missed altogether the principle which Darwin afterwards found to be the chief
factor in the theory of transformation—namely, the principle of natural
selection in the struggle for existence. It was partly owing to his failure to
detect this supremely important element, and partly to the poor condition of
all biological science at the time, that Lamarck did not

succeed in establishing more firmly his theory of the common descent of man and
the other animals.

Independently of Lamarck, the older German school of natural philosophy,
especially Reinhold Treviranus, in his Biologie (1802), and Lorentz
Oken, in his Naturphilosophie (1809), turned its attention to the
problem of evolution about the end of the eighteenth and beginning of the
nineteenth century. I have described its work in my History of Creation
(chap. iv). Here I can only deal with the brilliant genius whose evolutionary
ideas are of special interest—the greatest of German poets, Wolfgang
Goethe. With his keen eye for the beauties of nature, and his profound insight
into its life, Goethe was early attracted to the study of various natural
sciences. It was the favourite occupation of his leisure hours throughout life.
He gave particular and protracted attention to the theory of colours. But the
most valuable of his scientific studies are those which relate to that
“living, glorious, precious thing,” the organism. He made profound
research into the science of structures or morphology (morphæ = forms). Here,
with the aid of comparative anatomy, he obtained the most brilliant results,
and went far in advance of his time. I may mention, in particular, his
vertebral theory of the skull, his discovery of the pineal gland in man, his
system of the metamorphosis of plants, etc. These morphological studies led
Goethe on to research into the formation and modification of organic structures
which we must count as the first germ of the science of evolution. He
approaches so near to the theory of descent that we must regard him, after
Lamarck, as one of its earliest founders. It is true that he never formulated a
complete scientific theory of evolution, but we find a number of remarkable
suggestions of it in his splendid miscellaneous essays on morphology. Some of
them are really among the very basic ideas of the science of evolution. He
says, for instance (1807): “When we compare plants and animals in their
most rudimentary forms, it is almost impossible to distinguish between them.
But we may say that the plants and animals, beginning with an almost
inseparable closeness, gradually advance along two divergent lines, until the
plant at last grows in the solid, enduring tree and the animal attains in man
to the highest degree of mobility and freedom.” That Goethe was not
merely speaking in a poetical, but in a literal genealogical, sense of this
close affinity of organic forms is clear from other remarkable passages in
which he treats of their variety in outward form and unity in internal
structure. He believes that every living thing has arisen by the interaction of
two opposing formative forces or impulses. The internal or
“centripetal” force, the type or “impulse to
specification,” seeks to maintain the constancy of the specific forms in
the succession of generations: this is heredity. The external or
“centrifugal” force, the element of variation or “impulse to
metamorphosis,” is continually modifying the species by changing their
environment: this is adaptation. In these significant conceptions Goethe
approaches very close to a recognition of the two great mechanical factors
which we now assign as the chief causes of the formation of species.

However, in order to appreciate Goethe’s views on morphology, one must
associate his decidedly monistic conception of nature with his pantheistic
philosophy. The warm and keen interest with which he followed, in his last
years, the controversies of contemporary French scientists, and especially the
struggle between Cuvier and Geoffroy St. Hilaire (see chap. iv of The
History of Creation), is very characteristic. It is also necessary to be
familiar with his style and general tenour of thought in order to appreciate
rightly the many allusions to evolution found in his writings. Otherwise, one
is apt to make serious errors.

He approached so close, at the end of the eighteenth century, to the principles
of the science of evolution that he may well be described as the first
forerunner of Darwin, although he did not go so far as to formulate evolution
as a scientific system, as Lamarck did.

Chapter V.
THE MODERN SCIENCE OF EVOLUTION

We owe so much of the progress of scientific knowledge to Darwin’s
Origin of Species that its influence is almost without parallel in the
history of science. The literature of Darwinism grows from day to day, not only
on the side of academic zoology and botany, the sciences which were chiefly
affected by Darwin’s theory, but in a far wider circle, so that we find
Darwinism discussed in popular literature with a vigour and zest that are given
to no other scientific conception. This remarkable success is due chiefly to
two circumstances. In the first place, all the sciences, and especially
biology, have made astounding progress in the last half-century, and have
furnished a very vast quantity of proofs of the theory of evolution. In
striking contrast to the failure of Lamarck and the older scientists to attract
attention to their effort to explain the origin of living things and of man, we
have this second and successful effort of Darwin, which was able to gather to
its support a large number of established facts. Availing himself of the
progress already made, he had very different scientific proofs to allege than
Lamarck, or St. Hilaire, or Goethe, or Treviranus had had. But, in the second
place, we must acknowledge that Darwin had the special distinction of
approaching the subject from an entirely new side, and of basing the theory of
descent on a consistent system, which now goes by the name of Darwinism.

Lamarck had unsuccessfully attempted to explain the modification of organisms
that descend from a common form chiefly by the action of habit and the use of
organs, though with the aid of heredity. But Darwin’s success was
complete when he independently sought to give a mechanical explanation, on a
quite new ground, of this modification of plant and animal structures by
adaptation and heredity. He was impelled to his theory of selection on the
following grounds. He compared the origin of the various kinds of animals and
plants which we modify artificially—by the action of artificial selection
in horticulture and among domestic animals—with the origin of the species
of animals and plants in their natural state. He then found that the agencies
which we employ in the modification of forms by artificial selection are also
at work in Nature. The chief of these agencies he held to be “the
struggle for life.” The gist of this peculiarly Darwinian idea is given
in this formula: The struggle for existence produces new species without
premeditated design in the life of Nature, in the same way that the will of man
consciously selects new races in artificial conditions. The gardener or the
farmer selects new forms as he wills for his own profit, by ingeniously using
the agency of heredity and adaptation for the modification of structures; so,
in the natural state, the struggle for life is always unconsciously modifying
the various species of living things. This struggle for life, or competition of
organisms in securing the means of subsistence, acts without any conscious
design, but it is none the less effective in modifying structures. As heredity
and adaptation enter into the closest reciprocal action under its influence,
new structures, or alterations of structure, are produced; and these are
purposive in the sense that they serve the organism when formed, but they were
produced without any pre-conceived aim.

This simple idea is the central thought of Darwinism, or the theory of
selection. Darwin conceived this idea at an early date, and then, for more than
twenty years, worked at the collection of empirical evidence in support of it
before he published his theory. His grandfather, Erasmus Darwin, was an able
scientist of the older school of natural philosophy, who published a number of
natural-philosophic works about the end of the eighteenth century. The most
important of them is his Zoonomia, published in 1794, in which he
expounds views similar to those of Goethe and Lamarck, without really knowing
anything of the work of these

contemporaries. However, in the writings of the grandfather the plastic
imagination rather outran the judgment, while in Charles Darwin the two were
better balanced.

Darwin did not publish any account of his theory until 1858, when Alfred Russel
Wallace, who had independently reached the same theory of selection, published
his own work. In the following year appeared the Origin of Species, in
which he develops it at length and supports it with a mass of proof. Wallace
had reached the same conclusion, but he had not so clear a perception as Darwin
of the effectiveness of natural selection in forming species, and did not
develop the theory so fully. Nevertheless, Wallace’s writings, especially
those on mimicry, etc., and an admirable work on The Geographical
Distribution of Animals, contain many fine original contributions to the
theory of selection. Unfortunately, this gifted scientist has since devoted
himself to spiritism.^[10]

[10]
Darwin and Wallace arrived at the theory quite independently. Vide
Wallace’s Contributions to the Theory of Natural Selection (1870)
and Darwinism (1891).

Darwin’s Origin of Species had an extraordinary influence, though
not at first on the experts of the science. It took zoologists and botanists
several years to recover from the astonishment into which they had been thrown
through the revolutionary idea of the work. But its influence on the special
sciences with which we zoologists and botanists are concerned has increased
from year to year; it has introduced a most healthy fermentation in every
branch of biology, especially in comparative anatomy and ontogeny, and in
zoological and botanical classification. In this way it has brought about
almost a revolution in the prevailing views.

However, the point which chiefly concerns us here—the extension of the
theory to man—was not touched at all in Darwin’s first work in
1859. It was believed for several years that he had no thought of applying his
principles to man, but that he shared the current idea of man holding a special
position in the universe. Not only ignorant laymen (especially several
theologians), but also a number of men of science, said very naively that
Darwinism in itself was not to be opposed; that it was quite right to use it to
explain the origin of the various species of plants and animals, but that it
was totally inapplicable to man.

In the meantime, however, it seemed to a good many thoughtful people, laymen as
well as scientists, that this was wrong; that the descent of man from some
other animal species, and immediately from some ape-like mammal, followed
logically and necessarily from Darwin’s reformed theory of evolution.
Many of the acuter opponents of the theory saw at once the justice of this
position, and, as this consequence was intolerable, they wanted to get rid of
the whole theory.

The first scientific application of the Darwinian theory to man was made by
Huxley, the greatest zoologist in England. This able and learned scientist, to
whom zoology owes much of its progress, published in 1863 a small work entitled
Evidence as to Man’s Place in Nature. In the extremely important
and interesting lectures which made up this work he proved clearly that the
descent of man from the ape followed necessarily from the theory of descent. If
that theory is true, we are bound to conceive the animals which most closely
resemble man as those from which humanity has been gradually evolved. About the
same time Carl Vogt published a larger work on the same subject. We must also
mention Gustav Jaeger and Friedrich Rolle among the zoologists who accepted and
taught the theory of evolution immediately after the publication of
Darwin’s book, and maintained that the descent of man from the lower
animals logically followed from it. The latter published, in 1866, a work on
the origin and position of man.

About the same time I attempted, in the second volume of my General
Morphology (1866), to apply the theory of evolution to the whole organic
kingdom, including man.^[11] I endeavoured to sketch the probable
ancestral trees of the various classes of the animal world, the protists, and
the plants, as it seemed necessary to do on Darwinian principles, and as we can
actually do now with a high degree of confidence. If the theory of descent,
which Lamarck first clearly formulated and Darwin thoroughly established, is
true, we should be able to draw up a natural classification of plants and
animals in the light of their genealogy, and to conceive the large and small
divisions of

the system as the branches and twigs of an ancestral tree. The eight
genealogical tables which I inserted in the second volume of the General
Morphology are the first sketches of their kind. In Chapter 27,
particularly, I trace the chief stages in man’s ancestry, as far as it is
possible to follow it through the vertebrate stem. I tried especially to
determine, as well as one could at that time, the position of man in the
classification of the mammals and its genealogical significance. I have greatly
improved this attempt, and treated it in a more popular form, in chaps.
xxvi–xxviii of my History of Creation (1868).^[12]

[11]
Huxley spoke of this “as one of the greatest scientific works ever
published.”—Translator.

[12]
Of which Darwin said that the Descent of Man would probably never have
been written if he had seen it earlier.—Translator.

It was not until 1871, twelve years after the appearance of The Origin of
Species, that Darwin published the famous work which made the
much-contested application of his theory to man, and crowned the splendid
structure of his system. This important work was The Descent of Man, and
Selection in Relation to Sex. In this Darwin expressly drew the conclusion,
with rigorous logic, that man also must have been developed out of lower
species, and described the important part played by sexual selection in the
elevation of man and the other higher animals. He showed that the careful
selection which the sexes exercise on each other in regard to sexual relations
and procreation, and the æsthetic feeling which the higher animals develop
through this, are of the utmost importance in the progressive development of
forms and the differentiation of the sexes. The males choosing the handsomest
females in one class of animals, and the females choosing only the
finest-looking males in another, the special features and the sexual
characteristics are increasingly accentuated. In fact, some of the higher
animals develop in this connection a finer taste and judgment than man himself.
But, even as regards man, it is to this sexual selection that we owe the
family-life, which is the chief foundation of civilisation. The rise of the
human race is due for the most part to the advanced sexual selection which our
ancestors exercised in choosing their mates.

Darwin accepted in the main the general outlines of man’s ancestral tree,
as I gave it in the General Morphology and the History of
Creation, and admitted that his studies led him to the same conclusion.
That he did not at once apply the theory to man in his first work was a
commendable piece of discretion; such a sequel was bound to excite the
strongest opposition to the whole theory. The first thing to do was to
establish it as regards the animal and plant worlds. The subsequent extension
to man was bound to be made sooner or later.

It is important to understand this very clearly. If all living things come from
a common root, man must be included in the general scheme of evolution. On the
other hand, if the various species were separately created, man, too, must have
been created, and not evolved. We have to choose between these two
alternatives. This cannot be too frequently or too strongly emphasised.
Either all the species of animals and plants are of supernatural
origin—created, not evolved—and in that case man also is the
outcome of a creative act, as religion teaches, or the different species
have been evolved from a few common, simple ancestral forms, and in that case
man is the highest fruit of the tree of evolution.

We may state this briefly in the following principle—The descent of
man from the lower animals is a special deduction which inevitably follows from
the general inductive law of the whole theory of evolution. In this
principle we have a clear and plain statement of the matter. Evolution is in
reality nothing but a great induction, which we are compelled to make by the
comparative study of the most important facts of morphology and physiology. But
we must draw our conclusion according to the laws of induction, and not attempt
to determine scientific truths by direct measurement and mathematical
calculation. In the study of living things we can scarcely ever directly and
fully, and with mathematical accuracy, determine the nature of phenomena, as is
done in the simpler study of the inorganic world—in chemistry, physics,
mineralogy, and astronomy. In the latter, especially, we can always use the
simplest and absolutely safest method—that of mathematical determination.
But in biology this is quite impossible for various reasons; one very obvious
reason being that most of the facts of the science are very complicated and
much too intricate to allow a direct mathematical analysis. The greater part of
the phenomena that biology deals with are

complicated historical processes, which are related to a far-reaching
past, and as a rule can only be approximately estimated. Hence we have to
proceed by induction—that is to say, to draw general conclusions,
stage by stage, and with proportionate confidence, from the accumulation of
detailed observations. These inductive conclusions cannot command absolute
confidence, like mathematical axioms; but they approach the truth, and gain
increasing probability, in proportion as we extend the basis of observed facts
on which we build. The importance of these inductive laws is not diminished
from the circumstance that they are looked upon merely as temporary
acquisitions of science, and may be improved to any extent in the progress of
scientific knowledge. The same may be said of the attainments of many other
sciences, such as geology or archeology. However much they may be altered and
improved in detail in the course of time, these inductive truths may retain
their substance unchanged.

Now, when we say that the theory of evolution in the sense of Lamarck and
Darwin is an inductive law—in fact, the greatest of all biological
inductions—we rely, in the first place, on the facts of paleontology.
This science gives us some direct acquaintance with the historical phenomena of
the changes of species. From the situations in which we find the fossils in the
various strata of the earth we gather confidently, in the first place, that the
living population of the earth has been gradually developed, as clearly as the
earth’s crust itself; and that, in the second place, several different
populations have succeeded each other in the various geological periods. Modern
geology teaches that the formation of the earth has been gradual, and unbroken
by any violent revolutions. And when we compare together the various kinds of
animals and plants which succeed each other in the history of our planet, we
find, in the first place, a constant and gradual increase in the number of
species from the earliest times until the present day; and, in the second
place, we notice that the forms in each great group of animals and plants also
constantly improve as the ages advance. Thus, of the vertebrates there are at
first only the lower fishes; then come the higher fishes, and later the
amphibia. Still later appear the three higher classes of vertebrates—the
reptiles, birds, and mammals, for the first time; only the lowest and least
perfect forms of the mammals are found at first; and it is only at a very late
period that placental mammals appear, and man belongs to the latest and
youngest branch of these. Thus perfection of form increases as well as variety
from the earliest to the latest stage. That is a fact of the greatest
importance. It can only be explained by the theory of evolution, with which it
is in perfect harmony. If the different groups of plants and animals do really
descend from each other, we must expect to find this increase in their number
and perfection under the influence of natural selection, just as the succession
of fossils actually discloses it to us.

Comparative anatomy furnishes a second series of facts which are of great
importance for the forming of our inductive law. This branch of morphology
compares the adult structures of living things, and seeks in the great variety
of organic forms the stable and simple law of organisation, or the common type
or structure. Since Cuvier founded this science at the beginning of the
nineteenth century it has been a favourite study of the most distinguished
scientists. Even before Cuvier’s time Goethe had been greatly stimulated
by it, and induced to take up the study of morphology. Comparative osteology,
or the philosophic study and comparison of the bony skeleton of the
vertebrates—one of its most interesting sections—especially
fascinated him, and led him to form the theory of the skull which I mentioned
before. Comparative anatomy shows that the internal structure of the animals of
each stem and the plants of each class is the same in its essential features,
however much they differ in external appearance. Thus man has so great a
resemblance in the chief features of his internal organisation to the other
mammals that no comparative anatomist has ever doubted that he belongs to this
class. The whole internal structure of the human body, the arrangement of its
various systems of organs, the distribution of the bones, muscles,
blood-vessels, etc., and the whole structure of these organs in the larger and
the finer scale, agree so closely with those of the other mammals (such as the
apes, rodents, ungulates, cetacea, marsupials, etc.) that their external
differences are of no account whatever. We learn further from comparative
anatomy that the chief features of animal structure

are so similar in the various classes (fifty to sixty in number altogether)
that they may all be comprised in from eight to twelve great groups. But even
in these groups, the stem-forms or animal types, certain organs (especially the
alimentary canal) can be proved to have been originally the same for all. We
can only explain by the theory of evolution this essential unity in internal
structure of all these animal forms that differ so much in outward appearance.
This wonderful fact can only be really understood and explained when we regard
the internal resemblance as an inheritance from common-stem forms, and the
external differences as the effect of adaptation to different environments.

In recognising this, comparative anatomy has itself advanced to a higher stage.
Gegenbaur, the most distinguished of recent students of this science, says that
with the theory of evolution a new period began in comparative anatomy, and
that the theory in turn found a touch stone in the science. “Up to now
there is no fact in comparative anatomy that is inconsistent with the theory of
evolution; indeed, they all lead to it. In this way the theory receives back
from the science all the service it rendered to its method.” Until then
students had marvelled at the wonderful resemblance of living things in their
inner structure without being able to explain it. We are now in a position to
explain the causes of this, by showing that this remarkable agreement is the
necessary consequence of the inheriting of common stem-forms; while the
striking difference in outward appearance is a result of adaptation to changes
of environment. Heredity and adaptation alone furnish the true explanation.

But one special part of comparative anatomy is of supreme interest and of the
utmost philosophic importance in this connection. This is the science of
rudimentary or useless organs; I have given it the name of
“dysteleology” in view of its philosophic consequences. Nearly
every organism (apart from the very lowest), and especially every
highly-developed animal or plant, including man, has one or more organs which
are of no use to the body itself, and have no share in its functions or vital
aims. Thus we all have, in various parts of our frame, muscles which we never
use, as, for instance, in the shell of the ear and adjoining parts. In most of
the mammals, especially those with pointed ears, these internal and external
ear-muscles are of great service in altering the shell of the ear, so as to
catch the waves of sound as much as possible. But in the case of man and other
short-eared mammals these muscles are useless, though they are still present.
Our ancestors having long abandoned the use of them, we cannot work them at all
to-day. In the inner corner of the eye we have a small crescent-shaped fold of
skin; this is the last relic of a third inner eye-lid, called the nictitating
(winking) membrane. This membrane is highly developed and of great service in
some of our distant relations, such as fishes of the shark type and several
other vertebrates; in us it is shrunken and useless. In the intestines we have
a process that is not only quite useless, but may be very harmful—the
vermiform appendage. This small intestinal appendage is often the cause of a
fatal illness. If a cherry-stone or other hard body is unfortunately squeezed
through its narrow aperture during digestion, a violent inflammation is set up,
and often proves fatal. This appendix has no use whatever now in our frame; it
is a dangerous relic of an organ that was much larger and was of great service
in our vegetarian ancestors. It is still large and important in many vegetarian
animals, such as apes and rodents.

There are similar rudimentary organs in all parts of our body, and in all the
higher animals. They are among the most interesting phenomena to which
comparative anatomy introduces us; partly because they furnish one of the
clearest proofs of evolution, and partly because they most strikingly refute
the teleology of certain philosophers. The theory of evolution enables us to
give a very simple explanation of these phenomena.

We have to look on them as organs which have fallen into disuse in the course
of many generations. With the decrease in the use of its function, the organ
itself shrivels up gradually, and finally disappears. There is no other way of
explaining rudimentary organs. Hence they are also of great interest in
philosophy; they show clearly that the monistic or mechanical view of
the organism is the only correct one, and that the dualistic or
teleological conception is wrong. The ancient legend of the direct creation of
man according to a pre-conceived plan and the empty phrases about

“design” in the organism are completely shattered by them. It would
be difficult to conceive a more thorough refutation of teleology than is
furnished by the fact that all the higher animals have these rudimentary
organs.

The theory of evolution finds its broadest inductive foundation in the natural
classification of living things, which arranges all the various forms in larger
and smaller groups, according to their degree of affinity. These groupings or
categories of classification—the varieties, species, genera, families,
orders, classes, etc.—show such constant features of coordination and
subordination that we are bound to look on them as genealogical, and
represent the whole system in the form of a branching tree. This is the
genealogical tree of the variously related groups; their likeness in form is
the expression of a real affinity. As it is impossible to explain in any other
way the natural tree-like form of the system of organisms, we must regard it at
once as a weighty proof of the truth of evolution. The careful construction of
these genealogical trees is, therefore, not an amusement, but the chief task of
modern classification.

Among the chief phenomena that bear witness to the inductive law of evolution
we have the geographical distribution of the various species of animals and
plants over the surface of the earth, and their topographical distribution on
the summits of mountains and in the depths of the ocean. The scientific study
of these features—the “science of distribution,” or chorology
(chora = a place)—has been pursued with lively interest since the
discoveries made by Alexander von Humboldt. Until Darwin’s time the work
was confined to the determination of the facts of the science, and chiefly
aimed at settling the spheres of distribution of the existing large and small
groups of living things. It was impossible at that time to explain the causes
of this remarkable distribution, or the reasons why one group is found only in
one locality and another in a different place, and why there is this manifold
distribution at all. Here, again, the theory of evolution has given us the
solution of the problem. It furnishes the only possible explanation when it
teaches that the various species and groups of species descend from common
stem-forms, whose ever-branching offspring have gradually spread themselves by
migration over the earth. For each group of species we must admit a
“centre of production,” or common home; this is the original
habitat in which the ancestral form was developed, and from which its
descendants spread out in every direction. Several of these descendants became
in their turn the stem-forms for new groups of species, and these also
scattered themselves by active and passive migration, and so on. As each
migrating organism found a different environment in its new home, and adapted
itself to it, it was modified, and gave rise to new forms.

