.' ;.'.. ;-W THE EVOLUTION OF MAN A POPULAR SCIENTIFIC STUDY, BY ERNST HAECKEL. HUMAN EMBRYOLOGY or ONTOGENY. TRANSLATED FROM THE FIFTH ENLARGED) EDITION, By JOSEPH McCABE. NEW YORK. THE TRUTH SEEKER COMPANY, 62 VESEY STREET. Digitized by the Internet Archive in 2010 http://www.archive.org/details/evolutionofmanpoOOhaec CONTENTS i.isr of Illustrations Glossary ..... Tk inslator's Preface Table : Classification of the Animal World I. II. in. IV. v. vr. VII. VIII. IX. X. XI. XII. XIII. XIV. XV The Fundamental Law ok Organic Evolution Tin: Older Embryology . Modern Embryology Tin; Older Phylogeny The Modern Scienci of Evolution The Ovum and the Amceba Con>. BFTION ..... Tin-: Gastrjea Theory THE GasTRILATION OF THE VERTEBRATB . Tin. Gslom Theory Tin. Vertebrate Character of Man Embryonic Shield and Germinative Area Dorsal Boda and Ventral Body The Articulation of the Body . i >i i \i. Membranes and Circulation 9 '5 22 28 3* 5i 59 7' 90 103 .41 •56 CONTENTS List ok Illustrations Index . CHAP. XVI. SrRLXnur. or mi L.INCELB1 MID THE Sea-Squirt . XVII. EMBRYOLOGY OF THE LANCELBT AND THE SeA-SQUJRT XVI II. Dl RATION OF THE HISTORY OF OUR STEM XIX. Our Protist Ancestors XX. Our Worm-Like Ancestors . XXI. O.r Fish-Like Ancestors * XXII. Ojr Five-Toed Ancestors , XX1IL Our Ape Ancestors . XXIV. Evolution of the Nervous System . XXV. Evolution of the Sense-Organs XXVI. Evolution of the Organs of Movement . XXVII. Evolution of the Alimentary System XXVIII. Evolution of the Vascular System * •; XXIX. Evolution of the Sexual Organs XXX. Results of Anthropogenv ,. * rAGJt v 359 '70 191 '99 207 318 229 239 253 266 280 29a 308 3'8 33o 348 LIST OF ILLUSTRATIONS ru.i 1. The human ovum ... 36 2. Stem-cell of an echinoderm - 37 3. Three epithelial cells 38 4. Five spiny or grooved cells - 38 5. Ten liver-cells - - - - 38 6. Nino star-shaped bone-cells - 39 7. Eleven star-shaped rills 39 8. Unfertilised ovum of an echino- denn - .... 40 9. A large branching nerve-cell - 41 10. Blood-cells ----- 42 11. Indirect or mitotic cell-division - 43 12. Mobile cells- - - - - 44 13. Ova of various animals 45 14. The human ovum- - - - 46 15. Fertilised ovum of hen 47 16. A creeping amoeba - - - 47 17. Division of an amoeba- - - 48 18. Ovum of a sponge - - - 49 19. Blood-cells, Or phagocytes - - 50 jo. Spermia or spermatozoa 52 21. Spermatozoa of various animals - 53 22. A single human spermatozoon - 53 23. Fertilisation of the ovum - - 54 24. Impregnated echinoderm ovum - 55 25. Impregnation of the star -fish ovum ----- ^6 26 and 27. Impregnation of sea-urchin ovum 57 28. Stem-cell of a rabbit - - - 58 29. Gastrulation of a coral 61 30. Gastrula of a gastraead 63 31. Gastrula of a worm - - - 63 32. Gastrula of an echinoderm- - 63 ^2- Gastrula of an arthropod - - 63 34. Gastrula of a mollusc - - - 63 35. Gastrula of a vertebrate - - 63 36. Gastrula of a lower sponge - 64 37. Cells from the primary germinal layers ----- 65 38. Gastrulation of the amphioxus - 66 3<>. Gastrula of the amphioxus 67 PACE 40. Cleavage of the frog's ovum - 72 }i 14. Sections of fertilised toad ovum - - - - " 73 15 (S. Gastrulation of the salamander 74 49. Segmentation of the lamprey - 76 50. Gastrulation of the lamprey - 76 51. Gastrulation of Ceratodus 77 ~,2. Ovum of a deep-sea bony fish - 78 53. Segmentation of a bony fish - 79 54. Discoid