This very important branch of science that deals with active and passive
migration was founded by Darwin, with the aid of the theory of evolution; and
at the same time he advanced the true explanation of the remarkable relation or
similarity of the living population in any locality to the fossil forms found
in it. Moritz Wagner very ably developed his idea under the title of “the
theory of migration.” In my opinion, this famous traveller has rather
over-estimated the value of his theory of migration when he takes it to be an
indispensable condition of the formation of new species and opposes the theory
of selection. The two theories are not opposed in their main features.
Migration (by which the stem-form of a new species is isolated) is really only
a special case of selection. The striking and interesting facts of chorology
can be explained only by the theory of evolution, and therefore we must count
them among the most important of its inductive bases.

The same must be said of all the remarkable phenomena which we perceive in the
economy of the living organism. The many and various relations of plants and
animals to each other and to their environment, which are treated in
bionomy (from nomos, law or norm, and bios, life), the
interesting facts of parasitism, domesticity, care of the young, social habits,
etc., can only be explained by the action of heredity and adaptation. Formerly
people saw only the guidance of a beneficent Providence in these phenomena;
to-day we discover in them admirable proofs of the theory of evolution. It is
impossible to understand them except in the light of this theory and the
struggle for life.

Finally, we must, in my opinion, count among the chief inductive bases of the

theory of evolution the fœtal development of the individual organism, the whole
science of embryology or ontogeny. But as the later chapters will deal with
this in detail, I need say nothing further here. I shall endeavour in the
following pages to show, step by step, how the whole of the embryonic phenomena
form a massive chain of proof for the theory of evolution; for they can be
explained in no other way. In thus appealing to the close causal connection
between ontogenesis and phylogenesis, and taking our stand throughout on the
biogenetic law, we shall be able to prove, stage by stage, from the facts of
embryology, the evolution of man from the lower animals.

The general adoption of the theory of evolution has definitely closed the
controversy as to the nature or definition of the species. The word has no
absolute meaning whatever, but is only a group-name, or category of
classification, with a purely relative value. In 1857, it is true, a famous and
gifted, but inaccurate and dogmatic, scientist, Louis Agassiz, attempted to
give an absolute value to these “categories of classification.” He
did this in his Essay on Classification, in which he turns upside down
the phenomena of organic nature, and, instead of tracing them to their natural
causes, examines them through a theological prism. The true species (bona
species) was, he said, an “incarnate idea of the Creator.”
Unfortunately, this pretty phrase has no more scientific value than all the
other attempts to save the absolute or intrinsic value of the species.

The dogma of the fixity and creation of species lost its last great champion
when Agassiz died in 1873. The opposite theory, that all the different species
descend from common stem-forms, encounters no serious difficulty to-day. All
the endless research into the nature of the species, and the possibility of
several species descending from a common ancestor, has been closed to-day by
the removal of the sharp limits that had been set up between species and
varieties on the one hand, and species and genera on the other. I gave an
analytic proof of this in my monograph on the sponges (1872), having made a
very close study of variability in this small but highly instructive group, and
shown the impossibility of making any dogmatic distinction of species.
According as the classifier takes his ideas of genus, species, and variety in a
broader or in a narrower sense, he will find in the small group of the sponges
either one genus with three species, or three genera with 238 species, or 113
genera with 591 species. Moreover, all these forms are so connected by
intermediate forms that we can convincingly prove the descent of all the
sponges from a common stem-form, the olynthus.

Here, I think, I have given an analytic solution of the problem of the origin
of species, and so met the demand of certain opponents of evolution for an
actual instance of descent from a stem-form. Those who are not satisfied with
the synthetic proofs of the theory of evolution which are provided by
comparative anatomy, embryology, paleontology, dysteleology, chorology, and
classification, may try to refute the analytic proof given in my treatise on
the sponge, the outcome of five years of assiduous study. I repeat: It is now
impossible to oppose evolution on the ground that we have no convincing example
of the descent of all the species of a group from a common ancestor. The
monograph on the sponges furnishes such a proof, and, in my opinion, an
indisputable proof. Any man of science who will follow the protracted steps of
my inquiry and test my assertions will find that in the case of the sponges we
can follow the actual evolution of species in a concrete case. And if this is
so, if we can show the origin of all the species from a common form in one
single class, we have the solution of the problem of man’s origin,
because we are in a position to prove clearly his descent from the lower
animals.

At the same time, we can now reply to the often-repeated assertion, even heard
from scientists of our own day, that the descent of man from the lower animals,
and proximately from the apes, still needs to be “proved with
certainty.” These “certain proofs” have been available for a
long time; one has only to open one’s eyes to see them. It is a mistake
to seek them in the discovery of intermediate forms between man and the ape, or
the conversion of an ape into a human being by skilful education. The proofs
lie in the great mass of empirical material we have already collected. They are
furnished in the strongest form by the data of comparative anatomy and
embryology, completed by paleontology. It is not a question now of detecting
new proofs of the evolution of man, but of examining

and understanding the proofs we already have.

I was almost alone thirty-six years ago when I made the first attempt, in my
General Morphology, to put organic science on a mechanical foundation
through Darwin’s theory of descent. The association of ontogeny and
phylogeny and the proof of the intimate causal connection between these two
sections of the science of evolution, which I expounded in my work, met with
the most spirited opposition on nearly all sides. The next ten years were a
terrible “struggle for life” for the new theory. But for the last
twenty-five years the tables have been turned. The phylogenetic method has met
with so general a reception, and found so prolific a use in every branch of
biology, that it seems superfluous to treat any further here of its validity
and results. The proof of it lies in the whole morphological literature of the
last three decades. But no other science has been so profoundly modified in its
leading thoughts by this adoption, and been forced to yield such far-reaching
consequences, as that science which I am now seeking to
establish—monistic anthropogeny.

This statement may seem to be rather audacious, since the very next branch of
biology, anthropology in the stricter sense, makes very little use of these
results of anthropogeny, and sometimes expressly opposes them.^[13] This
applies especially to the attitude which has characterised the German
Anthropological Society (the Deutsche Gesellschaft fur Anthropologie)
for some thirty years. Its powerful president, the famous pathologist, Rudolph
Virchow, is chiefly responsible for this. Until his death (September 5th, 1902)
he never ceased to reject the theory of descent as unproven, and to ridicule
its chief consequence—the descent of man from a series of mammal
ancestors—as a fantastic dream. I need only recall his well-known
expression at the Anthropological Congress at Vienna in 1894, that “it
would be just as well to say man came from the sheep or the elephant as from
the ape.”

[13]
This does not apply to English anthropologists, who are almost all
evolutionists.

Virchow’s assistant, the secretary of the German Anthropological Society,
Professor Johannes Ranke of Munich, has also indefatigably opposed
transformism: he has succeeded in writing a work in two volumes (Der
Mensch), in which all the facts relating to his organisation are explained
in a sense hostile to evolution. This work has had a wide circulation, owing to
its admirable illustrations and its able treatment of the most interesting
facts of anatomy and physiology—exclusive of the sexual organs! But, as
it has done a great deal to spread erroneous views among the general public, I
have included a criticism of it in my History of Creation, as well as
met Virchow’s attacks on anthropogeny.

Neither Virchow, nor Ranke, nor any other “exact” anthropologist,
has attempted to give any other natural explanation of the origin of man. They
have either set completely aside this “question of questions” as a
transcendental problem, or they have appealed to religion for its solution. We
have to show that this rejection of the rational explanation is totally without
justification. The fund of knowledge which has accumulated in the progress of
biology in the nineteenth century is quite adequate to furnish a rational
explanation, and to establish the theory of the evolution of man on the solid
facts of his embryology.

Chapter VI.
THE OVUM AND THE AMŒBA

In order to understand clearly the course of human embryology, we must select
the more important of its wonderful and manifold processes for fuller
explanation, and then proceed from these to the innumerable features of less
importance. The most important feature in this sense, and the best
starting-point for ontogenetic study, is the fact that man is developed from an
ovum, and that this ovum is a simple cell. The human ovum does not materially
differ in form and composition from that of the other mammals, whereas there is
a distinct difference between the fertilised ovum of the mammal and that of any
other animal.

Fig. 1—The human ovum. The globular mass of
yelk (b) is enclosed by a transparent membrane (the ovolemma or zona
pellucida [a]), and contains a noncentral nucleus (the germinal vesicle,
c). Cf. Fig. 14.

This fact is so important that few should be unaware of its
extreme significance; yet it was quite unknown in the first quarter of the
nineteenth century. As we have seen, the human and mammal ovum was not
discovered until 1827, when Carl Ernst von Baer detected it. Up to that time
the larger vesicles, in which the real and much smaller ovum is contained, had
been wrongly regarded as ova. The important circumstance that this mammal ovum
is a simple cell, like the ovum of other animals, could not, of course, be
recognised until the cell theory was established. This was not done, by
Schleiden for the plant and Schwann for the animal, until 1838. As we have
seen, this cell theory is of the greatest service in explaining the human frame
and its embryonic development. Hence we must say a few words about the actual
condition of the theory and the significance of the views it has suggested.

In order properly to appreciate the cellular theory, the most important element
in our science, it is necessary to understand in the first place that the cell
is a unified organism, a self-contained living being. When we
anatomically dissect the fully-formed animal or plant into its various organs,
and then examine the finer structure of these organs with the microscope, we
are surprised to find that all these different parts are ultimately made up of
the same structural element or unit. This common unit of structure is the cell.
It does not matter whether we thus dissect a leaf, flower, or fruit, or a bone,
muscle, gland, or bit of skin, etc.; we find in every case the same ultimate
constituent, which has been called the cell since Schleiden’s discovery.
There are many opinions as to its real nature, but the essential point in our
view of the cell is to look upon it as a self-contained or independent living
unit. It is, in the words of Brucke, “an elementary organism.” We
may define it most precisely as the ultimate organic unit, and, as the cells
are the sole active principles in every vital function, we may call them the
“plastids,” or “formative elements.” This unity is
found in both the anatomic structure and the physiological function. In the
case of the protists, the entire organism usually consists of a single
independent cell throughout life. But in the tissue-forming animals and plants,
which are the great majority, the organism begins its career as a simple cell,
and then grows into a cell-community, or, more correctly, an organised
cell-state. Our own body is not really the simple unity that it is generally
supposed to be. On the contrary, it is a very elaborate social system of
countless microscopic organisms, a colony or commonwealth, made up of
innumerable independent units, or very different tissue-cells.

In reality, the term “cell,” which existed long before the cell
theory was formulated, is not happily chosen. Schleiden, who first brought it
into scientific use in the sense of the cell theory, gave this name to the
elementary organisms because, when you find them in the dissected plant, they
generally have the appearance of chambers, like the cells in a bee-hive, with
firm walls and a fluid or pulpy content. But some cells, especially young ones,
are entirely without the enveloping membrane, or stiff wall. Hence we now
generally describe the cell as a living, viscous particle of protoplasm,
enclosing a firmer nucleus in its albuminoid body. There may be an enclosing
membrane, as there actually is in the case of most of the plants; but it may be
wholly lacking, as is the case with most of the animals. There is no membrane
at all in the first stage. The young cells are usually round, but they vary
much in shape later on. Illustrations of this will be found in the cells of the
various parts of the body shown in Figs. 3–7.

Hence the essential point in the modern idea of the cell is that it is made up
of two different active constituents—an inner and an outer part. The
smaller and inner part is the nucleus (or caryon or cytoblastus,
Fig. 1c and Fig. 2k). The outer and larger part, which encloses
the other, is the body of the cell (celleus, cytos, or cytosoma).
The soft living substance of which the two are composed has a peculiar chemical
composition, and belongs to the group of the albuminoid plasma-substances
(“formative matter”), or protoplasm. The essential and
indispensable element of the nucleus is called nuclein (or caryoplasm); that of
the cell body is called plastin (or cytoplasm). In the most rudimentary cases
both substances seem to be quite simple and homogeneous, without any visible
structure. But, as a rule, when we examine them under a high power of the
microscope, we find a certain structure in the protoplasm. The chief and most
common form of this is the fibrous or net-like “thready structure”
(Frommann) and the frothy “honeycomb structure” (Bütschli).

Fig. 2—Stem-cell of one of the echinoderms (cytula, or
“first segmentation-cell” = fertilised ovum), after Hertwig.
k is the nucleus or caryon.

The shape or outer form of the cell is infinitely varied, in accordance with
its endless power of adapting itself to the most diverse activities or
environments. In its simplest form the cell is globular (Fig. 2). This normal
round form is especially found in cells of the simplest construction, and those
that are developed in a free fluid without any external pressure. In such cases
the nucleus also is not infrequently round, and located in the centre of the
cell-body (Fig. 2k). In other cases, the cells have no definite shape;
they are constantly changing their form owing to their automatic movements.
This is the case with the amœbæ (Fig. 15 and 16) and the amœboid travelling
cells (Fig. 11), and also with very young ova (Fig. 13).However, as a rule, the
cell assumes a definite form in the course of its career. In the tissues of the
multicellular organism, in which a number of similar cells are bound together
in virtue of certain laws of heredity, the shape is determined partly by the
form of their connection and partly by their special functions. Thus, for
instance, we find in the mucous lining of our tongue very thin and delicate
flat cells of roundish shape (Fig. 3). In the outer skin we find similar, but
harder, covering cells, joined together by saw-like edges (Fig. 4). In the
liver and other glands there are thicker and softer cells, linked together in
rows (Fig. 5).

The last-named tissues (Figs. 3–5) belong to the simplest and most
primitive type, the group of the “covering-tissues,” or epithelia.
In these “primary tissues” (to which the germinal layers belong)
simple cells of the same kind are arranged in layers. The arrangement and shape
are more complicated in the “secondary tissues,” which are
gradually developed out of the primary, as in the tissues of the muscles,
nerves, bones, etc. In the bones, for instance, which belong to the group of
supporting or connecting organs,

the cells (Fig. 6) are star-shaped, and are joined together by numbers of
net-like interlacing processes; so, also, in the tissues of the teeth (Fig. 7),
and in other forms of supporting-tissue, in which a soft or hard substance
(intercellular matter, or base) is inserted between the cells.

Fig. 3—Three epithelial cells from the mucous
lining of the tongue.
Fig. 4—Five spiny or grooved cells, with edges joined, from the
outer skin (epidermis): one of them (b) is isolated.
Fig. 5—Ten liver-cells: one of them (b) has two nuclei.

The cells also differ very much in size. The great majority of them are
invisible to the naked eye, and can be seen only through the microscope (being
as a rule between 1/2500 and 1/250 inch in diameter). There are many of the
smaller plastids—such as the famous bacteria—which only come into
view with a very high magnifying power. On the other hand, many cells attain a
considerable size, and run occasionally to several inches in diameter, as do
certain kinds of rhizopods among the unicellular protists (such as the
radiolaria and thalamophora). Among the tissue-cells of the animal body many of
the muscular fibres and nerve fibres are more than four inches, and sometimes
more than a yard, in length. Among the largest cells are the yelk-filled ova;
as, for instance, the yellow “yolk” in the hen’s egg, which
we shall describe later (Fig. 15).

Cells also vary considerably in structure. In this connection we must first
distinguish between the active and passive components of the cell. It is only
the former, or active parts of the cell, that really live, and effect
that marvellous world of phenomena to which we give the name of “organic
life.” The first of these is the inner nucleus (caryoplasm), and
the second the body of the cell (cytoplasm). The passive
portions come third; these are subsequently formed from the others, and I have
given them the name of “plasma-products.” They are partly external
(cell-membranes and intercellular matter) and partly internal (cell-sap and
cell-contents).

The nucleus (or caryon), which is usually of a simple roundish form, is quite
structureless at first (especially in very young cells), and composed of
homogeneous nuclear matter or caryoplasm (Fig. 2k). But, as a rule, it
forms a sort of vesicle later on, in which we can distinguish a more solid
nuclear base (caryobasis) and a softer or fluid nuclear sap
(caryolymph). In a mesh of the nuclear network (or it may be on the inner
side of the nuclear envelope) there is, as a rule, a dark, very opaque, solid
body, called the nucleolus. Many of the nuclei contain several of these
nucleoli (as, for instance, the germinal vesicle of the ova of fishes and
amphibia). Recently a very small, but particularly important, part of the
nucleus has been distinguished as the central body (centrosoma)—a
tiny particle that is originally found in the nucleus itself, but is usually
outside it, in the cytoplasm; as a rule, fine threads stream out from it in the
cytoplasm. From the position of the central body with regard to the other parts
it seems probable that it has a high physiological importance as a centre of
movement; but it is lacking in many cells.

The cell-body also consists originally, and in its simplest form, of a
homogeneous viscid plasmic matter. But, as a rule,

only the smaller part of it is formed of the living active cell-substance
(protoplasm); the greater part consists of dead, passive plasma-products
(metaplasm). It is useful to distinguish between the inner and outer of these.
External plasma-products (which are thrust out from the protoplasm as solid
“structural matter”) are the cell-membranes and the intercellular
matter. The internal plasma-products are either the fluid cell-sap or
hard structures. As a rule, in mature and differentiated cells these various
parts are so arranged that the protoplasm (like the caryoplasm in the round
nucleus) forms a sort of skeleton or framework. The spaces of this network are
filled partly with the fluid cell-sap and partly by hard structural products.

Fig. 6—Nine star-shaped bone-cells, with
interlaced branches.

The simple round ovum, which we take as the starting-point of our study (Figs.
1 and 2), has in many cases the vague, indifferent features of the typical
primitive cell. As a contrast to it, and as an instance of a very highly
differentiated plastid, we may consider for a moment a large nerve-cell, or
ganglionic cell, from the brain. The ovum stands potentially for the entire
organism—in other words, it has the faculty of building up out of itself
the whole multicellular body. It is the common parent of all the countless
generations of cells which form the different tissues of the body; it unites
all their powers in itself, though only potentially or in germ. In complete
contrast to this, the neural cell in the brain (Fig. 9) develops along one
rigid line. It cannot, like the ovum, beget endless generations of cells, of
which some will become skin-cells, others muscle-cells, and others again
bone-cells. But, on the other hand, the nerve-cell has become fitted to
discharge the highest functions of life; it has the powers of sensation, will,
and thought. It is a real soul-cell, or an elementary organ of the psychic
activity. It has, therefore, a most elaborate and delicate structure.

Fig. 7—Eleven star-shaped cells from the enamel
of a tooth, joined together by their branchlets.

Numbers of extremely fine threads, like the electric wires at a
large telegraphic centre, cross and recross in the delicate protoplasm of the
nerve cell, and pass out in the branching processes which proceed from it and
put it in communication with other nerve-cells or nerve-fibres (a, b).
We can only partly follow their intricate paths in the fine matter of the body
of the cell.

Here we have a most elaborate apparatus, the delicate structure of which we are
just beginning to appreciate through our most powerful microscopes, but whose
significance is rather a matter of

conjecture than knowledge. Its intricate structure corresponds to the very
complicated functions of the mind. Nevertheless, this elementary organ of
psychic activity—of which there are thousands in our brain—is
nothing but a single cell. Our whole mental life is only the joint result of
the combined activity of all these nerve-cells, or soul-cells. In the centre of
each cell there is a large transparent nucleus, containing a small and dark
nuclear body. Here, as elsewhere, it is the nucleus that determines the
individuality of the cell; it proves that the whole structure, in spite of its
intricate composition, amounts to only a single cell.

Fig. 8—Unfertilised ovum of an echinoderm (from
Hertwig). The vesicular nucleus (or “germinal vesicle”) is
globular, half the size of the round ovum, and encloses a nuclear framework, in
the central knot of which there is a dark nucleolus (the “germinal
spot”).

In contrast with this very elaborate and very strictly differentiated psychic
cell (Fig. 9), we have our ovum (Figs. 1 and 2), which has hardly any structure
at all. But even in the case of the ovum we must infer from its properties that
its protoplasmic body has a very complicated chemical composition and a fine
molecular structure which escapes our observation. This presumed molecular
structure of the plasm is now generally admitted; but it has never been seen,
and, indeed, lies far beyond the range of microscopic vision. It must not be
confused—as is often done—with the structure of the plasm (the
fibrous network, groups of granules, honey-comb, etc.) which does come within
the range of the microscope.

But when we speak of the cells as the elementary organisms, or structural
units, or “ultimate individualities,” we must bear in mind a
certain restriction of the phrases. I mean, that the cells are not, as is often
supposed, the very lowest stage of organic individuality. There are yet more
elementary organisms to which I must refer occasionally. These are what we call
the “cytodes” (cytos = cell), certain living, independent
beings, consisting only of a particle of plasson—an albuminoid
substance, which is not yet differentiated into caryoplasm and cytoplasm, but
combines the properties of both. Those remarkable beings called the
monera—especially the chromacea and bacteria—are specimens of
these simple cytodes. (Compare Chapter XIX.) To be quite accurate, then, we
must say: the elementary organism, or the ultimate individual, is found in two
different stages. The first and lower stage is the cytode, which consists
merely of a particle of plasson, or quite simple plasm. The second and higher
stage is the cell, which is already divided or differentiated into nuclear
matter and cellular matter. We comprise both kinds—the cytodes and the
cells—under the name of plastids (“formative
particles”), because they are the real builders of the organism. However,
these cytodes are not found, as a rule, in the higher animals and plants; here
we have only real cells with a nucleus. Hence, in these tissue-forming
organisms (both plant and animal) the organic unit always consists of two
chemically and anatomically different parts—the outer cell-body and the
inner nucleus.

In order to convince oneself that this cell is really an independent organism,
we have only to observe the development and vital phenomena of one of them. We
see then that it performs all the essential functions of life—both
vegetal and animal—which we find in the entire organism. Each of these
tiny beings grows and nourishes itself independently. It takes its food from
the surrounding fluid; sometimes, even, the naked cells take in solid particles
at certain points of their surface—in other words, “eat”
them—without needing any special mouth and stomach for the purpose (cf.
Fig. 19).

Further, each cell is able to reproduce itself. This multiplication, in most
cases, takes the form of a simple cleavage, sometimes direct, sometimes
indirect; the simple direct (or “amitotic”) division is less
common, and is found, for instance, in the blood cells (Fig. 10). In these the
nucleus first divides into two equal parts by constriction. The indirect (or
“mitotic”)


cleavage is much more frequent; in this the caryoplasm of the nucleus and the
cytoplasm of the cell-body act upon each other in a peculiar way, with a
partial dissolution (caryolysis), the formation of knots and loops
(mitosis), and a movement of the halved plasma-particles towards two
mutually repulsive poles of attraction (caryokinesis, Fig. 11.)

Fig. 9—A large branching nerve-cell, or
“soul-cell”, from the brain of an electric fish
(Torpedo). In the middle of the cell is the large transparent round
nucleus, one nucleolus, and, within the latter again, a
nucleolinus. The protoplasm of the cell is split into innumerable fine
threads (or fibrils), which are embedded in intercellular matter, and are
prolonged into the branching processes of the cell (b). One branch
(a) passes into a nerve-fibre. (From Max Schultze.)

Fig. 10—Blood-cells, multiplying by direct
division, from the blood of the embryo of a stag. Originally, each
blood-cell has a nucleus and is round (a). When it is going to multiply,
the nucleus divides into two (b, c, d). Then the protoplasmic body is
constricted between the two nuclei, and these move away from each other
(e). Finally, the constriction is complete, and the cell splits into two
daughter-cells (f). (From Frey.)

The intricate physiological processes which accompany this
“mitosis” have been very closely studied of late years. The inquiry
has led to the detection of certain laws of evolution which are of extreme
importance in connection with heredity. As a rule, two very different parts of
the nucleus play an important part in these changes. They are: the
chromatin, or coloured nuclear substance, which has a peculiar property
of tingeing itself deeply with certain colouring matters (carmine, hæmatoxylin,
etc.), and the achromin (or linin, or achromatin), a
colourless nuclear substance that lacks this property. The latter generally
forms in the dividing cell a sort of spindle, at the poles of which there is a
very small particle, also colourless, called the “central body”
(centrosoma). This acts as the centre or focus in a “sphere of
attraction” for the granules of protoplasm in the surrounding cell-body,
and assumes a star-like appearance (the cell-star, or monaster). The two
central bodies, standing opposed to each other at the poles of the nuclear
spindle, form “the double-star” (or amphiaster, Fig. 11, B
C). The chromatin often forms a long, irregularly-wound thread—“the
coil” (spirema, Fig. A). At the commencement of the cleavage it
gathers at the equator of the cell, between the stellar poles, and forms a
crown of U-shaped loops (generally four or eight, or some other definite
number). The loops split lengthwise into two halves (B), and these back away
from each other towards the poles of the spindle (C). Here each group forms a
crown once more, and this, with the corresponding half of the divided spindle,
forms a fresh nucleus (D). Then the protoplasm of the cell-body begins to
contract in the middle, and gather about the new daughter-nuclei, and at last
the two daughter-cells become independent beings.

Between this common mitosis, or indirect cell-division—which is
the normal cleavage-process in most cells of the higher animals and
plants—and the simple direct division (Fig. 10) we find every
grade of segmentation; in some circumstances even one kind of division may be
converted into another.

The plastid is also endowed with the functions of movement and sensation. The
single cell can move and creep about, when it has space for free movement and
is not prevented by a hard envelope; it then thrusts out at its surface
processes like fingers, and quickly withdraws them again, and thus changes its
shape (Fig. 12). Finally, the young cell is sensitive, or more or less
responsive to stimuli; it makes certain movements on the application of
chemical and mechanical irritation. Hence we can ascribe to the individual cell
all the chief functions which we comprehend under the general heading of
“life”—sensation, movement, nutrition, and reproduction. All
these properties of the multicellular and highly developed animal are also
found in the single animal-cell, at least in its younger stages. There is no
longer any doubt about this, and so we may regard it as a solid and important
base of our physiological conception of the elementary organism.

Without going any further here into these very interesting phenomena of the
life of the cell, we will pass on to consider the application of the cell
theory to the ovum. Here comparative research yields the important result that
every ovum is at first a simple cell. I say this is very important,
because our whole science of embryology now resolves itself into the problem:
“How does the multicellular

organism arise from the unicellular?” Every organic individual is at
first a simple cell, and as such an elementary organism, or a unit of
individuality. This cell produces a cluster of cells by segmentation, and from
these develops the multicellular organism, or individual of higher rank.

A. Mother-cell
(Knot, spirema)
1. Nuclear
threads (chromosomata) (coloured nuclear matter, chromatin)
2. Nuclear
membrane
3. Nuclear sap
4. Cytosoma
5. Protoplasm of the
cell-body

B. Mother-star, the loops beginning to split
lengthways (nuclear membrane gone)
1. Star-like appearance in
cytoplasm
2. Centrosoma (sphere of attraction)
3. Nuclear spindle
(achromin, colourless matter)
4. Nuclear loops (chromatin, coloured
matter)

C. The two daughter-stars,
produced
by the breaking of the loops of the mother-star (moving away)
1. Upper
daughter-crown
2. Connecting threads of the two crowns (achromin)
3.
Lower daughter-crown
4. Double-star (amphiaster)

D. The two daughter-cells,
produced by the complete division of the
two nuclear halves (cytosomata still connected at the equator) (Double-knot,
Dispirema)
1. Upper daughter-nucleus
2. Equatorial constriction of
the cell-body
3. Lower daughter-nucleus.

Fig. 11—Indirect or mitotic
cell-division (with caryolysis and caryokinesis) from the skin of the larva
of a salamander. (From Rabl.).

When we examine a little closer the original features of the ovum, we notice
the extremely significant fact that in its first stage the ovum is just the
same simple and indefinite structure in the case of man and all the animals
(Fig. 13). We are unable to detect any material difference between them, either
in outer shape or internal constitution. Later, though the ova remain
unicellular, they differ in size and shape, enclose various kinds of
yelk-particles, have different envelopes, and so on. But when we examine them
at their birth, in the ovary of the female animal, we find them to be always of
the same form in the first stages of their life. In the beginning each ovum is
a very simple, roundish, naked, mobile cell, without a membrane; it consists
merely of a particle of cytoplasm enclosing a nucleus (Fig. 13). Special names
have been given to these parts of the ovum; the cell-body is called the
yelk (vitellus), and the cell-nucleus the germinal
vesicle. As a rule, the

nucleus of the ovum is soft, and looks like a small pimple or vesicle. Inside
it, as in many other cells, there is a nuclear skeleton or frame and a third,
hard nuclear body (the nucleolus). In the ovum this is called the
germinal spot. Finally, we find in many ova (but not in all) a still
further point within the germinal spot, a “nucleolin,” which goes
by the name of the germinal point. The latter parts (germinal spot and germinal
point) have, apparently, a minor importance, in comparison with the other two
(the yelk and germinal vesicle). In the yelk we must distinguish the active
formative yelk (or protoplasm = first plasm) from the passive
nutritive yelk (or deutoplasm = second plasm).

Fig. 12—Mobile cells from the
inflamed eye of a frog (from the watery fluid of the eye, the humor
aqueus). The naked cells creep freely about, by (like the amœba or
rhizopods) protruding fine processes from the uncovered protoplasmic body.
These bodies vary continually in number, shape, and size. The nucleus of these
amœboid lymph-cells (“travelling cells,” or planocytes) is
invisible, because concealed by the numbers of fine granules which are
scattered in the protoplasm. (From Frey.)

In many of the lower animals (such as sponges, polyps, and medusæ) the naked
ova retain their original simple appearance until impregnation. But in most
animals they at once begin to change; the change consists partly in the
formation of connections with the yelk, which serve to nourish the ovum, and
partly of external membranes for their protection (the ovolemma, or
prochorion). A membrane of this sort is formed in all the mammals in the course
of the embryonic process. The little globule is surrounded by a thick capsule
of glass-like transparency, the zona pellucida, or ovolemma
pellucidum (Fig. 14). When we examine it closely under the microscope, we
see very fine radial streaks in it, piercing the zona, which are really
very narrow canals. The human ovum, whether fertilised or not, cannot be
distinguished from that of most of the other mammals. It is nearly the same
everywhere in form, size, and composition. When it is fully formed, it has a
diameter of (on an average) about 1/120 of an inch. When the mammal ovum has
been carefully isolated, and held against the light on a glass-plate, it may be
seen as a fine point even with the naked eye. The ova of most of the higher
mammals are about the same size. The diameter of the ovum is almost always
between 1/250 to 1/125 inch. It has always the same globular shape; the same
characteristic membrane; the same transparent germinal vesicle with its dark
germinal spot. Even when we use the most powerful microscope with its highest
power, we can detect no material difference between the ova of man, the ape,
the dog, and so on. I do not mean to say that there are no differences between
the ova of these different mammals. On the contrary, we are bound to assume
that there are such, at least as regards chemical composition. Even the ova of
different men must differ from each other; otherwise we should not have a
different individual from each ovum. It is true that our crude and imperfect
apparatus cannot detect these subtle individual differences, which are probably
in the molecular structure. However, such a striking resemblance of their ova
in form, so great as to seem to be a complete similarity, is a strong proof of
the common parentage of man and the other mammals. From the common germ-form we
infer a common stem-form. On the other hand, there are striking peculiarities
by which we can easily distinguish the fertilised ovum of the mammal from the
fertilised ovum of the birds, amphibia, fishes, and other vertebrates (see the
close of Chap. XXIX).

The fertilised bird-ovum (Fig. 15) is notably different. It is true that in its
earliest stage (Fig. 13 E) this ovum also is very like that of the mammal (Fig.
13 F). But afterwards, while still within the oviduct, it takes up a quantity
of nourishment and works this into the familiar large yellow yelk. When we
examine a very young ovum in the hen’s oviduct, we

find it to be a simple, small, naked, amœboid cell, just like the young ova of
other animals (Fig. 13). But it then grows to the size we are familiar with in
the round yelk of the egg. The nucleus of the ovum, or the germinal vesicle, is
thus pressed right to the surface of the globular ovum, and is embedded there
in a small quantity of transparent matter, the so-called white yelk. This forms
a round white spot, which is known as the “tread”
(cicatricula) (Fig. 15 b). From the tread a thin column of the
white yelk penetrates through the yellow yelk to the centre of the globular
cell, where it swells into a small, central globule (wrongly called the
yelk-cavity, or latebra, Fig. 15 d′). The yellow
yelk-matter which surrounds this white yelk has the appearance in the egg (when
boiled hard) of concentric layers (c). The yellow yelk is also enclosed
in a delicate structureless membrane (the membrana vitellina, a).

Fig. 13—Ova of various animals, executing
amœboid movements, magnified. All the ova are naked cells of varying shape.
In the dark fine-grained protoplasm (yelk) is a large vesicular nucleus (the
germinal vesicle), and in this is seen a nuclear body (the germinal spot), in
which again we often see a germinal point. Figs. A1–A4 represent
the ovum of a sponge (Leuculmis echinus) in four successive movements.
B1–B8 are the ovum of a parasitic crab (Chondracanthus
cornutus), in eight successive movements. (From Edward von Beneden.)
C1–C5 show the ovum of the cat in various stages of movement (from
Pflüger); Fig. D the ovum of a trout; E the ovum of a
chicken; F a human ovum.

As the large yellow ovum of the bird

attains a diameter of several inches in the bigger birds, and encloses round
yelk-particles, there was formerly a reluctance to consider it as a simple
cell. This was a mistake. Every animal that has only one cell-nucleus, every
amœba, every gregarina, every infusorium, is unicellular, and remains
unicellular whatever variety of matter it feeds on. So the ovum remains a
simple cell, however much yellow yelk it afterwards accumulates within its
protoplasm. It is, of course, different, with the bird’s egg when it has
been fertilised. The ovum then consists of as many cells as there are nuclei in
the tread. Hence, in the fertilised egg which we eat daily, the yellow yelk is
already a multicellular body. Its tread is composed of several cells, and is
now commonly called the germinal disc. We shall return to this
discogastrula in Chap. IX.

Fig. 14—The human ovum, taken from the female
ovary, magnified. The whole ovum is a simple round cell. The chief part of the
globular mass is formed by the nuclear yelk (deutoplasm), which is
evenly distributed in the active protoplasm, and consists of numbers of fine
yelk-granules. In the upper part of the yelk is the transparent round germinal
vesicle, which corresponds to the nucleus. This encloses a darker
granule, the germinal spot, which shows a nucleolus. The globular yelk
is surrounded by the thick transparent germinal membrane (ovolemma, or
zona pellucida). This is traversed by numbers of lines as fine as
hairs, which are directed radially towards the centre of the ovum. These are
called the pore-canals; it is through these that the moving spermatozoa
penetrate into the yelk at impregnation.

When the mature bird-ovum has left the ovary and been fertilised in the
oviduct, it covers itself with various membranes which are secreted from the
wall of the oviduct. First, the large clear albuminous layer is deposited
around the yellow yelk; afterwards, the hard external shell, with a fine inner
skin. All these gradually forming envelopes and processes are of no importance
in the formation of the embryo; they serve merely for the protection of the
original simple ovum. We sometimes find extraordinarily large eggs with strong
envelopes in the case of other animals, such as fishes of the shark type. Here,
also, the ovum is originally of the same character as it is in the mammal; it
is a perfectly simple and naked cell. But, as in the case of the bird, a
considerable quantity of nutritive yelk is accumulated inside the original yelk
as food for the developing embryo; and various coverings are formed round the
egg. The ovum of many other animals has the same internal and external
features. They have, however, only a physiological, not a morphological,
importance; they have no direct influence on the formation of the fœtus. They
are partly consumed as food by the embryo, and partly serve as protective
envelopes. Hence we may leave them out of consideration altogether here, and
restrict ourselves to material points—to the substantial identity of
the original ovum in man and the rest of the animals (Fig. 13).

Now, let us for the first time make use of our biogenetic law; and directly
apply this fundamental law of evolution to the human ovum. We reach a very
simple, but very important, conclusion. From

the fact that the human ovum and that of all other animals consists of a
single cell, it follows immediately, according to the biogenetic law, that all
the animals, including man, descend from a unicellular organism. If our
biogenetic law is true, if the embryonic development is a summary or condensed
recapitulation of the stem-history—and there can be no doubt about
it—we are bound to conclude, from the fact that all the ova are at first
simple cells, that all the multicellular organisms originally sprang from a
unicellular being. And as the original ovum in man and all the other animals
has the same simple and indefinite appearance, we may assume with some
probability that this unicellular stem-form was the common ancestor of the
whole animal world, including man. However, this last hypothesis does not seem
to me as inevitable and as absolutely certain as our first conclusion.

Fig. 15—A fertilised ovum from the oviduct of a
hen. The yellow yelk (c) consists of several concentric layers
(d), and is enclosed in a thin yelk-membrane (a). The nucleus or
germinal vesicle is seen above in the cicatrix or “tread”
(b). From that point the white yelk penetrates to the central
yelk-cavity (d′). The two kinds of yelk do not differ very much.

This inference from the unicellular embryonic form to the
unicellular ancestor is so simple, but so important, that we cannot
sufficiently emphasise it. We must, therefore, turn next to the question
whether there are to-day any unicellular organisms, from the features of which
we may draw some approximate conclusion as to the unicellular ancestors of the
multicellular organisms. The answer is: Most certainly there are. There are
assuredly still unicellular organisms which are, in their whole nature, really
nothing more than permanent ova. There are independent unicellular organisms of
the simplest character which develop no further, but reproduce themselves as
such, without any further growth. We know to-day of a great number of these
little beings, such as the gregarinæ, flagellata, acineta, infusoria, etc.
However, there is one of them that has an especial interest for us, because it
at once suggests itself when we raise our question, and it must be regarded as
the unicellular being that approaches nearest to the real ancestral form. This
organism is the Amœba.

Fig. 16—A creeping amœba (highly magnified).
The whole organism is a simple naked cell, and moves about by means of the
changing arms which it thrusts out of and withdraws into its protoplasmic body.
Inside it is the roundish nucleus with its nucleolus.

For a long time now we have comprised under the general name of
amœbæ a number of microscopic unicellular organisms, which are very widely
distributed, especially in fresh-water, but also in the ocean; in fact, they
have lately been discovered in damp soil. There are also parasitic amœbæ which
live inside other animals. When we place one of these amœbæ in a drop of water
under the microscope and examine it with a high power, it generally appears as
a roundish particle of a very irregular and varying shape (Figs. 16 and 17). In
its soft, slimy, semi-fluid substance, which consists of protoplasm, we see
only the solid globular particle it contains, the nucleus. This unicellular
body moves about continually, creeping in every direction on the glass on which
we are examining it. The movement is effected by the shapeless body thrusting
out finger-like processes at various parts of its surface; and these are slowly
but continually changing, and drawing the rest of the body after them. After a
time, perhaps, the action changes. The amœba suddenly stands still, withdraws
its projections, and assumes a globular shape. In a little while, however, the
round body begins to expand again, thrusts out arms in another

direction, and moves on once more. These changeable processes are called
“false feet,” or pseudopodia, because they act physiologically as
feet, yet are not special organs in the anatomic sense. They disappear as
quickly as they come, and are nothing more than temporary projections of the
semi-fluid and structureless body.

Fig. 17—Division of a
unicellular amœba (Amœba polypodia) in six stages. (From F. E.
Schultze.) the dark spot is the nucleus, the lighter spot a contractile
vacuole in the protoplasm. The latter reforms in one of the daughter-cells.)

If you touch one of these creeping amœbæ with a needle, or put a drop of acid
in the water, the whole body at once contracts in consequence of this
mechanical or physical stimulus. As a rule, the body then resumes its globular
shape. In certain circumstances—for instance, if the impurity of the
water lasts some time—the amœba begins to develop a covering. It exudes a
membrane or capsule, which immediately hardens, and assumes the appearance of a
round cell with a protective membrane. The amœba either takes its food directly
by imbibition of matter floating in the water, or by pressing into its
protoplasmic body solid particles with which it comes in contact. The latter
process may be observed at any moment by forcing it to eat. If finely ground
colouring matter, such as carmine or indigo, is put into the water, you can see
the body of the amœba pressing these coloured particles into itself, the
substance of the cell closing round them. The amœba can take in food in this
way at any point on its surface, without having any special organs for
intussusception and digestion, or a real mouth or gut.

The amœba grows by thus taking in food and dissolving the particles eaten in
its protoplasm. When it reaches a certain size by this continual feeding, it
begins to reproduce. This is done by the simple process of cleavage (Fig. 17).
First, the nucleus divides into two parts. Then the protoplasm is separated
between the two new nuclei, and the whole cell splits into two daughter-cells,
the protoplasm gathering about each of the nuclei. The thin bridge of
protoplasm which at first connects the daughter-cells soon breaks. Here we have
the simple form of direct cleavage of the nuclei. Without mitosis, or formation
of threads, the homogeneous nucleus divides into two halves. These move away
from each other, and become centres of attraction for the enveloping matter,
the protoplasm. The same direct cleavage of the nuclei is also witnessed in the
reproduction of many other protists, while other unicellular organisms show the
indirect division of the cell.

Hence, although the amœba is nothing but a simple cell, it is evidently able to
accomplish all the functions of the multicellular organism. It moves, feels,
nourishes itself, and reproduces. Some kinds of these amœbæ can be seen with
the naked eye, but most of them are microscopically small. It is for the
following reasons that we regard the amœbæ as the unicellular organisms which
have

special phylogenetic (or evolutionary) relations to the ovum. In many of the
lower animals the ovum retains its original naked form until fertilisation,
develops no membranes, and is then often indistinguishable from the ordinary
amœba. Like the amœbæ, these naked ova may thrust out processes, and move about
as travelling cells. In the sponges these mobile ova move about freely in the
maternal body like independent amœbæ (Fig. 17). They had been observed by
earlier scientists, but described as foreign bodies—namely, parasitic
amœbæ, living parasitically on the body of the sponge. Later, however, it was
discovered that they were not parasites, but the ova of the sponge. We also
find this remarkable phenomenon among other animals, such as the graceful,
bell-shaped zoophytes, which we call polyps and medusæ. Their ova remain naked
cells, which thrust out amœboid projections, nourish themselves, and move
about. When they have been fertilised, the multicellular organism is formed
from them by repeated segmentation.

It is, therefore, no audacious hypothesis, but a perfectly sound conclusion, to
regard the amœba as the particular unicellular organism which offers us an
approximate illustration of the ancient common unicellular ancestor of all the
metazoa, or multicellular animals. The simple naked amœba has a less definite
and more original character than any other cell. Moreover, there is the fact
that recent research has discovered such amœba-like cells everywhere in the
mature body of the multicellular animals. They are found, for instance, in the
human blood, side by side with the red corpuscles, as colourless blood-cells;
and it is the same with all the vertebrates. They are also found in many of the
invertebrates—for instance, in the blood of the snail. I showed, in 1859,
that these colourless blood-cells can, like the independent amœbæ, take up
solid particles, or “eat” (whence they are called phagocytes
= “eating-cells,” Fig. 19). Lately, it has been discovered that
many different cells may, if they have room enough, execute the same movements,
creeping about and eating. They behave just like amœbæ (Fig. 12). It has also
been shown that these “travelling-cells,” or planocytes,
play an important part in man’s physiology and pathology (as means of
transport for food, infectious matter, bacteria, etc.).

The power of the naked cell to execute these characteristic amœba-like
movements comes from the contractility (or automatic mobility) of its
protoplasm. This seems to be a universal property of young cells. When they are
not enclosed by a firm membrane, or confined in a “cellular
prison,” they can always accomplish these amœboid movements. This is true
of the naked ova as well as of any other naked cells, of the
“travelling-cells,” of various kinds in connective tissue,
lymph-cells, mucus-cells, etc.

Fig. 18—Ovum of a sponge (Olynthus).
The ovum creeps about in a body of the sponge by thrusting out ever-changing
processes. It is indistinguishable from the common amœba.)

We have now, by our study of the ovum and the comparison of it
with the amœba, provided a perfectly sound and most valuable foundation for
both the embryology and the evolution of man. We have learned that the human
ovum is a simple cell, that this ovum is not materially different from that of
other mammals, and that we may infer from it the existence of a primitive
unicellular ancestral form, with a substantial resemblance to the amœba.

The statement that the earliest progenitors of the human race were simple cells
of this kind, and led an independent unicellular life like the amœba, has not
only been ridiculed as the dream of a natural philosopher, but also been
violently censured in theological journals as “shameful and
immoral.” But, as I observed in my essay On the Origin and Ancestral
Tree of the Human Race in 1870, this offended piety must equally protest
against the “shameful and immoral” fact that each human individual
is developed from a simple ovum, and that this human ovum is indistinguishable
from those of the other mammals, and in its earliest stage is like a naked
amœba.

We can show this to be a fact any day with the microscope, and it is little use
to close one’s eyes to “immoral” facts of this kind. It is as
indisputable as the momentous conclusions we draw from it and as the vertebrate
character of man (see Chap. XI).