gastrula of a bony fish - 80 55 and 56. Sections of blastula of shark 80 57. Discoid segmentation of birds ovum ----- 82 58-61. Gastrulation of the bird - - 83 62. Germinal disk of the lizard - - 84 63 and 64. Gastrulation of the opossum ----- 85 65-67. Gastrulation of the opossum - 86 68-71. Gastrulation of the rabbit - 88 72. Gastrula of the placental mammal 89 73. Gastrula of the rabbit - - 90 74 and 75. Diagram of the four secon- dary germinal layers - - 91 76 and 77. Ccelomula of Sagitta - 92 78. Section of young Sagitta - - 93 79 and 80. Section of amphioxus- larvae . ... 94 81 and 82. Section of amphioxus- larvae ----- 95 83 and 84. Chordula of the amphioxus 96 85 and 86. Chordula of the amphibia 96 87 and 88. Section of ccelomula-em- bryos of vertebrates - - 97 89 and 90. Section of ccelomula-em- bryo of Triton 97 91. Dorsal part of three Triton-em- bryos ... ... 98 92. Chordula-embryo of a bird - - 99 93. Vertebrate-embryo of a bird - 100 94 and 95. Section of the primitive streak of a chick - - - 101 / As T OF ILLUSTRA TIONS ru.r Section of the primitive groove of a rabbit ----- ior Section of primitive mouth of ■ human embryo ... 102 ioj. The ideal primitive vertebrate 106 10* Redundant mammary glands - us 104. A Greek gynecomast - - - 114 105. Severance of the discoid mammal embryo - - - - -116 106 and 107. The visceral embryonic vesicle - - - - - 1 is - Four en todermic cells - - - 119 hk). Two entodermic cells - - - 119 110-114. Ovum of a rabbit ... 120 115-118. Embryonic vesicle of a rabbit 121 in). Section of the gastrula of four vertebrates - - - - 123 120-123. Embryonic shield of a rabbit 125 124. Coslomula of the amphioxus - 127 125. Chordula of a fro^ - - - 127 uo. Section of frog-embryo - - 128 127 and 128. Dorsal shield of a chick 128 129. Section of hind end of a chick - 129 130. Germinal area of the rabbit - 129 131. Embryo of the opossum - - 130 132. Embryonic shield of the rabbit - 130 133. Human embryo at the sandal- stage 131 134. Embryonic shield of rabbit - - 131 135. Embryonic shield of opossum - 132 136. Embryonic disk of a chick - - 133 137. Embryonic disk of a higher verte- brate - - - - 134 138-142. Sections of maturing mammal embryo 135 143-146. Sections of embryonic chicks 136 147. Section of embryonic chick - 138 148. Section of fore-half of chick- embryo - - - - 139 149 and 150. Sections of human em- bryos 140 151. Section ofa shark-embryo - - 140 152. Section of a duck-embryo - - 141 153-155. Sole-shaped embryonic disk of chick - - - - - 143 156 and 157. Embryoofthe amphioxus 144 158-160. Embryo of the amphioxus - l6l and 162. Sections of shark-em- bryos - !<>,;. Section ofa Triton-embryo 164- id>«i. Wrtobrae - 1(17. Head of a shark-embryo 168 and 169. Head of a chick embryo 170. Head of a dog -embryo 171. Human embryo of fourth week - 172. Section of shoulder of chick-em- bryo 173. Section of pelvic region of chick- embryo - 174. Development of the lizard's le#s 175. Human embryo five weeks old - 176-178. Embryos of the bat 179. Human embryos - - 180. Human embryo of fourth week - 181. Human embryo of fifth week 182. Section of tail of human embryo - 183 and 184. Human embryo dissected [85. .Miss Julia Pastrana 186-190. Human embryos ... 