Fig. 19—Blood-cells that eat, or phagocytes,
from a naked sea-snail (Thetis), greatly magnified. I was the first
to observe in the blood-cells of this snail the important fact that “the
blood-cells of the invertebrates are unprotected pieces of plasm, and take in
food, by means of their peculiar movements, like the amœbæ.” I had (in
Naples, on May 10th, 1859) injected into the blood-vessels of one of these
snails an infusion of water and ground indigo, and was greatly astonished to
find the blood-cells themselves more or less filled with the particles of
indigo after a few hours. After repeated injections I succeeded in
“observing the very entrance of the coloured particles in the
blood-cells, which took place just in the same way as with the amœba.” I
have given further particulars about this in my Monograph on the
Radiolaria.

We now see very clearly how extremely important the cell theory has been for
our whole conception of organic nature. “Man’s place in
nature” is settled beyond question by it. Apart from the cell theory, man
is an insoluble enigma to us. Hence philosophers, and especially physiologists,
should be thoroughly conversant with it. The soul of man can only be really
understood in the light of the cell-soul, and we have the simplest form of this
in the amœba. Only those who are acquainted with the simple psychic functions
of the unicellular organisms and their gradual evolution in the series of lower
animals can understand how the elaborate mind of the higher vertebrates, and
especially of man, was gradually evolved from them. The academic psychologists
who lack this zoological equipment are unable to do so.

This naturalistic and realistic conception is a stumbling-block to our modern
idealistic metaphysicians and their theological colleagues. Fenced about with
their transcendental and dualistic prejudices, they attack not only the
monistic system we establish on our scientific knowledge, but even the plainest
facts which go to form its foundation. An instructive instance of this was seen
a few years ago, in the academic discourse delivered by a distinguished
theologian, Willibald Beyschlag, at Halle, January 12th, 1900, on the occasion
of the centenary festival. The theologian protested violently against the
“materialistic dustmen of the scientific world who offer our people the
diploma of a descent from the ape, and would prove to them that the genius of a
Shakespeare or a Goethe is merely a distillation from a drop of primitive
mucus.” Another well-known theologian protested against “the
horrible idea that the greatest of men, Luther and Christ, were descended from
a mere globule of protoplasm.” Nevertheless, not a single informed and
impartial scientist doubts the fact that these greatest men were, like all
other men—and all other vertebrates—developed from an impregnated
ovum, and that this simple nucleated globule of protoplasm has the same
chemical constitution in all the mammals.

Chapter VII.
CONCEPTION

The recognition of the fact that every man begins his individual existence as a
simple cell is the solid foundation of all research into the genesis of man.
From this fact we are forced, in virtue of our biogenetic law, to draw the
weighty phylogenetic conclusion that the earliest ancestors of the human race
were also unicellular organisms; and among these protozoa we may single out the
vague form of the amœba as particularly important (cf. Chapter VI). That these
unicellular ancestral forms did once exist follows directly from the phenomena
which we perceive every day in the fertilised ovum. The development of the
multicellular organism from the ovum, and the formation of the germinal layers
and the tissues, follow the same laws in man and all the higher animals. It
will, therefore, be our next task to consider more closely the impregnated ovum
and the process of conception which produces it.

The process of impregnation or sexual conception is one of those phenomena that
people love to conceal behind the mystic veil of supernatural power. We shall
soon see, however, that it is a purely mechanical process, and can be reduced
to familiar physiological functions. Moreover, this process of conception is of
the same type, and is effected by the same organs, in man as in all the other
mammals. The pairing of the male and female has in both cases for its main
purpose the introduction of the ripe matter of the male seed or sperm into the
female body, in the sexual canals of which it encounters the ovum. Conception
then ensues by the blending of the two.

We must observe, first, that this important process is by no means so widely
distributed in the animal and plant world as is commonly supposed. There is a
very large number of lower organisms which propagate unsexually, or by
monogamy; these are especially the sexless monera (chromacea, bacteria, etc.)
but also many other protists, such as the amœbæ, foraminifera, radiolaria,
myxomycetæ, etc. In these the multiplication of individuals takes place by
unsexual reproduction, which takes the form of cleavage, budding, or
spore-formation. The copulation of two coalescing cells, which in these cases
often precedes the reproduction, cannot be regarded as a sexual act unless the
two copulating plastids differ in size or structure. On the other hand, sexual
reproduction is the general rule with all the higher organisms, both animal and
plant; very rarely do we find asexual reproduction among them. There are, in
particular, no cases of parthenogenesis (virginal conception) among the
vertebrates.

Sexual reproduction offers an infinite variety of interesting forms in the
different classes of animals and plants, especially as regards the mode of
conception, and the conveyance of the spermatozoon to the ovum. These features
are of great importance not only as regards conception itself, but for the
development of the organic form, and especially for the differentiation of the
sexes. There is a particularly curious correlation of plants and animals in
this respect. The splendid studies of Charles Darwin and Hermann Müller on the
fertilisation of flowers by insects have given us very interesting particulars
of this.^[14] This reciprocal service has given rise to a most intricate
sexual apparatus. Equally elaborate structures have been developed in man and
the higher animals, serving partly for the isolation of the sexual products on
each side, partly for bringing them together in conception. But, however
interesting these phenomena are in themselves, we cannot go into them here, as
they have only a minor importance—if any at all—in the real process
of conception. We must, however, try to get a very clear idea of this process
and the meaning of sexual reproduction.

[14]
See Darwin’s work, On the Various Contrivances by which Orchids are
Fertilised (1862).

In every act of conception we have, as I said, to consider two different kinds
of cells—a female and a male cell. The female cell of the animal organism
is always called the ovum (or ovulum, egg, or egg-cell); the male cells
are known as the sperm or seed-cells, or the spermatozoa (also spermium and
zoospermium). The ripe ovum is, on the whole, one of the largest cells we know.
It attains colossal dimensions when it absorbs great quantities of nutritive
yelk, as is the case with birds and reptiles and many of the fishes. In the
great majority of the animals the ripe ovum is rich in yelk and much larger
than the other cells. On the other hand, the next cell which we have to
consider in the process of conception, the male sperm-cell or spermatozoon, is
one of the smallest cells in the animal body. Conception usually consists in
the bringing into contact with the ovum of a slimy fluid secreted by the male,
and this may take place either inside or out of the female body. This fluid is
called sperm, or the male seed. Sperm, like saliva or blood, is not a simple
fluid, but a thick agglomeration of innumerable cells, swimming about in a
comparatively small quantity of fluid. It is not the fluid, but the independent
male cells that swim in it, that cause conception.

Fig. 20—Spermia or spermatozoa of various
mammals. The pear-shaped flattened nucleus is seen from the front in
I and sideways in II. k is the nucleus, m its middle part
(protoplasm), s the mobile, serpent-like tail (or whip); M four
human spermatozoa, A four spermatozoa from the ape; K from the
rabbit; H from the mouse; C from the dog; S from the
pig.

The spermatozoa of the great majority of animals have two characteristic
features. Firstly, they are extraordinarily small, being usually the smallest
cells in the body; and, secondly, they have, as a rule, a peculiarly lively
motion, which is known as spermatozoic motion. The shape of the cell has a good
deal to do with this motion. In most of the animals, and also in many of the
lower plants (but not the higher) each of these spermatozoa has a very small,
naked cell-body, enclosing an elongated nucleus, and a long thread hanging from
it (Fig. 20). It was long before we could recognise that these structures are
simple cells. They were formerly held to be special organisms, and were called
“seed animals” (spermato-zoa, or spermato-zoidia); they are now
scientifically known as spermia or spermidia, or as
spermatosomata (seed-bodies) or spermatofila (seed threads). It
took a good deal of comparative research to convince us that each of these
spermatozoa is really a simple cell. They have the same shape as in many other
vertebrates and most of the invertebrates. However, in many of the lower
animals they have quite a different shape. Thus, for instance, in the craw fish
they are large round cells, without any movement, equipped with stiff
outgrowths like bristles (Fig. 21 f ). They have also a peculiar
form in some of the worms, such as the thread-worms (filaria); in this
case they are sometimes

amœboid and like very small ova (Fig. 21 c to e). But in most of
the lower animals (such as the sponges and polyps) they have the same pine-cone
shape as in man and the other animals (Fig. 21 a, h).

Fig. 21—Spermatozoa or spermidia of various
animals. (From Lang). a of a fish, b of a turbellaria
worm (with two side-lashes), c to e of a nematode worm (amœboid
spermatozoa), f from a craw fish (star-shaped), g from the
salamander (with undulating membrane), h of an annelid (a and
h are the usual shape).

When the Dutch naturalist Leeuwenhoek discovered these thread-like lively
particles in 1677 in the male sperm, it was generally believed that they were
special, independent, tiny animalcules, like the infusoria, and that the whole
mature organism existed already, with all its parts, but very small and packed
together, in each spermatozoon (see p.12). We now know that the mobile
spermatozoa are nothing but simple and real cells, of the kind that we call
“ciliated” (equipped with lashes, or cilia). In the previous
illustrations we have distinguished in the spermatozoon a head, trunk, and
tail. The “head” (Fig. 20 k) is merely the oval nucleus of
the cell; the body or middle-part (m) is an accumulation of cell-matter;
and the tail (s) is a thread-like prolongation of the same.

Moreover, we now know that these spermatozoa are not at all a peculiar form of
cell; precisely similar cells are found in various other parts of the body. If
they have many short threads projecting, they are called ciliated; if
only one long, whip-shaped process (or, more rarely, two or four),
caudate (tailed) cells.

Very careful recent examination of the spermia, under a very high microscopic
power (Fig. 22 a, b), has detected some further details in the finer structure
of the ciliated cell, and these are common to man and the anthropoid ape. The
head (k) encloses the elliptic nucleus in a thin envelope of cytoplasm;
it is a little flattened on one side, and thus looks rather pear-shaped from
the front (b). In the central piece (m) we can distinguish a
short neck and a longer connective piece (with central body). The tail consists
of a long main section (h) and a short, very fine tail (e).

Fig. 22—A single human spermatozoon
magnified; a shows it from the broader and b from the narrower side. k
head (with nucleus), m middle-stem, h long-stem, and e
tail. (From Retzius.)

The process of fertilisation by sexual conception consists, therefore,
essentially in the coalescence and fusing together of two different cells. The
lively spermatozoon travels towards the ovum by its serpentine movements, and
bores its way into the female cell (Fig. 23). The nuclei of both sexual cells,
attracted by a certain “affinity,” approach each other and melt
into one.

The fertilised cell is quite another thing from the unfertilised cell. For if
we must regard the spermia as real cells no less than the ova, and the process
of conception as a coalescence of the two, we must consider the resultant cell
as a quite new and independent organism. It bears in the cell and nuclear
matter of the penetrating spermatozoon a part of the father’s body, and
in the protoplasm and caryoplasm of the ovum a part of the mother’s body.
This is clear from the fact that the child inherits many features from both
parents. It inherits from the father by means of the spermatozoon, and from the
mother by means of the ovum. The

actual blending of the two cells produces a third cell, which is the germ of
the child, or the new organism conceived. One may also say of this sexual
coalescence that the stem-cell is a simple hermaphrodite; it unites
both sexual substances in itself.

Fig. 23—The fertilisation of the ovum by the
spermatozoon (of a mammal). One of the many thread-like, lively spermidia
pierces through a fine pore-canal into the nuclear yelk. The nucleus of the
ovum is invisible.

I think it necessary to emphasise the fundamental importance of
this simple, but often unappreciated, feature in order to have a correct and
clear idea of conception. With that end, I have given a special name to the new
cell from which the child develops, and which is generally loosely called
“the fertilised ovum,” or “the first segmentation
sphere.” I call it “the stem-cell” (cytula). The name
“stem-cell” seems to me the simplest and most suitable, because all
the other cells of the body are derived from it, and because it is, in the
strictest sense, the stem-father and stem-mother of all the countless
generations of cells of which the multicellular organism is to be composed.
That complicated molecular movement of the protoplasm which we call
“life” is, naturally, something quite different in this stem-cell
from what we find in the two parent-cells, from the coalescence of which it has
issued. The life of the stem-cell or cytula is the product or resultant of
the paternal life-movement that is conveyed in the spermatozoon and the
maternal life-movement that is contributed by the ovum.

The admirable work done by recent observers has shown that the individual
development, in man and the other animals, commences with the formation of a
simple “stem-cell” of this character, and that this then passes, by
repeated segmentation (or cleavage), into a cluster of cells, known as
“the segmentation sphere” or “segmentation cells.” The
process is most clearly observed in the ova of the echinoderms (star-fishes,
sea-urchins, etc.). The investigations of Oscar and Richard Hertwig were
chiefly directed to these. The main results may be summed up as follows:—

Conception is preceded by certain preliminary changes, which are very
necessary—in fact, usually indispensable—for its occurrence. They
are comprised under the general heading of “Changes prior to
impregnation.” In these the original nucleus of the ovum, the germinal
vesicle, is lost. Part of it is extruded, and part dissolved in the cell
contents; only a very small part of it is left to form the basis of a fresh
nucleus, the pronucleus femininus. It is the latter alone that combines
in conception with the invading nucleus of the fertilising spermatozoon (the
pronucleus masculinus).

The impregnation of the ovum commences with a decay of the germinal vesicle, or
the original nucleus of the ovum (Fig. 8). We have seen that this is in most
unripe ova a large, transparent, round vesicle. This germinal vesicle contains
a viscous fluid (the caryolymph). The firm nuclear frame
(caryobasis) is formed of the enveloping membrane and a mesh-work of
nuclear threads running across the interior, which is filled with the nuclear
sap. In a knot of the network is contained the dark, stiff, opaque nuclear
corpuscle or nucleolus. When the impregnation of the ovum sets in, the greater
part of the germinal vesicle is dissolved in the cell; the nuclear membrane and
mesh-work disappear; the nuclear sap is distributed in the protoplasm; a small
portion of the nuclear base is extruded; another small portion is left, and is
converted into the secondary nucleus, or the female pro-nucleus (Fig. 24 e
k).

The small portion of the nuclear base which is extruded from the impregnated
ovum is known as the “directive bodies” or “polar
cells”; there are many disputes as to their origin and significance, but
we are as yet imperfectly acquainted with them. As a rule, they are two small
round granules, of the same size and appearance as the remaining pro-nucleus.
They are detached cell-buds; their separation from the large mother-cell takes

place in the same way as in ordinary “indirect cell-division.”
Hence, the polar cells are probably to be conceived as “abortive
ova,” or “rudimentary ova,” which proceed from a simple
original ovum by cleavage in the same way that several sperm-cells arise from
one “sperm-mother-cell,” in reproduction from sperm. The male
sperm-cells in the testicles must undergo similar changes in view of the coming
impregnation as the ova in the female ovary. In this maturing of the sperm each
of the original seed-cells divides by double segmentation into four
daughter-cells, each furnished with a fourth of the original nuclear matter
(the hereditary chromatin); and each of these four descendant cells becomes a
spermatozoon, ready for impregnation. Thus is prevented the doubling of
the chromatin in the coalescence of the two nuclei at conception. As the two
polar cells are extruded and lost, and have no further part in the
fertilisation of the ovum, we need not discuss them any further. But we must
give more attention to the female pro-nucleus which alone remains after the
extrusion of the polar cells and the dissolving of the germinal vesicle (Fig.
23 e k). This tiny round corpuscle of chromatin now acts as a centre of
attraction for the invading spermatozoon in the large ripe ovum, and coalesces
with its “head,” the male pro-nucleus. The product of this
blending, which is the most important part of the act of impregnation, is the
stem-nucleus, or the first segmentation nucleus (archicaryon)—that
is to say, the nucleus of the new-born embryonic stem-cell or “first
segmentation cell.” This stem-cell is the starting point of the
subsequent embryonic processes.

Hertwig has shown that the tiny transparent ova of the echinoderms are the most
convenient for following the details of this important process of impregnation.
We can, in this case, easily and successfully accomplish artificial
impregnation, and follow the formation of the stem-cell step by step within the
space of ten minutes. If we put ripe ova of the star-fish or sea-urchin in a
watch glass with sea-water and add a drop of ripe sperm-fluid, we find each
ovum impregnated within five minutes. Thousands of the fine, mobile ciliated
cells, which we have described as “sperm-threads” (Fig. 20), make
their way to the ova, owing to a sort of chemical sensitive action which may be
called “smell.” But only one of these innumerable spermatozoa is
chosen—namely, the one that first reaches the ovum by the serpentine
motions of its tail, and touches the ovum with its head. At the spot where the
point of its head touches the surface of the ovum the protoplasm of the latter
is raised in the form of a small wart, the “impregnation rise”
(Fig. 25 A). The spermatozoon then bores its way into this with its
head, the tail outside wriggling about all the time (Fig. 25 B, C).
Presently the tail also disappears within the ovum. At the same time the ovum
secretes a thin external yelk-membrane (Fig. 25 C), starting from the
point of impregnation; and this prevents any more spermatozoa from entering.

Fig. 24—An impregnated echinoderm ovum, with
small homogeneous nucleus (e k).
(From Hertwig.)

Inside the impregnated ovum we now see a rapid series of most important
changes. The pear-shaped head of the sperm-cell, or the “head of the
spermatozoon,” grows larger and rounder, and is converted into the male
pro-nucleus (Fig. 26 s k). This has an attractive influence on the fine
granules or particles which are distributed in the protoplasm of the ovum; they
arrange themselves in lines in the figure of a star. But the attraction or the
“affinity” between the two nuclei is even stronger. They move
towards each other inside the yelk with increasing speed, the male (Fig. 27
s k) going more quickly than the female nucleus (e k). The tiny
male nucleus takes with it the radiating mantle which spreads like a star about
it. At last the two sexual nuclei touch (usually in the centre of the globular
ovum), lie close together, are flattened at the points of contact, and coalesce
into a common mass. The small central particle of

nuclein which is formed from this combination of the nuclei is the
stem-nucleus, or the first segmentation nucleus; the new-formed cell, the
product of the impregnation, is our stem-cell, or “first segmentation
sphere” (Fig. 2).

Fig. 25—Impregnation of the ovum of a
star-fish. (From Hertwig.) Only a small part of the surface of the
ovum is shown. One of the numerous spermatozoa approaches the
“impregnation rise” (A), touches it (B), and then
penetrates into the protoplasm of the ovum (C).

Hence the one essential point in the process of sexual reproduction or
impregnation is the formation of a new cell, the stem-cell, by the combination
of two originally different cells, the female ovum and the male spermatozoon.
This process is of the highest importance, and merits our closest attention;
all that happens in the later development of this first cell and in the life of
the organism that comes of it is determined from the first by the chemical and
morphological composition of the stem-cell, its nucleus and its body. We must,
therefore, make a very careful study of the rise and structure of the
stem-cell.

The first question that arises is as to the two different active elements, the
nucleus and the protoplasm, in the actual coalescence. It is obvious that the
nucleus plays the more important part in this. Hence Hertwig puts his theory of
conception in the principle: “Conception consists in the copulation of
two cell-nuclei, which come from a male and a female cell.” And as the
phenomenon of heredity is inseparably connected with the reproductive process,
we may further conclude that these two copulating nuclei “convey the
characteristics which are transmitted from parents to offspring.” In this
sense I had in 1866 (in the ninth chapter of the General Morphology)
ascribed to the reproductive nucleus the function of generation and
heredity, and to the nutritive protoplasm the duties of nutrition and
adaptation. As, moreover, there is a complete coalescence of the
mutually attracted nuclear substances in conception, and the new nucleus formed
(the stem-nucleus) is the real starting-point for the development of the fresh
organism, the further conclusion may be drawn that the male nucleus conveys to
the child the qualities of the father, and the female nucleus the features of
the mother. We must not forget, however, that the protoplasmic bodies of the
copulating cells also fuse together in the act of impregnation; the cell-body
of the invading spermatozoon (the trunk and tail of the male ciliated cell) is
dissolved in the yelk of the female ovum. This coalescence is not so important
as that of the nuclei, but it must not be overlooked; and, though this process
is not so well known to us, we see clearly at least the formation of the
star-like figure (the radial arrangement of the particles in the plasma) in it
(Figs. 26–27).

The older theories of impregnation generally went astray in regarding the large
ovum as the sole base of the new organism, and only ascribed to the
spermatozoon the work of stimulating and originating its development. The
stimulus which it gave to the ovum was sometimes thought to be purely chemical,
at other times rather physical (on the principle of transferred movement), or
again a mystic and transcendental process. This error was partly due to the
imperfect knowledge at that time of the facts of impregnation, and partly to
the striking

difference in the sizes of the two sexual cells. Most of the earlier observers
thought that the spermatozoon did not penetrate into the ovum. And even when
this had been demonstrated, the spermatozoon was believed to disappear in the
ovum without leaving a trace. However, the splendid research made in the last
three decades with the finer technical methods of our time has completely
exposed the error of this. It has been shown that the tiny sperm-cell is not
subordinated to, but coordinated with, the large ovum. The nuclei of the
two cells, as the vehicles of the hereditary features of the parents, are of
equal physiological importance. In some cases we have succeeded in proving that
the mass of the active nuclear substance which combines in the copulation of
the two sexual nuclei is originally the same for both.

These morphological facts are in perfect harmony with the familiar
physiological truth that the child inherits from both parents, and that on the
average they are equally distributed. I say “on the average,”
because it is well known that a child may have a greater likeness to the father
or to the mother; that goes without saying, as far as the primary sexual
characters (the sexual glands) are concerned. But it is also possible that the
determination of the latter—the weighty determination whether the child
is to be a boy or a girl—depends on a slight qualitative or quantitative
difference in the nuclein or the coloured nuclear matter which comes from both
parents in the act of conception.

Figs. 26 and 27.—Impregnation of the ovum of the
sea-urchin. (From Hertwig.) In Fig. 26 the little sperm-nucleus
(sk) moves towards the larger nucleus of the ovum (ek). In Fig.
27 they nearly touch, and are surrounded by the radiating mantle of
protoplasm.

The striking differences of the respective sexual cells in size and shape,
which occasioned the erroneous views of earlier scientists, are easily
explained on the principle of division of labour. The inert, motionless ovum
grows in size according to the quantity of provision it stores up in the form
of nutritive yelk for the development of the germ. The active swimming
sperm-cell is reduced in size in proportion to its need to seek the ovum and
bore its way into its yelk. These differences are very conspicuous in the
higher animals, but they are much less in the lower animals. In those protists
(unicellular plants and animals) which have the first rudiments of sexual
reproduction the two copulating cells are at first quite equal. In these cases
the act of impregnation is nothing more than a sudden growth, in which
the originally simple cell doubles its volume, and is thus prepared for
reproduction (cell-division). Afterwards slight differences are seen in the
size of the copulating cells; though the smaller ones still have the same shape
as the larger ones. It is only when the difference in size is very pronounced
that a notable difference in shape is found: the sprightly sperm-cell changes
more in shape and the ovum in size.

Quite in harmony with this new conception of the equivalence of the two
gonads, or the equal physiological importance of the male and female
sex-cells and their equal share in the process of heredity, is the important
fact established by Hertwig (1875), that in normal impregnation only one single
spermatozoon

copulates with one ovum; the membrane which is raised on the surface of the
yelk immediately after one sperm-cell has penetrated (Fig. 25 C)
prevents any others from entering. All the rivals of the fortunate penetrator
are excluded, and die without. But if the ovum passes into a morbid state, if
it is made stiff by a lowering of its temperature or stupefied with narcotics
(chloroform, morphia, nicotine, etc.), two or more spermatozoa may penetrate
into its yelk-body. We then witness polyspermism. The more Hertwig
chloroformed the ovum, the more spermatozoa were able to bore their way into
its unconscious body.

Fig. 28—Stem-cell of a rabbit, magnified. In
the centre of the granular protoplasm of the fertilised ovum (d) is seen
the little, bright stem-nucleus, z is the ovolemma, with a mucous
membrane (h). s are dead spermatozoa.

These remarkable facts of impregnation are also of the greatest
interest in psychology, especially as regards the theory of the cell-soul,
which I consider to be its chief foundation. The phenomena we have described
can only be understood and explained by ascribing a certain lower degree of
psychic activity to the sexual principles. They feel each other’s
proximity, and are drawn together by a sensitive impulse (probably
related to smell); they move towards each other, and do not rest until
they fuse together. Physiologists may say that it is only a question of a
peculiar physico-chemical phenomenon, and not a psychic action; but the two
cannot be separated. Even the psychic functions, in the strict sense of the
word, are only complex physical processes, or “psycho-physical”
phenomena, which are determined in all cases exclusively by the chemical
composition of their material substratum.

The monistic view of the matter becomes clear enough when we remember the
radical importance of impregnation as regards heredity. It is well known that
not only the most delicate bodily structures, but also the subtlest traits of
mind, are transmitted from the parents to the children. In this the chromatic
matter of the male nucleus is just as important a vehicle as the large
caryoplasmic substance of the female nucleus; the one transmits the mental
features of the father, and the other those of the mother. The blending of the
two parental nuclei determines the individual psychic character of the child.

But there is another important psychological question—the most important
of all—that has been definitely answered by the recent discoveries in
connection with conception. This is the question of the immortality of the
soul. No fact throws more light on it and refutes it more convincingly than the
elementary process of conception that we have described. For this copulation of
the two sexual nuclei (Figs. 26 and 27) indicates the precise moment at which
the individual begins to exist. All the bodily and mental features of the
new-born child are the sum-total of the hereditary qualities which it has
received in reproduction from parents and ancestors. All that man acquires
afterwards in life by the exercise of his organs, the influence of his
environment, and education—in a word, by adaptation—cannot
obliterate that general outline of his being which he inherited from his
parents. But this hereditary disposition, the essence of every human soul, is
not “eternal,” but “temporal”; it comes into being only
at the moment when the sperm-nucleus of the father and the nucleus of the
maternal ovum meet and fuse together. It is clearly irrational to assume an
“eternal life without end” for an individual phenomenon, the
commencement of which we can indicate to a moment by direct visual observation.

The great importance of the process of impregnation in answering such questions
is quite clear. It is true that conception has never been studied
microscopically in all its details in the human case—notwithstanding its
occurrence at every moment—for reasons that are

obvious enough. However, the two cells which need consideration, the female
ovum and the male spermatozoon, proceed in the case of man in just the same way
as in all the other mammals; the human fœtus or embryo which results from
copulation has the same form as with the other animals. Hence, no scientist who
is acquainted with the facts doubts that the processes of impregnation are just
the same in man as in the other animals.

The stem-cell which is produced, and with which every man begins his career,
cannot be distinguished in appearance from those of other mammals, such as the
rabbit (Fig. 28). In the case of man, also, this stem-cell differs materially
from the original ovum, both in regard to form (morphologically), in regard to
material composition (chemically), and in regard to vital properties
(physiologically). It comes partly from the father and partly from the mother.
Hence it is not surprising that the child who is developed from it inherits
from both parents. The vital movements of each of these cells form a sum of
mechanical processes which in the last analysis are due to movements of the
smallest vital parts, or the molecules, of the living substance. If we agree to
call this active substance plasson, and its molecules
plastidules, we may say that the individual physiological character of each
of these cells is due to its molecular plastidule-movement. Hence, the
plastidule-movement of the cytula is the resultant of the combined
plastidule-movements of the female ovum and the male
sperm-cell.^[15]

[15]
The plasson of the stem-cell or cytula may, from the anatomical point of view,
be regarded as homogeneous and structureless, like that of the monera. This is
not inconsistent with our hypothetical ascription to the plastidules (or
molecules of the plasson) of a complex molecular structure. The complexity of
this is the greater in proportion to the complexity of the organism that is
developed from it and the length of the chain of its ancestry, or to the
multitude of antecedent processes of heredity and adaptation.

Chapter VIII.
THE GASTRÆA THEORY

There is a substantial agreement throughout the animal world in the first
changes which follow the impregnation of the ovum and the formation of the
stem-cell; they begin in all cases with the segmentation of the ovum and the
formation of the germinal layers. The only exception is found in the protozoa,
the very lowest and simplest forms of animal life; these remain unicellular
throughout life. To this group belong the amœbae, gregarinæ, rhizopods,
infusoria, etc. As their whole organism consists of a single cell, they can
never form germinal layers, or definite strata of cells. But all the other
animals—all the tissue-forming animals, or metazoa, as we call
them, in contradistinction to the protozoa—construct real germinal layers
by the repeated cleavage of the impregnated ovum. This we find in the lower
cnidaria and worms, as well as in the more highly-developed molluscs,
echinoderms, articulates, and vertebrates.

In all these metazoa, or multicellular animals, the chief embryonic processes
are substantially alike, although they often seem to a superficial observer to
differ considerably. The stem-cell that proceeds from the impregnated ovum
always passes by repeated cleavage into a number of simple cells. These cells
are all direct descendants of the stem-cell, and are, for reasons we shall see
presently, called segmentation-cells. The repeated cleavage of the stem-cell,
which gives rise to these segmentation-spheres, has long been known as
“segmentation.” Sooner or later the segmentation-cells join
together to form a round (at first, globular) embryonic sphere
(blastula); they then form into two very different groups, and arrange
themselves

in two separate strata—the two primary germinal layers. These
enclose a digestive cavity, the primitive gut, with an opening, the primitive
mouth. We give the name of the gastrula to the important embryonic form
that has these primitive organs, and the name of gastrulation to the
formation of it. This ontogenetic process has a very great significance, and is
the real starting-point of the construction of the multicellular animal body.

The fundamental embryonic processes of the cleavage of the ovum and the
formation of the germinal layers have been very thoroughly studied in the last
thirty years, and their real significance has been appreciated. They present a
striking variety in the different groups, and it was no light task to prove
their essential identity in the whole animal world. But since I formulated the
gastræa theory in 1872, and afterwards (1875) reduced all the various forms of
segmentation and gastrulation to one fundamental type, their identity may be
said to have been established. We have thus mastered the law of unity which
governs the first embryonic processes in all the animals.

Man is like all the other higher animals, especially the apes, in regard to
these earliest and most important processes. As the human embryo does not
essentially differ, even at a much later stage of development—when we
already perceive the cerebral vesicles, the eyes, ears, gill-arches,
etc.—from the similar forms of the other higher mammals, we may
confidently assume that they agree in the earliest embryonic processes,
segmentation and the formation of germinal layers. This has not yet, it is
true, been established by observation. We have never yet had occasion to
dissect a woman immediately after impregnation and examine the stem-cell or the
segmentation-cells in her oviduct. However, as the earliest human embryos we
have examined, and the later and more developed forms, agree with those of the
rabbit, dog, and other higher mammals, no reasonable man will doubt but that
the segmentation and formation of layers are the same in both cases.

But the special form of segmentation and layer formation which we find in the
mammal is by no means the original, simple, palingenetic form. It has been much
modified and cenogenetically altered by a very complex adaptation to embryonic
conditions. We cannot, therefore, understand it altogether in itself. In order
to do this, we have to make a comparative study of segmentation and
layer-formation in the animal world; and we have especially to seek the
original, palingenetic form from which the modified cenogenetic
(see p. 4) form has gradually been developed.

This original unaltered form of segmentation and layer-formation is found
to-day in only one case in the vertebrate-stem to which man belongs—the
lowest and oldest member of the stem, the wonderful lancelet or amphioxus (cf.
Chapters XVI and XVII). But we find a precisely similar palingenetic form of
embryonic development in the case of many of the invertebrate animals, as, for
instance, the remarkable ascidia, the pond-snail (Limnæus), and
arrow-worm (Sagitta), and many of the echinoderms and cnidaria, such as
the common star-fish and sea-urchin, many of the medusæ and corals, and the
simpler sponges (Olynthus). We may take as an illustration the
palingenetic segmentation and germinal layer-formation in an eight-fold insular
coral, which I discovered in the Red Sea, and described as Monoxenia
Darwinii.

The impregnated ovum of this coral (Fig. 29 A, B) first splits into two equal
cells (C). First, the nucleus of the stem-cell and its central body divide into
two halves. These recede from and repel each other, and act as centres of
attraction on the surrounding protoplasm; in consequence of this, the
protoplasm is constricted by a circular furrow, and, in turn, divides into two
halves. Each of the two segmentation-cells thus produced splits in the same way
into two equal cells. The four segmentation-cells (grand-daughters of the
stem-cell) lie in one plane. Now, however, each of them subdivides into two
equal halves, the cleavage of the nucleus again preceding that of the
surrounding protoplasm. The eight cells which thus arise break into sixteen,
these into thirty-two, and then (each being constantly halved) into sixty-four,
128, and so on.^[16] The final result of this

repeated cleavage is the formation of a globular cluster of similar
segmentation-cells, which we call the mulberry-formation or morula. The cells
are thickly pressed together like the parts of a mulberry or blackberry, and
this gives a lumpy appearance to the surface of the sphere (Fig.
E).^[17]

[16]
The number of segmentation-cells thus produced increases geometrically in the
original gastrulation, or the purest palingenetic form of cleavage. However, in
different animals the number reaches a different height, so that the morula,
and also the blastula, may consist sometimes of thirty-two, sometimes of
sixty-four, and sometimes of 128, or more, cells.

[17]
The segmentation-cells which make up the morula after the close of the
palingenetic cleavage seem usually to be quite similar, and to present no
differences as to size, form, and composition. That, however, does not prevent
them from differentiating into animal and vegetative cells, even during the
cleavage.

Fig. 29—Gastrulation of a coral (Monoxenia
Darwinii). A, B, stem-cell (cytula) or impregnated ovum. In Figure A
(immediately after impregnation) the nucleus is invisible. In Figure B (a
little later) it is quite clear. C two segmentation-cells. D four
segmentation-cells. E mulberry-formation (morula). F blastosphere (blastula). G
blastula (transverse section). H depula, or hollowed blastula (transverse
section). I gastrula (longitudinal section). K gastrula, or cup-sphere,
external appearance.)

When the cleavage is thus ended, the mulberry-like mass changes into a hollow
globular sphere. Watery fluid or jelly gathers inside the globule; the
segmentation-cells are loosened, and all rise to the surface. There they are
flattened by mutual pressure, and assume the shape of truncated pyramids, and
arrange themselves side by side in one regular layer (Figs. F, G). This layer
of cells is called the germinal membrane (or blastoderm); the homogeneous cells
which compose its simple structure are called blastodermic cells; and the whole
hollow sphere, the walls of which are made of the preceding, is called the
blastula or blastosphere.^[18]

[18]
The blastula of the lower animals must not be confused with the very different
blastula of the mammal, which is properly called the gastrocystis or
blastocystis. This cenogenetic gastrocystis and the
palingenetic blastula are sometimes very wrongly comprised under the
common name of blastula or vesicula blastodermica.

In the case of our coral, and of many other lower forms of animal life, the
young embryo begins at once to move independently and swim about in the water.
A fine, long, thread-like process, a sort of whip or lash, grows out of each
blastodermic cell, and this independently executes vibratory movements, slow at
first, but quicker after a time (Fig. F). In this way each blastodermic cell
becomes a ciliated cell. The combined force of all these vibrating lashes
causes the whole blastula to move about in a rotatory fashion. In many other
animals, especially those in which the embryo develops within enclosed
membranes, the ciliated cells are only formed at a later stage, or even not
formed at all. The blastosphere may grow and expand by the blastodermic cells
(at the surface of the sphere) dividing and increasing, and more fluid is
secreted in the internal cavity. There are still to-day some organisms that
remain throughout life at the structural stage of the blastula—hollow
vesicles that swim about by a ciliary movement in the water, the wall of which
is composed of a single layer of cells, such as the volvox, the magosphæra,
synura, etc. We shall speak further of the great phylogenetic significance of
this fact in Chapter XIX.

A very important and remarkable process now follows—namely, the curving
or invagination of the blastula (Fig. H). The vesicle with a single layer of
cells for wall is converted into a cup with a wall of two layers of cells (cf.
Figs. G, H, I). A certain spot at the surface of the sphere is flattened, and
then bent inward. This depression sinks deeper and deeper, growing at the cost
of the internal cavity. The latter decreases as the hollow deepens. At last the
internal cavity disappears altogether, the inner side of the blastoderm (that
which lines the depression) coming to lie close on the outer side. At the same
time, the cells of the two sections assume different sizes and shapes; the
inner cells are more round and the outer more oval (Fig. I). In this way the
embryo takes the form of a cup or jar-shaped body, with a wall made up of two
layers of cells, the inner cavity of which opens to the outside at one end (the
spot where the depression was originally formed). We call this very important
and interesting embryonic form the “cup-embryo” or
“cup-larva” (gastrula, Fig. 29, I longitudinal section, K
external view). I have in my Natural History of Creation given the name
of depula to the remarkable intermediate form which appears at the
passage of the blastula into the gastrula. In this intermediate stage there are
two cavities in the embryo—the original cavity (blastocœl) which
is disappearing, and the primitive gut-cavity (progaster) which is
forming.

I regard the gastrula as the most important and significant embryonic form in
the animal world. In all real animals (that is, excluding the unicellular
protists) the segmentation of the ovum produces either a pure, primitive,
palingenetic gastrula (Fig. 29 I, K) or an equally instructive cenogenetic
form, which has been developed in time from the first, and can be directly
reduced to it. It is certainly a fact of the greatest interest and
instructiveness that animals of the most different stems—vertebrates and
tunicates, molluscs and articulates, echinoderms and annelids, cnidaria and
sponges—proceed from one and the same embryonic form. In illustration I
give a few

pure gastrula forms from various groups of animals (Figs. 30–35,
explanation given below each).

Fig. 30 (A)—Gastrula of a very simple
primitive-gut animal or gastræad (gastrophysema).
(Haeckel.)
Fig. 31 (B)—Gastrula of a worm
(Sagitta). (From Kowalevsky.)
Fig. 32
(C)—Gastrula of an echinoderm (star-fish, Uraster),
not completely folded in (depula). (From Alexander Agassiz.)
Fig.
33 (D)—Gastrula of an arthropod (primitive crab,
Nauplius) (as 32).
Fig. 34 (E)—Gastrula of a
mollusc (pond-snail, Linnæus). (From Karl Rabl.)
Fig. 35
(F)—Gastrula of a vertebrate (lancelet, Amphioxus).
(From Kowalevsky.) (Front view.)
In each figure d is the
primitive-gut cavity, o primitive mouth,
s
segmentation-cavity, i entoderm (gut-layer), e ectoderm (skin
layer).

In view of this extraordinary significance of the gastrula, we must make a very
careful study of its original structure. As a rule, the typical gastrula is
very small, being invisible to the naked eye, or at the most only visible as a
fine point under very favourable conditions, and measuring generally 1/500 to
1/250 of an inch (less frequently 1/50 inch, or even more) in diameter. In
shape it is usually like a roundish drinking-cup. Sometimes it is rather oval,
at other times more ellipsoid or spindle-shaped; in some cases it is half
round, or even almost round, and in others lengthened out, or almost
cylindrical.

I give the name of primitive gut (progaster) and primitive mouth
(prostoma) to the internal cavity of the gastrula-body and its opening;
because this cavity is the first rudiment of the digestive cavity of the
organism, and the opening originally served to take food into it. Naturally,
the primitive gut and mouth change very considerably afterwards in the various
classes of animals. In most of the cnidaria and many of the annelids (worm-like
animals) they remain unchanged throughout life. But in most of the

higher animals, and so in the vertebrates, only the larger central part of the
later alimentary canal develops from the primitive gut; the later mouth is a
fresh development, the primitive mouth disappearing or changing into the anus.
We must therefore distinguish carefully between the primitive gut and mouth of
the gastrula and the later alimentary canal and mouth of the fully developed
vertebrate.^[19]

[19]
My distinction (1872) between the primitive gut and mouth and the later
permanent stomach (metagaster) and mouth (metastoma) has been
much criticised; but it is as much justified as the distinction between the
primitive kidneys and the permanent kidneys. Professor E. Ray-Lankester
suggested three years afterwards (1875) the name archenteron for the
primitive gut, and blastoporus for the primitive mouth.

Fig. 36—Gastrula of a lower sponge (lynthus). A
external view, B longitudinal section through the axis, g
primitive-gut cavity, a primitive mouth-aperture, i inner cell-layer
(entoderm, endoblast, gut-layer), e external cell-layer (outer germinal
layer, ectoderm, ectoblast, or skin-layer).

The two layers of cells which line the gut-cavity and compose its wall are of
extreme importance. These two layers, which are the sole builders of the whole
organism, are no other than the two primary germinal layers, or the primitive
germ-layers. I have spoken in the introductory section (Chapter III) of their
radical importance. The outer stratum is the skin-layer, or ectoderm
(Figs. 30–35e); the inner stratum is the gut-layer, or
entoderm (i). The former is often also called the ectoblast, or
epiblast, and the latter the endoblast, or hypoblast. From these two primary
germinal layers alone is developed the entire organism of all the metazoa or
multicellular animals. The skin-layer forms the external skin, the
gut-layer forms the internal skin or lining of the body. Between these two
germinal layers are afterwards developed the middle germinal layer
(mesoderma) and the body-cavity (cœloma) filled with blood or
lymph.

The two primary germinal layers were first distinguished by Pander in 1817 in
the incubated chick. Twenty years later (1849) Huxley pointed out that in many
of the lower zoophytes, especially the medusæ, the whole body consists
throughout life of these two primary germinal layers. Soon afterwards (1853)
Allman introduced the names which have come into general use; he called the
outer layer the ectoderm (“outer-skin”), and the inner the
entoderm (“inner-skin”). But in 1867 it was shown,
particularly by Kowalevsky, from comparative observation, that even in
invertebrates, also, of the most different classes—annelids, molluscs,
echinoderms, and articulates—the body is developed out of the same two
primary layers. Finally, I discovered them (1872) in the lowest tissue-forming
animals, the sponges, and proved in my gastræa theory that these two layers
must be regarded as identical throughout the animal world, from the sponges and
corals to the insects and vertebrates, including man. This fundamental
“homology

[identity] of the primary germinal layers and the primitive gut” has been
confirmed during the last thirty years by the careful research of many able
observers, and is now pretty generally admitted for the whole of the metazoa.

As a rule, the cells which compose the two primary germinal layers show
appreciable differences even in the gastrula stage. Generally (if not always)
the cells of the skin-layer or ectoderm (Figs. 36 c and 37 e) are
the smaller, more numerous, and clearer; while the cells of the gut-layer, or
entoderm (i), are larger, less numerous, and darker. The protoplasm of
the ectodermic (outer) cells is clearer and firmer than the thicker and softer
cell-matter of the entodermic (inner) cells; the latter are, as a rule, much
richer in yelk-granules (albumen and fatty particles) than the former. Also the
cells of the gut-layer have, as a rule, a stronger affinity for colouring
matter, and take on a tinge in a solution of carmine, aniline, etc., more
quickly and appreciably than the cells of the skin-layer. The nuclei of the
entoderm-cells are usually roundish, while those of the ectoderm-cells are
oval.

When the doubling-process is complete, very striking histological differences
between the cells of the two layers are found (Fig. 37). The tiny, light
ectoderm-cells (e) are sharply distinguished from the larger and darker
entoderm-cells (i). Frequently this differentiation of the cell-forms
sets in at a very early stage, during the segmentation-process, and is already
very appreciable in the blastula.

We have, up to the present, only considered that form of segmentation and
gastrulation which, for many and weighty reasons, we may regard as the
original, primordial, or palingenetic form. We might call it
“equal” or homogeneous segmentation, because the divided cells
retain a resemblance to each other at first (and often until the formation of
the blastoderm). We give the name of the “bell-gastrula,” or
archigastrula, to the gastrula that succeeds it. In just the same form as
in the coral we considered (Monoxenia, Fig. 29), we find it in the
lowest zoophyta (the gastrophysema, Fig. 30), and the simplest sponges
(olynthus, Fig. 36); also in many of the medusæ and hydrapolyps, lower types of
worms of various classes (brachiopod, arrow-worm, Fig. 31), tunicates
(ascidia), many of the echinoderms (Fig. 32), lower articulates (Fig. 33), and
molluscs (Fig. 34), and, finally, in a slightly modified form, in the lowest
vertebrate (the amphioxus, Fig. 35).

Fig. 37—Cells from the two primary germinal
layers of the mammal (from both layers of the blastoderm). i larger
and darker cells of the inner stratum, the vegetal layer or entoderm. e
smaller and clearer cells from the outer stratum, the animal layer or
ectoderm.

The gastrulation of the amphioxus is especially interesting because this lowest
and oldest of all the vertebrates is of the highest significance in connection
with the evolution of the vertebrate stem, and therefore with that of man
(compare Chapters XVI and XVII). Just as the comparative anatomist traces the
most elaborate features in the structures of the various classes of vertebrates
to divergent development from this simple primitive vertebrate, so comparative
embryology traces the various secondary forms of vertebrate gastrulation to the
simple, primary formation of the germinal layers in the amphioxus. Although
this formation, as distinguished from the cenogenetic modifications of the
vertebrate, may on the whole be regarded as palingenetic, it is nevertheless
different in some features from the quite primitive gastrulation such as we
have, for instance, in the Monoxenia (Fig. 29) and the Sagitta.
Hatschek rightly observes that the segmentation of the ovum in the amphioxus is
not strictly equal, but almost equal, and approaches the unequal. The
difference in size between the two groups of cells continues to be very
noticeable in the further course of the segmentation; the smaller animal cells
of the upper hemisphere divide more quickly than the larger vegetal cells of
the lower (Fig. 38 A, B). Hence the blastoderm, which forms the
single-layer wall of the globular blastula at the end of the cleavage-process,
does not consist of

homogeneous cells of equal size, as in the Sagitta and the Monoxenia; the cells
of the upper half of the blastoderm (the mother-cells of the ectoderm) are more
numerous and smaller, and the cells of the lower half (the mother-cells of the
entoderm) less numerous and larger. Moreover, the segmentation-cavity of the
blastula (Fig. 38 C, h) is not quite globular, but forms a flattened
spheroid with unequal poles of its vertical axis. While the blastula is being
folded into a cup at the vegetal pole of its axis, the difference in the size
of the blastodermic cells increases (Fig. 38 D, E); it is most
conspicuous when the invagination is complete and the segmentation-cavity has
disappeared (Fig. 38 F). The larger vegetal cells of the entoderm are
richer in granules, and so darker than the smaller and lighter animal cells of
the ectoderm.

Fig. 38—Gastrulation of the amphioxus, from
Hatschek (vertical section through the axis of the ovum). A, B,
C three stages in the formation of the blastula; D, E curving of the
blastula; F complete gastrula. h segmentation-cavity. g
primitive gut-cavity.

But the unequal gastrulation of the amphioxus diverges from the typical equal
cleavage of the Sagitta, the Monoxenia (Fig. 29), and the
Olynthus (Fig. 36), in another important particular. The pure
archigastrula of the latter forms is uni-axial, and it is round in its whole
length in transverse section. The vegetal pole of the vertical axis is just in
the centre of the primitive mouth. This is not the case in the gastrula of the
amphioxus. During the folding of the blastula the ideal axis is already bent on
one side, the growth of the blastoderm (or the increase of its cells) being
brisker on one side than on the other; the side that grows more quickly, and so
is more curved (Fig. 39 v), will be the anterior or belly-side, the
opposite, flatter side will form the back (d). The primitive mouth,
which at first, in the typical archigastrula, lay at the vegetal pole of the
main axis, is forced away to the dorsal side; and whereas its two lips lay at
first in a plane at right angles to the chief axis, they are now so far thrust
aside that their plane cuts the axis at a sharp angle. The dorsal lip is
therefore the upper and more forward, the ventral lip the lower and hinder. In
the latter, at the ventral passage of the entoderm into the ectoderm, there lie
side by side a pair of very large cells, one to the right and one to the left
(Fig. 39 p): these are the important polar cells of the primitive mouth,
or “the primitive cells of the mesoderm.” In consequence of these
considerable variations arising in the course of the gastrulation, the
primitive uni-axial form of the archigastrula in the amphioxus has already
become tri-axial, and thus the two-sidedness, or bilateral symmetry, of the
vertebrate body has already been determined. This has been transmitted from the
amphioxus to all the other modified gastrula-forms of the vertebrate stem.

Apart from this bilateral structure, the gastrula of the amphioxus resembles
the typical archigastrula of the lower animals (Figs. 30–36) in
developing the two primary germinal layers from a single layer of cells. This
is clearly the oldest and original form of the metazoic embryo. Although the
animals I have mentioned belong to the most diverse classes, they nevertheless
agree with each other, and many more animal forms, in having retained to the
present day, by a conservative heredity, this palingenetic form of gastrulation
which they have from their

earliest common ancestors. But this is not the case with the great majority of
the animals. With these the original embryonic process has been gradually more
or less altered in the course of millions of years by adaptation to new
conditions of development. Both the segmentation of the ovum and the subsequent
gastrulation have in this way been considerably changed. In fact, these
variations have become so great in the course of time that the segmentation was
not rightly understood in most animals, and the gastrula was unrecognised. It
was not until I had made an extensive comparative study, lasting a considerable
time (in the years 1866–75), in animals of the most diverse classes, that
I succeeded in showing the same common typical process in these apparently very
different forms of gastrulation, and tracing them all to one original form. I
regard all those that diverge from the primary palingenetic gastrulation as
secondary, modified, and cenogenetic. The more or less divergent form of
gastrula that is produced may be called a secondary, modified gastrula, or a
metagastrula. The reader will find a scheme of these different kinds of
segmentation and gastrulation at the close of this chapter.

By far the most important process that determines the various cenogenetic forms
of gastrulation is the change in the nutrition of the ovum and the accumulation
in it of nutritive yelk. By this we understand various chemical substances
(chiefly granules of albumin and fat-particles) which serve exclusively as
reserve-matter or food for the embryo. As the metazoic embryo in its earlier
stages of development is not yet able to obtain its food and so build up the
frame, the necessary material has to be stored up in the ovum. Hence we
distinguish in the ova two chief elements—the active formative yelk
(protoplasm) and the passive food-yelk (deutoplasm, wrongly spoken of as
“the yelk”). In the little palingenetic ova, the segmentation of
which we have already considered, the yelk-granules are so small and so
regularly distributed in the protoplasm of the ovum that the even and repeated
cleavage is not affected by them. But in the great majority of the animal ova
the food-yelk is more or less considerable, and is stored in a certain part of
the ovum, so that even in the unfertilised ovum the “granary” can
clearly be distinguished from the formative plasm. As a rule, the
formative-yelk (with the germinal vesicle) then usually gathers at one pole and
the food-yelk at the other. The first is the animal, and the second the
vegetal, pole of the vertical axis of the ovum.

Fig. 39—Gastrula of the amphioxus, seen from
left side (diagrammatic median section). (From Hatschek.) g
primitive gut, u primitive mouth, p peristomal pole-cells,
i entoderm, e ectoderm, d dorsal side, v ventral
side.

In these “telolecithal” ova, or ova with the yelk at
one end (for instance, in the cyclostoma and amphibia), the gastrulation then
usually takes place in such a way that in the cleavage of the impregnated ovum
the animal (usually the upper) half splits up more quickly than the vegetal
(lower). The contractions of the active protoplasm, which effect this continual
cleavage of the cells, meet a greater resistance in the lower vegetal half from
the passive deutoplasm than in the upper animal half. Hence we find in the
latter more but smaller, and in the former fewer but larger, cells. The animal
cells produce the external, and the vegetal cells the internal, germinal layer.

Although this unequal segmentation of the cyclostoma, ganoids, and amphibia
seems at first sight to differ from the original equal segmentation (for
instance, in the monoxenia, Fig. 29), they both have this in common, that the
cleavage process throughout affects the whole cell; hence Remak called
it total segmentation, and the ova in question holoblastic, or
“whole-cleaving.” It is otherwise with the second chief group of
ova, which he distinguished from these as meroblastic, or
“partially-cleaving ”: to this class belong the familiar large eggs
of birds and reptiles, and of most fishes. The inert mass of the passive
food-yelk is so

large in these cases that the protoplasmic contractions of the active yelk
cannot effect any further cleavage. In consequence, there is only a partial
segmentation. While the protoplasm in the animal section of the ovum continues
briskly to divide, multiplying the nuclei, the deutoplasm in the vegetal
section remains more or less undivided; it is merely consumed as food by the
forming cells. The larger the accumulation of food, the more restricted is the
process of segmentation. It may, however, continue for some time (even after
the gastrulation is more or less complete) in the sense that the vegetal
cell-nuclei distributed in the deutoplasm slowly increase by cleavage; as each
of them is surrounded by a small quantity of protoplasm, it may afterwards
appropriate a portion of the food-yelk, and thus form a real
“yelk-cell” (merocyte). When this vegetal cell-formation
continues for a long time, after the two primary germinal layers have been
formed, it takes the name of the “after-segmentation.”

The meroblastic ova are only found in the larger and more highly developed
animals, and only in those whose embryo needs a longer time and richer
nourishment within the fœtal membranes. According as the yelk-food accumulates
at the centre or at the side of the ovum, we distinguish two groups of dividing
ova, periblastic and discoblastic. In the periblastic the food-yelk is in the
centre, enclosed inside the ovum (hence they are also called
“centrolecithal” ova): the formative yelk surrounds the food-yelk,
and so suffers itself a superficial cleavage. This is found among the
articulates (crabs, spiders, insects, etc.). In the discoblastic ova the
food-yelk gathers at one side, at the vegetal or lower pole of the vertical
axis, while the nucleus of the ovum and the great bulk of the formative yelk
lie at the upper or animal pole (hence these ova are also called
“telolecithal”). In these cases the cleavage of the ovum begins at
the upper pole, and leads to the formation of a dorsal discoid embryo. This is
the case with all meroblastic vertebrates, most fishes, the reptiles and birds,
and the oviparous mammals (the monotremes).

The gastrulation of the discoblastic ova, which chiefly concerns us, offers
serious difficulties to microscopic investigation and philosophic
consideration. These, however, have been mastered by the comparative
embryological research which has been conducted by a number of distinguished
observers during the last few decades—especially the brothers Hertwig,
Rabl, Kupffer, Selenka, Rückert, Goette, Rauber, etc. These thorough and
careful studies, aided by the most perfect modern improvements in technical
method (in tinting and dissection), have given a very welcome support to the
views which I put forward in my work, On the Gastrula and the Segmentation
of the Animal Ovum [not translated], in 1875. As it is very important to
understand these views and their phylogenetic foundation clearly, not only as
regards evolution in general, but particularly in connection with the genesis
of man, I will give here a brief statement of them as far as they concern the
vertebrate-stem:—

1. All the vertebrates, including man, are phylogenetically (or genealogically)
related—that is, are members of one single natural stem.

2. Consequently, the embryonic features in their individual development must
also have a genetic connection.

3. As the gastrulation of the amphioxus shows the original palingenetic form in
its simplest features, that of the other vertebrates must have been derived
from it.

4. The cenogenetic modifications of the latter are more appreciable the more
food-yelk is stored up in the ovum.

5. Although the mass of the food-yelk may be very large in the ova of the
discoblastic vertebrates, nevertheless in every case a blastula is developed
from the morula, as in the holoblastic ova.

6. Also, in every case, the gastrula develops from the blastula by curving or
invagination.

7. The cavity which is produced in the fœtus by this curving is, in each case,
the primitive gut (progaster), and its opening the primitive mouth (prostoma).

8. The food-yelk, whether large or small, is always stored in the ventral wall
of the primitive gut; the cells (called “merocytes”) which may be
formed in it subsequently (by “after-segmentation”) also belong to
the inner germinal layer, like the cells which immediately enclose the
primitive gut-cavity.

9. The primitive mouth, which at first lies below at the lower pole of the
vertical axis, is forced, by the growth of the yelk, backwards and then
upwards,

towards the dorsal side of the embryo; the vertical axis of the primitive gut
is thus gradually converted into horizontal.

10. The primitive mouth is closed sooner or later in all the vertebrates, and
does not evolve into the permanent mouth-aperture; it rather corresponds to the
“properistoma,” or region of the anus. From this important point
the formation of the middle germinal layer proceeds, between the two primary
layers.

The wide comparative studies of the scientists I have named have further shown
that in the case of the discoblastic higher vertebrates (the three classes of
amniotes) the primitive mouth of the embryonic disc, which was long looked for
in vain, is found always, and is nothing else than the familiar
“primitive groove.” Of this we shall see more as we proceed.
Meantime we realise that gastrulation may be reduced to one and the same
process in all the vertebrates. Moreover, the various forms it takes in the
invertebrates can always be reduced to one of the four types of segmentation
described above. In relation to the distinction between total and partial
segmentation, the grouping of the various forms is as follows:—

The lowest metazoa we know—namely, the lower zoophyta (sponges, simple
polyps, etc.)—remain throughout life at a stage of development which
differs little from the gastrula; their whole body consists of two layers of
cells. This is a fact of extreme importance. We see that man, and also other
vertebrates, pass quickly through a stage of development in which they consist
of two layers, just as these lower zoophyta do throughout life. If we apply our
biogenetic law to the matter, we at once reach this important conclusion.
“Man and all the other animals which pass through the two-layer stage, or
gastrula-form, in the course of their embryonic development, must descend from
a primitive simple stem-form, the whole body of which consisted throughout life
(as is the case with the lower zoophyta to-day) merely of two cell-strata or
germinal layers.” We will call this primitive stem-form, with which we
shall deal more fully later on, the gastræa—that is to say,
“primitive-gut animal.”

According to this gastræa-theory there was originally in all the multicellular
animals one organ with the same structure and function. This was the
primitive gut; and the two primary germinal layers which form its wall must
also be regarded as identical in all. This important homology or identity of
the primary germinal layers is proved, on the one hand, from the fact that the
gastrula was originally formed in the same way in all cases—namely, by
the curving of the blastula; and, on the other hand, by the fact that in every
case the same fundamental organs arise from the germinal layers. The outer or
animal layer, or ectoderm, always forms the chief organs of animal
life—the skin, nervous system, sense-organs, etc.; the inner or vegetal
layer, or entoderm, gives rise to the chief organs of vegetative life—the
organs of nourishment, digestion, blood-formation, etc.

In the lower zoophyta, whose body remains at the two-layer stage throughout
life, the gastræads, the simplest sponges (Olynthus), and polyps
(Hydra), these two groups of functions, animal and vegetative, are
strictly divided between the two simple primary layers. Throughout life the
outer or animal layer acts simply as a covering for the body, and accomplishes
its movement and sensation. The inner or vegetative layer of cells acts
throughout life as a gut-lining, or nutritive layer of enteric cells, and often
also yields the reproductive cells.

The best known of these “gastræads,” or “gastrula-like
animals,” is the common fresh-water polyp (Hydra). This simplest
of all the cnidaria has, it is true, a crown of tentacles round its mouth. Also
its outer germinal layer has certain special modifications. But these are
secondary additions, and the inner germinal layer is a simple stratum of cells.
On the whole, the hydra has preserved to our day by heredity the simple
structure of our primitive ancestor, the gastræa (cf. Chapter XIX).

In all other
animals, particularly the vertebrates, the gastrula is merely a brief
transitional stage. Here the two-layer stage of the embryonic development is
quickly succeeded by a three-layer, and then a four-layer, stage. With the
appearance of the four superimposed germinal layers we reach again a firm and
steady standing-ground, from which we may follow the further, and much more
difficult and complicated, course of embryonic development.

SUMMARY OF THE CHIEF DIFFERENCES IN THE OVUM-SEGMENTATION AND GASTRULATION OF
ANIMALS.
The animal stems are indicated by the
letters a–g: a Zoophyta. b Annelida.
c
Mollusca. d Echinoderma. e Articulata. f Tunicata.
g Vertebrata.

Chapter IX.
THE GASTRULATION OF THE VERTEBRATE^[20]

[20]
Cf. Balfour’s Manual of Comparative Embryology, vol. ii; Theodore
Morgan’s The Development of the Frog’s Egg.

The remarkable processes of gastrulation, ovum-segmentation, and formation of
germinal layers present a most conspicuous variety. There is to-day only the
lowest of the vertebrates, the amphioxus, that exhibits the original form of
those processes, or the palingenetic gastrulation which we have considered in
the preceding chapter, and which culminates in the formation of the
archigastrula (Fig. 38). In all other extant vertebrates these fundamental
processes have been more or less modified by adaptation to the conditions of
embryonic development (especially by changes in the food-yelk); they exhibit
various cenogenetic types of the formation of germinal layers. However, the
different classes vary considerably from each other. In order to grasp the
unity that underlies the manifold differences in these phenomena and their
historical connection, it is necessary to bear in mind always the unity of the
vertebrate-stem. This “phylogenetic unity,” which I developed in my
General Morphology in 1866, is now generally admitted. All impartial
zoologists agree to-day that all the vertebrates, from the amphioxus and the
fishes to the ape and man, descend from a common ancestor, “the primitive
vertebrate.” Hence the embryonic processes, by which each individual
vertebrate is developed, must also be capable of being reduced to one common
type of embryonic development; and this primitive type is most certainly
exhibited to-day by the amphioxus.

It must, therefore, be our next task to make a comparative study of the various
forms of vertebrate gastrulation, and trace them backwards to that of the
lancelet. Broadly speaking, they fall first into two groups: the older
cyclostoma, the earliest fishes, most of the amphibia, and the viviparous
mammals, have holoblastic ova—that is to say, ova with total,
unequal segmentation; while the younger cyclostoma, most of the fishes, the
cephalopods, reptiles, birds, and monotremes, have meroblastic ova, or
ova with partial discoid segmentation. A closer study of them shows, however,
that these two groups do not present a natural unity, and that the historical
relations between their several divisions are very complicated. In order to
understand them properly, we must first consider the various modifications of
gastrulation in these classes. We may begin with that of the amphibia.

The most suitable and most available objects of study in this class are the
eggs of our indigenous amphibia, the tailless frogs and toads, and the tailed
salamander. In spring they are to be found in clusters in every pond, and
careful examination of the ova with a lens is sufficient to show at least the
external features of the segmentation. In order to understand the whole process
rightly and follow the formation of the germinal layers and the gastrula, the
ova of the frog and salamander must be carefully hardened; then the thinnest
possible sections must be made of the hardened ova with the microtome, and the
tinted sections must be very closely compared under a powerful microscope.

The ova of the frog or toad are globular in shape, about the twelfth of an inch
in diameter, and are clustered in jelly-like masses, which are lumped together
in the case of the frog, but form long strings in the case of the toad. When we
examine the opaque, grey, brown, or blackish ova closely, we find that the
upper half is darker than the lower. The middle of the upper half is in many
species black, while the middle of the lower half is white.^[21] In this
way we get a definite axis of the ovum with two poles. To give a clear

idea of the segmentation of this ovum, it is best to compare it with a globe,
on the surface of which are marked the various parallels of longitude and
latitude. The superficial dividing lines between the different cells, which
come from the repeated segmentation of the ovum, look like deep furrows on the
surface, and hence the whole process has been given the name of furcation. In
reality, however, this “furcation,” which was formerly regarded as
a very mysterious process, is nothing but the familiar, repeated
cell-segmentation. Hence also the segmentation-cells which result from it are
real cells.

[21]
The colouring of the eggs of the amphibia is caused by the accumulation of
dark-colouring matter at the animal pole of the ovum. In consequence of this,
the animal cells of the ectoderm are darker than the vegetal cells of the
entoderm. We find the reverse of this in the case of most animals, the
protoplasm of the entoderm cells being usually darker and coarser-grained.

Fig. 40—The cleavage of the frog’s ovum
(magnified). A stem-cell. B the first two segmentation-cells. C four cells.
D eight cells (4 animal and 4 vegetative). E twelve cells (8 animal
and 4 vegetative). F sixteen cells (8 animal and 8 vegetative). G
twenty-four cells (16 animal and 8 vegetative). H thirty-two cells.
I forty-eight cells. K sixty-four cells. L ninety-six
cells. M 160 cells (128 animal and 32 vegetative).

The unequal segmentation which we observe in the ovum of the amphibia has the
special feature of beginning at the upper and darker pole (the north pole of
the terrestrial globe in our illustration), and slowly advancing towards the
lower and brighter pole (the south pole). Also the upper and darker hemisphere
remains in this position throughout the course of the segmentation, and its
cells multiply much more briskly. Hence the cells of the lower hemisphere are
found to be larger and less numerous. The cleavage of the stem-cell (Fig. 40
A) begins with the formation of a complete furrow, which starts from the
north pole and reaches to the south (B). An hour later a second furrow
arises in the same way, and this cuts the first at a right angle (Fig. 40
C). The ovum is thus divided into four equal parts. Each of these four
“segmentation cells” has an upper and darker and a lower, brighter
half. A few hours later a third furrow appears, vertically to the first two
(Fig. 40 D). The globular germ now consists of eight cells, four smaller
ones above (northern) and four larger ones below (southern). Next, each of the
four upper ones divides into two halves by a cleavage beginning from the north
pole, so that we now have eight above and four below (Fig. 40 E). Later,
the

four new longitudinal divisions extend gradually to the lower cells, and the
number rises from twelve to sixteen (F). Then a second circular furrow
appears, parallel to the first, and nearer to the north pole, so that we may
compare it to the north polar circle. In this way we get twenty-four
segmentation-cells—sixteen upper, smaller, and darker ones, and eight
smaller and brighter ones below (G). Soon, however, the latter also
sub-divide into sixteen, a third or “meridian of latitude”
appearing, this time in the southern hemisphere: this makes thirty-two cells
altogether (H). Then eight new longitudinal lines are formed at the
north pole, and these proceed to divide, first the darker cells above and
afterwards the lighter southern cells, and finally reach the south pole. In
this way we get in succession forty, forty-eight, fifty-six, and at last
sixty-four cells (I, K). In the meantime, the two hemispheres differ
more and more from each other. Whereas the sluggish lower hemisphere long
remains at thirty-two cells, the lively northern hemisphere briskly sub-divides
twice, producing first sixty-four and then 128 cells (L, M). Thus we
reach a stage in which we count on the surface of the ovum 128 small cells in
the upper half and thirty-two large ones in the lower half, or 160 altogether.
The dissimilarity of the two halves increases: while the northern breaks up
into a great number of small cells, the southern consists of a much smaller
number of larger cells. Finally, the dark cells of the upper half grow almost
over the surface of the ovum, leaving only a small circular spot

at the south pole, where the large and clear cells of the lower half are
visible. This white region at the south pole corresponds, as we shall see
afterwards, to the primitive mouth of the gastrula. The whole mass of the inner
and larger and clearer cells (including the white polar region) belongs to the
entoderm or ventral layer. The outer envelope of dark smaller cells forms the
ectoderm or skin-layer.

Figs. 41–44—Four vertical sections of the
fertilised ovum of the toad, in four successive stages of development. The
letters have the same meaning throughout: F segmentation-cavity.
D covering of same (D dorsal half of the embryo, P ventral
half). P yelk-stopper (white round field at the lower pole). Z
yelk-cells of the entoderm (Remak’s “glandular embryo”).
N primitive gut cavity (progaster or Rusconian alimentary cavity). The
primitive mouth (prostoma) is closed by the yelk-stopper, P. s partition
between the primitive gut cavity (N) and the segmentation cavity
(F). k k′, section of the large circular lip-border of the
primitive mouth (the Rusconian anus). The line of dots between k and
k′ indicates the earlier connection of the yelk-stopper
(P) with the central mass of the yelk-cells (Z). In Fig. 44 the
ovum has turned 90°, so that the back of the embryo is uppermost and the
ventral side down. (From Stricker.).

Fig. 45—Blastula of the water-salamander
(Triton). fh segmentation-cavity, dz yelk-cells, rz
border-zone. (From Hertwig.)

In the meantime, a large cavity, full of fluid, has been formed within the
globular body—the segmentation-cavity or embryonic cavity
(blastocœl, Figs. 41–44 F). It extends considerably as the
cleavage proceeds, and afterwards assumes an almost semi-circular form (Fig. 41
F). The frog-embryo now represents a modified embryonic vesicle or
blastula, with hollow animal half and solid vegetal half.

Now a second, narrower but longer, cavity arises by a process of folding at the
lower pole, and by the falling away from each other of the white entoderm-cells
(Figs. 41–44 N). This is the primitive gut-cavity or the gastric
cavity of the gastrula, progaster or archenteron. It was first observed in the
ovum of the amphibia by Rusconi, and so called the Rusconian cavity. The reason
of its peculiar narrowness here is that it is, for the most part, full of
yelk-cells of the entoderm. These also stop up the whole of the wide opening of
the primitive mouth, and form what is known as the “yelk-stopper,”
which is seen freely at the white round spot at the south pole (P).
Around it the ectoderm is much thicker, and forms the border of the primitive
mouth, the most important part of the embryo (Fig. 44 k, k′).
Soon the primitive gut-cavity stretches further and further at the expense of
the segmentation-cavity (F), until at last the latter disappears
altogether. The two cavities are only separated by a thin partition (Fig. 43
s). With the formation of the primitive gut our frog-embryo has reached
the gastrula stage, though it is clear that this cenogenetic amphibian gastrula
is very different from the real palingenetic gastrula we have considered (Figs.
30–36).

In the growth of this hooded gastrula we cannot sharply mark off the various
stages which we distinguish successively in the bell-gastrula as morula and
gastrula. Nevertheless, it is not difficult to reduce the whole cenogenetic or
disturbed development of this amphigastrula to the true palingenetic formation
of the archigastrula of the amphioxus.

Fig. 46—Embryonic vesicle of triton
(blastula), outer view, with the transverse fold of the primitive mouth
(u). (From Hertwig.)

This reduction becomes easier if, after considering the
gastrulation of the tailless amphibia (frogs and toads), we glance for a moment
at that of the tailed amphibia, the salamanders. In some of the latter, that
have only recently been carefully studied, and that are phylogenetically older,
the process is much simpler and clearer than is the case with the former and
longer known. Our common water-salamander (Triton taeniatus) is a
particularly good subject for observation. Its nutritive yelk is much smaller
and its formative yelk less obscured with black pigment-cells than in the case
of the frog; and its gastrulation has better retained the original palingenetic
character. It was first described by Scott and Osborn (1879), and Oscar Hertwig
especially made a careful study of it (1881), and rightly pointed out its great
importance in helping us to understand the vertebrate development. Its globular
blastula (Fig. 45) consists of loosely-aggregated,

yelk-filled entodermic cells or yelk-cells (dz) in the lower vegetal
half; the upper, animal half encloses the hemispherical segmentation-cavity
(fh), the curved roof of which is formed of two or three strata of small
ectodermic cells. At the point where the latter pass into the former (at the
equator of the globular vesicle) we have the border zone (rz). The
folding which leads to the formation of the gastrula takes place at a spot in
this border zone, the primitive mouth (Fig. 46 u).

Fig. 47—Sagittal section of a hooded-embryo
(depula) of triton (blastula at the commencement of
gastrulation). ak outer germinal layer, ik inner germinal layer,
fh segmentation-cavity, ud primitive gut, u primitive mouth,
dl and vl dorsal and ventral lips of the mouth, dz
yelk-cells. (From Hertwig.)

Unequal segmentation takes place in some of the cyclostoma and in the oldest
fishes in just the same way as in most of the amphibia. Among the cyclostoma
(“round-mouthed”) the familiar lampreys are particularly
interesting. In respect of organisation and development they are half-way
between the acrania (lancelet) and the lowest real fishes (Selachii);
hence I divided the group of the cyclostoma in 1886 from the real fishes with
which they were formerly associated, and formed of them a special class of
vertebrates. The ovum-segmentation in our common river-lamprey (Petromyzon
fluviatilis) was described by Max Schultze in 1856, and afterwards by Scott
(1882) and Goette (1890).

Unequal total segmentation follows the same lines in the oldest fishes, the
selachii and ganoids, which are directly descended from the cyclostoma. The
primitive fishes (Selachii), which we must regard as the ancestral group
of the true fishes, were generally considered, until a short time ago, to be
discoblastic. It was not until the beginning of the twentieth century that
Bashford Dean made the important discovery in Japan that one of the oldest
living fishes of the shark type (Cestracion japonicus) has the same
total unequal segmentation as the amphiblastic plated fishes
(ganoides).^[22] This is particularly interesting in connection
with our subject, because the few remaining survivors of this division, which
was so numerous in paleozoic times, exhibit three different types of
gastrulation.

[22]
Bashford Dean, Holoblastic Cleavage in the Egg of a Shark, Cestracion
japonicus Macleay. Annotationes zoologicae japonenses, vol. iv, Tokio,
1901.

Fig. 48—Sagittal section of the gastrula of the
water-salamander (Triton). (From Hertwig.) Letters as in Fig.
47; except—p yelk-stopper, mk beginning of the middle
germinal layer.)

The oldest and most conservative forms of the modern ganoids are the scaly
sturgeons (Sturiones), plated fishes of great evolutionary importance, the eggs
of which are eaten as caviar; their cleavage is not essentially different from
that of the lampreys and the amphibia. On the other hand, the most modern of
the plated fishes, the beautifully scaled bony pike of the North American
rivers (Lepidosteus), approaches the osseous fishes, and is discoblastic like
them. A third genus (Amia) is midway between the sturgeons and the latter.

The group of the lung-fishes (Dipneusta or Dipnoi) is closely
connected with the older ganoids. In respect of their whole organisation they
are midway between the gill-breathing fishes and the lung-breathing amphibia;
they share with the former the shape of the body and limbs, and with the latter
the form of the heart

and lungs. Of the older dipnoi (Paladipneusta) we have now only one
specimen, the remarkable Ceratodus of East Australia; its amphiblastic
gastrulation has been recently explained by Richard Semon (cf. Chapter XXI).
That of the two modern dipneusta, of which Protopterus is found in
Africa and Lepidosiren in America, is not materially different. (Cf.
Fig. 51.)

Fig. 49—Ovum-segmentation of the lamprey (Petromyzon
fluviatalis), in four successive stages. The small cells of the upper
(animal) hemisphere divide much more quickly than the cells of the lower
(vegetal) hemisphere.

Fig. 50—Gastrulation of the lamprey (Petromyzon
fluviatilis). A blastula, with wide embryonic cavity (blastocoel,
bl), g incipient invagination. B depula, with advanced
invagination, from the primitive mouth (g). C gastrula, with
complete primitive gut: the embryonic cavity has almost disappeared in
consequence of invagination.

All these amphiblastic vertebrates, Petromyzon and Cestracion,
Accipenser and Ceratodus, and also the salamanders and batrachia,
belong to the old, conservative groups of our stem. Their unequal
ovum-segmentation and gastrulation have many peculiarities in detail, but can
always be reduced with comparative ease to the original cleavage and
gastrulation of the lowest vertebrate, the amphioxus; and this is little
removed, as we have seen, from the very simple archigastrula of the
Sagitta and Monoxenia (see Fig. 29–36). All these and many
other classes of animals generally agree in the circumstance that in
segmentation their

ovum divides into a large number of cells by repeated cleavage. All such ova
have been called, after Remak, “whole-cleaving”
(holoblasta), because their division into cells is complete or total.

Fig.
51—Gastrulation of ceratodus (from Semon). A and
C stage with four cells, B and D with sixteen cells.
A and B are seen from above, C and D sideways.
E stage with thirty-two cells; F blastula; G gastrula in
longitudinal section. fh segmentation-cavity. gh primitive gut
or gastric cavity.

In a great many other classes of animals this is not the case, as we find (in
the vertebrate stem) among the birds, reptiles, and most of the fishes; among
the insects and most of the spiders and crabs (of the articulates); and the
cephalopods (of the molluscs). In all these animals the mature ovum, and the
stem-cell that arises from it in fertilisation, consist of two different and
separate parts, which we have called formative yelk and nutritive yelk. The
formative yelk alone consists of living protoplasm, and is the active,
evolutionary, and nucleated part of the ovum; this alone divides in
segmentation, and produces the numerous cells which make up the embryo. On the
other hand, the nutritive yelk is merely a passive part of the contents of the
ovum, a subordinate element which contains nutritive material (albumin, fat,
etc.), and so represents in a sense the provision-store of the developing
embryo. The latter takes a quantity of food out of this store, and finally
consumes it all. Hence the nutritive yelk is of great indirect importance in
embryonic development, though it has no direct share in it. It either does not
divide at all, or only later on, and does not generally consist of cells. It is
sometimes large and sometimes small, but generally many times larger than the
formative yelk; and hence it is

that it was formerly thought the more important of the two. As the respective
significance of these two parts of the ovum is often wrongly described, it must
be borne in mind that the nutritive yelk is only a secondary addition to the
primary cell, it is an inner enclosure, not an external appendage. All ova that
have this independent nutritive yelk are called, after Remak,
“partially-cleaving” (meroblasta). Their segmentation is
incomplete or partial.

Fig. 52—Ovum of a deep-sea bony fish.
b protoplasm of the stem-cell, k nucleus of same, d clear
globule of albumin, the nutritive yelk, f fat-globule of same, c
outer membrane of the ovum, or ovolemma.)

There are many difficulties in the way of understanding this partial
segmentation and the gastrula that arises from it. We have only recently
succeeded, by means of comparative research, in overcoming these difficulties,
and reducing this cenogenetic form of gastrulation to the original palingenetic
type. This is comparatively easy in the small meroblastic ova which contain
little nutritive yelk—for instance, in the marine ova of a bony fish, the
development of which I observed in 1875 at Ajaccio in Corsica. I found them
joined together in lumps of jelly, floating on the surface of the sea; and, as
the little ovula were completely transparent, I could easily follow the
development of the germ step by step. These ovula are glossy and colourless
globules of little more than the 50th of an inch. Inside a structureless, thin,
but firm membrane (ovolemma, Fig. 52 c) we find a large, quite
clear, and transparent globule of albumin (d). At both poles of its axis
this globule has a pit-like depression. In the pit at the upper, animal pole
(which is turned downwards in the floating ovum) there is a bi-convex lens
composed of protoplasm, and this encloses the nucleus (k); this is the
formative yelk of the stem-cell, or the germinal disk (b). The small
fat-globule (f) and the large albumin-globule (d) together form
the nutritive yelk. Only the formative yelk undergoes cleavage, the nutritive
yelk not dividing at all at first.

The segmentation of the lens-shaped formative yelk (b) proceeds quite
independently of the nutritive yelk, and in perfect geometrical order.

When the mulberry-like cluster of cells has been formed, the border-cells of
the lens separate from the rest and travel into the yelk and the border-layer.
From this the blastula is developed; the regular bi-convex lens being converted
into a disk, like a watch-glass, with thick borders. This lies on the upper and
less curved polar surface of the nutritive yelk like the watch glass on the
yelk. Fluid gathers between the outer layer and the border, and the
segmentation-cavity is formed. The gastrula is then formed by invagination, or
a kind of turning-up of the edge of the blastoderm. In this process the
segmentation-cavity disappears.

The space underneath the entoderm corresponds to the primitive gut-cavity, and
is filled with the decreasing food-yelk (n). Thus the formation of the
gastrula of our fish is complete. In contrast to the two chief forms of
gastrula we considered previously, we give the name of discoid gastrula
(discogastrula, Fig. 54) to this third principal type.

Very similar to the discoid gastrulation of the bony fishes is that of the hags
or myxinoida, the remarkable cyclostomes that live parasitically in the
body-cavity of fishes, and are distinguished by several notable peculiarities
from their nearest relatives, the lampreys. While the amphiblastic ova of the
latter are small and develop like those of the amphibia, the cucumber-shaped
ova of the hag are about an inch long, and form a discoid gastrula. Up to the
present it has only been observed in one species (Bdellostoma Stouti),
by Dean and Doflein (1898).

It is clear that the important features which distinguish the discoid gastrula
from the other chief forms we have considered are determined by the large
food-yelk. This takes no direct part in the building of the germinal layers,
and completely fills the primitive gut-cavity of the gastrula, even protruding
at the mouth-opening. If we imagine the original bell-gastrula (Figs.
30–36) trying to swallow a

ball of food which is much bigger than itself, it would spread out round it in
discoid shape in the attempt, just as we find to be the case here (Fig. 54).
Hence we may derive the discoid gastrula from the original bell-gastrula,
through the intermediate stage of the hooded gastrula. It has arisen through
the accumulation of a store of food-stuff at the vegetal pole, a
“nutritive yelk” being thus formed in contrast to the
“formative yelk.” Nevertheless, the gastrula is formed here, as in
the previous cases, by the folding or invagination of the blastula. We can,
therefore, reduce this cenogenetic form of the discoid segmentation to the
palingenetic form of the primitive cleavage.

Fig. 53—Ovum-segmentation of a bony fish. A first
cleavage of the stem-cell (cytula), B division of same into four
segmentation-cells (only two visible), C the germinal disk divides into
the blastoderm (b) and the periblast (p). d nutritive
yelk, f fat-globule, c ovolemma, z space between the
ovolemma and the ovum, filled with a clear fluid.)

This reduction is tolerably easy and confident in the case of the small ovum of
our deep-sea bony fish, but it becomes difficult and uncertain in the case of
the large ova that we find in the majority of the other fishes and in all the
reptiles and birds. In these cases the food-yelk is, in the first place,
comparatively colossal, the formative yelk being almost invisible beside it;
and, in the second place, the food-yelk contains a quantity of different
elements, which are known as “yelk-granules, yelk-globules, yelk-plates,
yelk-flakes, yelk-vesicles,” and so on. Frequently these definite
elements in the yelk have been described as real cells, and it has been wrongly
stated that a portion of the embryonic body is built up from these cells. This
is by no means the case. In every case, however large it is—and even when
cell-nuclei travel into it during the cleavage of the border—the
nutritive yelk remains a dead accumulation of food, which is taken into the gut
during embryonic development and consumed by the embryo. The latter develops
solely from the living formative yelk of the stem-cell. This is equally true of
the ova of our small bony fishes and of the colossal ova of the primitive
fishes, reptiles, and birds.

The gastrulation of the primitive fishes or selachii (sharks and rays) has been
carefully studied of late years by Ruckert, Rabl, and H.E. Ziegler in
particular, and is very important in the sense that this group is the oldest
among living fishes, and their gastrulation can be derived directly from that
of the cyclostoma by the accumulation of a large quantity of food-yelk. The
oldest sharks (Cestracion) still have the unequal segmentation inherited
from the cyclostoma. But while in this case, as in the case of the amphibia,
the small ovum completely divides into cells in segmentation, this is no longer
so in the great majority of the selachii (or Elasmobranchii). In these
the contractility of the active protoplasm no longer suffices to break up the
huge mass of the passive deutoplasm completely into cells; this is only
possible in the upper or dorsal part, but not in the lower or ventral section.
Hence we find in the primitive fishes a blastula with a small eccentric
segmentation-cavity (Fig. 55 b), the wall of which varies greatly in
composition. The circular border of the germinal disk which connects the roof
and floor of the segmentation-cavity corresponds to the border-zone at the
equator of the amphibian ovum. In the middle of its hinder border we have the
beginning of the invagination of the primitive gut

(Fig. 56 ud); it extends gradually from this spot (which corresponds to
the Rusconian anus of the amphibia) forward and around, so that the primitive
mouth becomes first crescent-shaped and then circular, and, as it opens wider,
surrounds the ball of the larger food-yelk.

Fig. 54—Discoid gastrula
(discogastrula) of a bony fish. e ectoderm, i
entoderm, w border-swelling or primitive mouth, n albuminous
globule of the nutritive yelk, f fat-globule of same, c external
membrane (ovolemma), d partition between entoderm and ectoderm (earlier
the segmentation-cavity.)

Essentially different from the wide-mouthed discoid gastrula of most of the
selachii is the narrow-mouthed discoid gastrula (or epigastrula) of the
amniotes, the reptiles, birds, and monotremes; between the two—as an
intermediate stage—we have the amphigastrula of the amphibia. The
latter has developed from the amphigastrula of the ganoids and dipneusts,
whereas the discoid amniote gastrula has been evolved from the amphibian
gastrula by the addition of food-yelk. This change of gastrulation is still
found in the remarkable ophidia (Gymnophiona, Cœcilia, or
Peromela), serpent-like amphibia that live in moist soil in the tropics,
and in many respects represent the transition from the gill-breathing amphibia
to the lung-breathing reptiles. Their embryonic development has been explained
by the fine studies of the brothers Sarasin of Ichthyophis glutinosa at
Ceylon (1887), and those of August Brauer of the Hypogeophis rostrata in
the Seychelles (1897). It is only by the historical and comparative study of
these that we can understand the difficult and obscure gastrulation of the
amniotes.

Fig. 55—Longitudinal section through the
blastula of a shark (Pristiuris). (From Ruckert.) (Looked at
from the left; to the right is the hinder end, H, to the left the fore
end, V.) B segmentation-cavity, kz cells of the germinal
membrane, dk yelk-nuclei.

The bird’s egg is particularly important for our purpose, because most of
the chief studies of the development of the vertebrates are based on
observations of the hen’s egg during hatching. The mammal ovum is much
more difficult to obtain and study, and for this practical and obvious reason
very rarely thoroughly investigated. But we can get hens’ eggs in any
quantity at any time, and, by means of artificial incubation, follow the
development of the embryo step by step. The bird’s egg differs
considerably from the tiny mammal ovum in size, a large quantity of food-yelk
accumulating within the original yelk or the protoplasm of the ovum. This is
the yellow ball which we commonly call the yolk of the egg. In order to
understand the bird’s egg aright—for it is very often quite wrongly
explained—we must examine it in its original condition, and follow it
from the very beginning of its development in the bird’s ovary. We then
see that the original ovum is a quite small, naked, and simple cell with a
nucleus, not differing in either size or shape from the original ovum of the
mammals and other animals (cf. Fig. 13 E). As in the case of all the
craniota (animals with a skull), the original or primitive ovum
(protovum) is covered with a continuous layer of small cells. This
membrane is the follicle, from which the ovum afterwards issues. Immediately
underneath it the structureless yelk-membrane is secreted from the yelk.

The small primitive ovum of the bird begins very early to take up into itself a
quantity of food-stuff through the yelk-membrane, and work it up into the
“yellow yelk.” In this way the ovum

enters on its second stage (the metovum), which is many times larger than the
first, but still only a single enlarged cell. Through the accumulation of the
store of yellow yelk within the ball of protoplasm the nucleus it contains (the
germinal vesicle) is forced to the surface of the ball. Here it is surrounded
by a small quantity of protoplasm, and with this forms the lens-shaped
formative yelk (Fig. 15 b). This is seen on the yellow yelk-ball, at a
certain point of the surface, as a small round white spot—the
“tread” (cicatricula). From this point a thread-like column
of white nutritive yelk (d), which contains no yellow yelk-granules, and
is softer than the yellow food-yelk, proceeds to the middle of the yellow
yelk-ball, and forms there a small central globule of white yelk (Fig. 15
d). The whole of this white yelk is not sharply separated from the yellow
yelk, which shows a slight trace of concentric layers in the hard-boiled egg
(Fig. 15 c). We also find in the hen’s egg, when we break the
shell and take out the yelk, a round small white disk at its surface which
corresponds to the tread. But this small white “germinal disk” is
now further developed, and is really the gastrula of the chick. The body of the
chick is formed from it alone. The whole white and yellow yelk-mass is without
any significance for the formation of the embryo, it being merely used as food
by the developing chick. The clear, glarous mass of albumin that surrounds the
yellow yelk of the bird’s egg, and also the hard chalky shell, are only
formed within the oviduct round the impregnated ovum.

Fig.
56—Longitudinal section of the blastula of a shark
(Pristiurus) at the beginning of gastrulation. (From Ruckert.)
(Seen from the left.) V fore end, H hind end, B
segmentation-cavity, ud first trace of the primitive gut, dk
yelk-nuclei, fd fine-grained yelk, gd coarse-grained yelk.

When the fertilisation of the bird’s ovum has taken place within the
mother’s body, we find in the lens-shaped stem-cell the progress of flat,
discoid segmentation (Fig. 57). First two equal segmentation-cells (A)
are formed from the ovum. These divide into four (B), then into eight,
sixteen (C), thirty-two, sixty-four, and so on. The cleavage of the
cells is always preceded by a division of their nuclei. The cleavage surfaces
between the segmentation-cells appear at the free surface of the tread as
clefts. The first two divisions are vertical to each other, in the form of a
cross (B). Then there are two more divisions, which cut the former at an
angle of forty-five degrees. The tread, which thus becomes the germinal disk,
now has the appearance of an eight-rayed star. A circular cleavage next taking
place round the middle, the eight triangular cells divide into sixteen, of
which eight are in the middle and eight distributed around (C).
Afterwards circular clefts and radial clefts, directed towards the centre,
alternate more or less irregularly (D, E). In most of the amniotes the
formation of concentric and radial clefts is irregular from the very first; and
so also in the hen’s egg. But the final outcome of the cleavage-process
is once more the formation of a large number of small cells of a similar
nature. As in the case of the fish-ovum, these segmentation-cells form a round,
lens-shaped disk, which corresponds to the morula, and is embedded in a small
depression of the white yelk. Between the lens-shaped disk of the morula-cells
and the underlying white yelk a small cavity is now formed by the accumulation
of fluid, as in the fishes. Thus we get the peculiar and not easily
recognisable blastula of the bird (Fig. 58). The small segmentation-cavity
(fh) is very flat and much compressed. The upper or dorsal wall
(dw) is formed of a single layer of clear, distinctly separated cells;
this

corresponds to the upper or animal hemisphere of the triton-blastula (Fig. 45).
The lower or ventral wall of the flat dividing space (vw) is made up of
larger and darker segmentation-cells; it corresponds to the lower or vegetal
hemisphere of the blastula of the water-salamander (Fig. 45 dz). The
nuclei of the yelk-cells, which are in this case especially numerous at the
edge of the lens-shaped blastula, travel into the white yelk, increase by
cleavage, and contribute even to the further growth of the germinal disk by
furnishing it with food-stuff.

Fig. 57—Diagram of discoid segmentation in the bird’s
ovum (magnified). Only the formative yelk (the tread) is shown in these six
figures (A to F), because cleavage only takes place in this. The
much larger food-yelk, which does not share in the cleavage, is left out and
merely indicated by the dark ring without.

The invagination or the folding inwards of the bird-blastula takes place in
this case also at the hinder pole of the subsequent chief axis, in the middle
of the hind border of the round germinal disk (Fig. 59 s). At this spot
we have the most brisk cleavage of the cells; hence the cells are more numerous
and smaller here than in the fore-half of the germinal disk. The
border-swelling or thick edge of the disk is less clear but whiter behind, and
is more sharply separated from contiguous parts. In the middle of its hind
border there is a white, crescent-shaped groove—Koller’s
sickle-groove (Fig. 59 s); a small projecting process in the centre of
it is called the sickle-knob (sk). This important cleft is the primitive
mouth, which was described for a long time as the “primitive
groove.” If we make a vertical section through this part, we see that a
flat and broad cleft stretches under the germinal disk forwards from the
primitive mouth; this is the primitive gut (Fig. 60 ud). Its roof or
dorsal wall is formed by the folded upper part of the blastula, and its floor
or ventral wall by the white yelk (wd), in which a number of yelk-nuclei
(dk) are distributed. There is a brisk multiplication of these at the
edge of the germinal disk, especially in the neighbourhood of the sickle-shaped
primitive mouth.

We learn from sections through later stages of this discoid bird-gastrula that
the primitive gut-cavity, extending forward from the primitive mouth as a flat
pouch, undermines the whole region of the round flat lens-shaped blastula (Fig.
61 ud). At the same time, the segmentation-cavity gradually disappears
altogether, the folded inner germinal layer (ik) placing itself from
underneath on the overlying outer germinal layer (ak). The typical
process of invagination, though greatly disguised, can thus be clearly seen in
this case, as Goette and Rauber, and more recently Duval (Fig. 61), have shown.

The older embryologists (Pander, Baer, Remak), and, in recent times especially,

His, Kölliker, and others, said that the two primary germinal layers of the
hen’s ovum—the oldest and most frequent subject of
observation!—arose by horizontal cleavage of a simple germinal disk. In
opposition to this accepted view, I affirmed in my Gastræa Theory (1873)
that the discoid bird-gastrula, like that of all other vertebrates, is formed
by folding (or invagination), and that this typical process is merely altered
in a peculiar way and disguised by the immense accumulation of food-yelk and
the flat spreading of the discoid blastula at one part of its surface. I
endeavoured to establish this view by the derivation of the vertebrates from
one source, and especially by proving that the birds descend from the reptiles,
and these from the amphibia. If this is correct, the discoid gastrula of the
amniotes must have been formed by the folding-in of a hollow blastula, as has
been shown by Remak and Rusconi of the discoid gastrula of the amphibia, their
direct ancestors. The accurate and extremely careful observations of the
authors I have mentioned (Goette, Rauber, and Duval) have decisively proved
this

recently for the birds; and the same has been done for the reptiles by the fine
studies of Kupffer, Beneke, Wenkebach, and others. In the shield-shaped
germinal disk of the lizard (Fig. 62), the crocodile, the tortoise, and other
reptiles, we find in the middle of the hind border (at the same spot as the
sickle groove in the bird) a transverse furrow (u), which leads into a
flat, pouch-like, blind sac, the primitive gut. The fore (dorsal) and hind
(ventral) lips of the transverse furrow correspond exactly to the lips of the
primitive mouth (or sickle-groove) in the birds.

Fig.
58—Vertical section of the blastula of a hen
(discoblastula). fh segmentation-cavity, dw dorsal wall
of same, vw ventral wall, passing directly into the white yelk
(wd). (From Duval.)
Fig. 59—The germinal disk of
the hen’s ovum at the beginning of gastrulation; A before
incubation, B in the first hour of incubation. (From Koller.)
ks germinal-disk, V its fore and H its hind border;
es embryonic shield, s sickle-groove, sk sickle knob,
d yelk.
Fig. 60—Longitudinal section of the germinal disk
of a siskin (discogastrula). (From Duval.) ud
primitive gut, vl, hl fore and hind lips of the primitive mouth (or
sickle-edge); ak outer germinal layer, ik inner germinal layer,
dk yelk-nuclei, wd white yelk.

Fig. 61—Longitudinal section of the discoid
gastrula of the nightingale. (From Duval.) ud primitive gut,
vl, hl fore and hind lips of the primitive mouth; ak, ik outer
and inner germinal layers; vr fore-border of the discogastrula.

Fig. 62—Germinal disk of the lizard
(Lacerta agilis). (From Kupffer.) u primitive mouth,
s sickle, es embryonic shield, hf and df light and
dark germinative area.

The gastrulation of the mammals must be derived from this special embryonic
development of the reptiles and birds. This latest and most advanced class of
the vertebrates has, as we shall see afterwards, evolved at a comparatively
recent date from an older group of reptiles; and all these amniotes must have
come originally from a common stem-form. Hence the distinctive embryonic
process of the mammal must have arisen by cenogenetic modifications from the
older form of gastrulation of the reptiles and birds. Until we admit this
thesis we cannot understand the formation of the germinal layers in the mammal,
and therefore in man.

I first advanced this fundamental principle in my essay On the Gastrulation
of Mammals (1877), and sought to show in this way that I assumed a gradual
degeneration of the food-yelk and the yelk-sac on the way from the proreptiles
to the mammals. “The cenogenetic process of adaptation,” I said,
“which has occasioned the atrophy of the rudimentary yelk-sac of the
mammal, is perfectly clear. It is due to the fact that the young of the mammal,
whose ancestors were certainly oviparous, now remain a long time in the womb.
As the great store of food-yelk, which the oviparous ancestors gave to the egg,
became superfluous in their descendants owing to the long carrying in the womb,
and the maternal blood in the wall of the uterus made itself the chief source
of nourishment, the now useless yelk-sac was bound to atrophy by embryonic
adaptation.”

My opinion met with little approval at the time; it was vehemently attacked by
Kölliker, Hensen, and His in particular. However, it has been gradually
accepted, and has recently been firmly established by a large number of
excellent studies of mammal gastrulation, especially by Edward Van
Beneden’s studies of the rabbit and bat, Selenka’s on the
marsupials and rodents, Heape’s and Lieberkühn’s on the mole,
Kupffer and Keibel’s on the rodents, Bonnet’s on the ruminants,
etc. From the general comparative point of view, Carl Rabl in his theory of the
mesoderm, Oscar Hertwig in the latest edition of his Manual (1902), and
Hubrecht in his Studies in Mammalian Embryology (1891), have supported
the opinion, and sought to derive the peculiarly modified gastrulation of the
mammal from that of the reptile.

In the meantime (1884) the studies of Wilhelm Haacke and Caldwell provided a
proof of the long-suspected and very interesting fact, that the lowest mammals,
the monotremes, lay eggs, like the birds and reptiles, and are not
viviparous like the other mammals. Although the gastrulation of the monotremes
was not really known until studied by Richard

Semon in 1894, there could be little doubt, in view of the great size of their
food-yelk, that their ovum-segmentation was discoid, and led to the formation
of a sickle-mouthed discogastrula, as in the case of the reptiles and birds.
Hence I had, in 1875 (in my essay on The Gastrula and Ovum-segmentation of
Animals), counted the monotremes among the discoblastic vertebrates. This
hypothesis was established as a fact nineteen years afterwards by the careful
observations of Semon; he gave in the second volume of his great work,
Zoological Journeys in Australia (1894), the first description and
correct explanation of the discoid gastrulation of the monotremes. The
fertilised ova of the two living monotremes (Echidna and
Ornithorhynchus) are balls of one-fifth of an inch in diameter, enclosed in
a stiff shell; but they grow considerably during development, so that when laid
the egg is three times as large. The structure of the plentiful yelk, and
especially the relation of the yellow and the white yelk, are just the same as
in the reptiles and birds. As with these, partial cleavage takes place at a
spot on the surface at which the small formative yelk and the nucleus it
encloses are found. First is formed a lens-shaped circular germinal disk. This
is made up of several strata of cells, but it spreads over the yelk-ball, and
thus becomes a one-layered blastula.

Fig. 63—Ovum of the opossum
(Didelphys) divided into four. (From Selenka.) b
the four segmentation-cells, r directive body, c unnucleated
coagulated matter, p, albumin-membrane.

If we then imagine the yelk it contains to be dissolved and replaced by a clear
liquid, we have the characteristic blastula of the higher mammals. In these the
gastrulation proceeds in two phases, as Semon rightly observes: firstly,
formation of the entoderm by cleavage at the centre and further growth at the
edge; secondly, invagination. In the monotremes more primitive conditions have
been retained better than in the reptiles and birds. In the latter, before the
commencement of the gastrula-folding, we have, at least at the periphery, a
two-layered embryo forming from the cleavage. But in the monotremes the
formation of the cenogenetic entoderm does not precede the invagination; hence
in this case the construction of the germinal layers is less modified than in
the other amniota.

Fig. 64—Blastula of the opossum
(Didelphys). (From Selenka.) a animal pole of the
blastula, v vegetal pole, en mother-cell of the entoderm,
ex ectodermic cells, s spermia, ib unnucleated yelk-balls
(remainder of the food-yelk), p albumin membrane.

The marsupials, a second sub-class, come next to the oviparous monotremes, the
oldest of the mammals. But as in their case the food-yelk is already atrophied,
and the little ovum develops within the mother’s body, the partial
cleavage has been reconverted into total. One section of the marsupials still
show points of agreement with the monotremes, while another section of them,
according to the splendid investigations of Selenka, form a connecting-link
between these and the placentals.

The fertilised ovum of the opossum (Didelphys) divides, according to
Selenka, first into two, then four, then eight equal cells; hence the
segmentation is at first equal or homogeneous. But in the course of the
cleavage a larger cell, distinguished by its less clear plasm and its
containing more yelk-granules (the mother cell of the entoderm, Fig. 64
en),

separates from the others; the latter multiply more rapidly than the former.
As, further, a quantity of fluid gathers in the morula, we get a round
blastula, the wall of which is of varying thickness, like that of the amphioxus
(Fig. 38 E) and the amphibia (Fig. 45). The upper or animal hemisphere
is formed of a large number of small cells; the lower or vegetal hemisphere of
a small number of large cells. One of the latter, distinguished by its size
(Fig. 64 en), lies at the vegetal pole of the blastula-axis, at the
point where the primitive mouth afterwards appears. This is the mother-cell of
the entoderm; it now begins to multiply by cleavage, and the daughter-cells
(Fig. 65 i) spread out from this spot over the inner surface of the
blastula, though at first only over the vegetal hemisphere. The less clear
entodermic cells (i) are distinguished at first by their rounder shape
and darker nuclei from the higher, clearer, and longer entodermic cells
(e), afterwards both are greatly flattened, the inner blastodermic cells
more than the outer.

Fig. 65—Blastula of the opossum
(Didelphys) at the beginning of gastrulation. (From Selenka.)
e ectoderm, i entoderm; a animal pole, u primitive
mouth at the vegetal pole, f segmentation-cavity, d unnucleated
yelk-balls (relics of the reduced food-yelk), c nucleated curd (without
yelk-granules)
Fig. 66—Oval gastrula of the opossum
(Didelphys), about eight hours old. (From Selenka) (external
view).)

The unnucleated yelk-balls and curd (Fig. 65 d) that we find in the
fluid of the blastula in these marsupials are very remarkable; they are the
relics of the atrophied food-yelk, which was developed in their ancestors, the
monotremes, and in the reptiles.

In the further course of the gastrulation of the opossum the oval shape of the
gastrula (Fig. 66) gradually changes into globular, a larger quantity of fluid
accumulating in the vesicle. At the same time, the entoderm spreads further and
further over the inner surface of the ectoderm (e). A globular vesicle
is formed, the wall of which consists of two thin simple strata of cells; the
cells of the outer germinal layer are rounder, and those of the inner layer
flatter. In the region of the primitive mouth (p) the cells are less
flattened, and multiply briskly. From this point—from the hind (ventral)
lip of the primitive mouth, which extends in a central cleft, the primitive
groove—the construction of the mesoderm proceeds.

Gastrulation is still more modified and curtailed cenogenetically in the
placentals than in the marsupials. It was first accurately known to us by the
distinguished investigations of Edward Van Beneden in 1875, the first object of
study being the ovum of the rabbit. But as man also belongs to this sub-class,
and as his as yet unstudied gastrulation cannot be materially different from
that of the other placentals, it merits the closest attention. We have, in the
first place, the peculiar feature that the two first segmentation-cells that
proceed from the cleavage of the fertilised ovum (Fig. 68) are of different
sizes and natures; the difference is sometimes greater, sometimes less (Fig.
69). One of these first daughter-cells of the ovum is a little

larger, clearer, and more transparent than the other. Further, the smaller cell
takes a colour in carmine, osmium, etc., more strongly than the larger. By
repeated cleavage of it a morula is formed, and from this a blastula, which
changes in a very characteristic way into the greatly modified gastrula. When
the number of the segmentation-cells in the mammal embryo has reached
ninety-six (in the rabbit, about seventy hours after impregnation) the fœtus
assumes a form very like the archigastrula (Fig. 72). The spherical embryo
consists of a central mass of thirty-two soft, round cells with dark nuclei,
which are flattened into polygonal shape by mutual pressure, and colour
dark-brown with osmic acid (Fig. 72 i). This dark central group of cells
is surrounded by a lighter spherical membrane, consisting of sixty-four
cube-shaped, small, and fine-grained cells which lie close together in a single
stratum, and only colour slightly in osmic acid (Fig. 72 e). The authors
who regard this embryonic form as the primary gastrula of the placental
conceive the outer layer as the ectoderm and the inner as the entoderm. The
entodermic membrane is only interrupted at one spot, one, two, or three of the
ectodermic cells being loose there. These form the yelk-stopper, and fill up
the mouth of the gastrula (a). The central primitive gut-cavity
(d) is full of entodermic cells. The uni-axial type of the mammal
gastrula is accentuated in this way. However, opinions still differ
considerably as to the real nature of this “provisional gastrula”
of the placental and its relation to the blastula into which it is converted.

As the gastrulation proceeds a large spherical blastula is formed from this
peculiar solid amphigastrula of the placental, as we saw in the case of the
marsupial. The accumulation of fluid in the solid gastrula (Fig. 73 A) leads to
the formation of an eccentric cavity, the group of the darker entodermic cells
(hy) remaining directly attached at one spot with the round enveloping
stratum of the lighter ectodermic cells (ep). This spot corresponds to
the original primitive mouth (prostoma or blastoporus). From this important
spot the inner germinal layer spreads all round on the inner surface of the
outer layer, the cell-stratum of which forms the wall of the hollow sphere; the
extension proceeds from the vegetal towards the animal pole.

Fig. 67—Longitudinal section through the oval
gastrula of the opossum (Fig. 69). (From Selenka.) p
primitive mouth, e ectoderm, i entoderm, d yelk remains in
the primitive gut-cavity (u).

The cenogenetic gastrulation of the placental has been greatly modified by
secondary adaptation in the various groups of this most advanced and youngest
sub-class of the mammals. Thus, for instance, we find in many of the rodents
(guinea-pigs, mice, etc.) apparently a temporary inversion of the two
germinal layers. This is due to a folding of the blastodermic wall by what is
called the “girder,” a plug-shaped growth of Rauber’s
“roof-layer.” It is a thin layer of flat epithelial cells, that is
freed from the surface of the blastoderm in some of the rodents; it has no more
significance in connection with the general course of placental gastrulation
than the conspicuous departure from the usual globular shape in the blastula of
some of the ungulates. In some pigs and ruminants it grows into a thread-like,
long and thin tube.

Thus the gastrulation of the placentals, which diverges most from that of the
amphioxus, the primitive form, is reduced to the original type, the
invagination of a modified blastula. Its chief peculiarity is that the folded
part of the blastoderm does not form a completely closed (only open at the
primitive mouth) blind sac, as is usual; but this blind sac has a wide opening
at the ventral curve (opposite to the dorsal mouth); and through this opening
the primitive gut communicates from the first with the embryonic cavity of the
blastula. The folded crest-shaped

entoderm grows with a free circular border on the inner surface of the entoderm
towards the vegetal pole; when it has reached this, and the inner surface of
the blastula is completely grown over, the primitive gut is closed. This
remarkable direct transition of the primitive gut-cavity into the
segmentation-cavity is explained simply by the assumption that in most of the
mammals the yelk-mass, which is still possessed by the oldest forms of the
class (the monotremes) and their ancestors (the reptiles), is atrophied. This
proves the essential unity of gastrulation in all the vertebrates, in spite of
the striking differences in the various classes.

Fig. 68—Stem-cell of the mammal ovum (from the
rabbit).
k stem-nucleus, n nuclear corpuscle, p
protoplasm of the stem-cell, z modified zona pellucida, h
outer albuminous membrane, s dead sperm-cells.

Fig. 69—Incipient cleavage of the mammal ovum (from
the rabbit). The stem-cell has divided into two unequal cells, one lighter
(e) and one darker (i). z zona pellucida, h outer
albuminous membrane, s dead sperm-cell.

Fig. 70—The first four
segmentation-cells of the mammal ovum (from the rabbit).
e the
two larger (and lighter) cells, i the two smaller (and darker)
cells, z zona pellucida, h outer albuminous membrane.

Fig. 71—Mammal ovum with eight segmentation-cells
(from the rabbit). e four larger and lighter cells, i four smaller
and darker cells, z zona pellucida, h outer albuminous membrane.

In order to complete our consideration of the important processes of
segmentation and gastrulation, we will, in conclusion, cast a brief glance at
the fourth chief type—superficial segmentation. In the vertebrates this
form is not found at all. But it plays the chief part in the large stem of the
articulates—the insects,

spiders, myriapods, and crabs. The distinctive form of gastrula that comes of
it is the “vesicular gastrula” (Perigastrula).

In the ova which undergo this superficial cleavage the formative yelk is
sharply divided from the nutritive yelk, as in the preceding cases of the ova
of birds, reptiles, fishes, etc.; the formative yelk alone undergoes cleavage.
But while in the ova with discoid gastrulation the formative yelk is not in the
centre, but at one pole of the uni-axial ovum, and the food-yelk gathered at
the other pole, in the ova with superficial cleavage we find the formative yelk
spread over the whole surface of the ovum; it encloses spherically the
food-yelk, which is accumulated in the middle of the ova. As the segmentation
only affects the former and not the latter, it is bound to be entirely
“superficial”; the store of food in the middle is quite untouched
by it. As a rule, it proceeds in regular geometrical progression. In the end
the whole of the formative yelk divides into a number of small and homogeneous
cells, which lie close together in a single stratum on the entire surface of
the ovum, and form a superficial blastoderm. This blastoderm is a simple,
completely closed vesicle, the internal cavity of which is entirely full of
food-yelk. This real blastula only differs from that of the primitive ova in
its chemical composition. In the latter the content is water or a watery jelly;
in the former it is a thick mixture, rich in food-yelk, of albuminous and fatty
substances. As this quantity of food-yelk fills the centre of the ovum before
cleavage begins, there is no difference in this respect between the morula and
the blastula. The two stages rather agree in this.

When the blastula is fully formed, we have again in this case the important
folding or invagination that determines gastrulation. The space between the
skin-layer and the gut-layer (the remainder of the segmentation-cavity) remains
full of food-yelk, which is gradually used up. This is the only material
difference between our vesicular gastrula (perigastrula) and the
original form of the bell-gastrula (archigastrula). Clearly the one has
been developed from the other in the course of time, owing to the accumulation
of food-yelk in the centre of the ovum.^[23]

[23]
On the reduction of all forms of gastrulation to the original palingenetic form
see especially the lucid treatment of the subject in Arnold Lang’s
Manual of Comparative Anatomy (1888), Part I.

We must count it an important advance that we are thus in a position to reduce
all the various embryonic phenomena in the different groups of animals to these
four principal forms of segmentation and gastrulation. Of these four forms we
must regard one only as the original palingenetic, and the other three as
cenogenetic and derivative. The unequal, the discoid, and the superficial
segmentation have all clearly arisen by secondary adaptation from the primary
segmentation; and the chief cause of their development has been the gradual
formation of the food-yelk, and the increasing antithesis between animal and
vegetal halves of the ovum, or between ectoderm (skin-layer) and entoderm
(gut-layer).

Fig. 72—Gastrula of the placental mammal
(epigastrula from the rabbit), longitudinal section through the axis. e
ectodermic cells (sixty-four, lighter and smaller), i entodermic cells
(thirty-two, darker and larger), d central entodermic cell, filling the
primitive gut-cavity, o peripheral entodermic cell, stopping up the
opening of the primitive mouth (yelk-stopper in the Rusconian anus).

The numbers of careful studies of animal gastrulation that have been made in
the last few decades have completely established the views I have expounded,
and which I first advanced in the years 1872–76. For a time they were
greatly disputed by many embryologists. Some said that the original embryonic
form of the metazoa was not the gastrula, but the “planula”—a
double-walled vesicle with closed cavity and without mouth-aperture; the latter
was supposed to pierce through gradually. It was afterwards shown that this
planula (found in several sponges, etc.) was a later evolution from the
gastrula.

Fig.
73—Gastrula of the rabbit. A as a solid, spherical cluster of
cells, B changing into the embryonic vesicle, bp primitive mouth,
ep ectoderm, hy entoderm.

It was also shown that what is called delamination—the rise of the two
primary germinal layers by the folding of the surface of the blastoderm (for
instance, in the Geryonidæ and other medusæ)—was a secondary
formation, due to cenogenetic variations from the original invagination of the
blastula. The same may be said of what is called “immigration,” in
which certain cells or groups of cells are detached from the simple layer of
the blastoderm, and travel into the interior of the blastula; they attach
themselves to the inner wall of the blastula, and form a second internal
epithelial layer—that is to say, the entoderm. In these and many other
controversies of modern embryology the first requisite for clear and natural
explanation is a careful and discriminative distinction between palingenetic
(hereditary) and cenogenetic (adaptive) processes. If this is properly attended
to, we find evidence everywhere of the biogenetic law.