191. Human embryos of sixteen to eighteen days - - - - 192 and 193. Human embryo of fourth week - 194. Human embryo with its mem- branes - 195. Diagram of the embryonic organs 196. Section of the pregnant womb - 197. Embryo of Siamang-gibbon 198. Section of pregnant womb - 199 and 200. Human foetus and pla- centa 201. Vitelline vessels in gcrminativc area ----.. 202. Boat-shaped embryo of the dog 203. Lar or white-handed gibbon 204. Young orang - 205. Wild orang ----- 206. Bald-headed chimpanzee - 207. Fcetal membranes and circula- tion ------ 208. Female gorilla - 209. Male giant-gorilla PACE '45 .46 '47 '47 148 149 '49 '5o '5' '52 '53 '54 '55 •56 '57 '58 '59 160 161 161 162 162 164 •65 166 167 168 169 170 171 172 173 174 '75 176 '77 ,78 LIST OF ILLUSTRATIONS 210. The lancelet - 21 1. Section of tbe beadoftbelAncelet 212. Section of an amphioxus-larva - 213. Diagram of preceding 214. Section of a young amphioxus - 215. Diagram of preceding 216. Transverse section of lancelel - a 1 7. Section through the middle of the lancelet - 218. Section of a primitive-fish embryo ------ 219. Section of the head of the lancelet 2J0-2.M. Organisation of an ascidia 1 223 224. Sections of young amphi- oxus-larva; - 225. An appendicaria - 226. Chroococcus minor - 22-. Aphanocapsa primordialis 2.'S. Protamaeba - 229. Original ovum-cleavage - 230. Morula ..... 231-232. Magospbasra planula - 233. Modern gastraeads ... 234-235. Prophysema primordiale - 236 J37. Ascula of gastropbysema - 238. Olynthus 231). Aphanostomum Langii 240-241. A turbellarian 2\2 243. Cbaetonotus - 244. A nemertine worm ... 245. An enteropneust 246. Section of the branchial gut 247. The marine lamprey - 24S. Fossil primitive fish - - - 24<). Embryo of a shark - 250. Man-eating shark ... 251. Fossil angel-shark 252. Tooth of a gigantic shark - 253~2.S5- Crossopterygii - 256. Fossil dipneust - - - - 257. The Australian dipneust - 258-259. Young ceratodus PACE 182 260 l82 261. 1S4 262 184 -'63 •85 264. 185 265. 186 266. 267. .87 268. .87 269 188 271. 89, 190 272. 273- '95 197 274, 209 275- 210 276. 211 277- 212 278- 212 214 283. 216 284. 217 285. 217 286. 218 287. 222 288. 223 224 289. 225 290. 2 -'5 29 r. 226 230 292- 231 294- 232 233 297. 234 298. 235 299. 236 300. 237 301. 237 302- 238 3°4- PACE 24O 241 242 243 245 246 247 248 . Fossil amphibian - . Larva of the spotted salamander . Larva of common frog . Fossil mailed amphibian - . The New Zealand lizard - . Homceosaurus pulchellus . Skull of a Permian lizard - . Skull of a theromorphum - . Lower jaw of a primitive mammal -270. The ornithorhyncus - . Lower jaw of a promammal . The crab-eating opossum - . Foetal membranes of the human embryo ----- , Skull of a fossil lemur . The slender lori ... . The white-nosed ape The drill-baboon - -282. Skeletons of man and the anthropoid apes ... Skull of the Java ape-man - . Section of the human skin - Epidermic cells . Rudimentary lachrymal glands The female breast * Mammary gland of a new-born infant , Embryo of a bear ... Human embryo .... Central marrow of a human embryo - -293. The human brain - 274-275 -296. Central marrow of human embryo Head of a chick embryo - Brain of three craniote embryos Brain of a shark ... , Brain and spinal cord of a frog Brain of an ox-embryo -303. Brain of a human embryo 278-279 Brain of the rabbit ... 279 249 250 25' 252 254 255 256 257 258 260 263 267 268 269 270 271 272 273 273 2-6 277 2/7 277 278 278 LIST < )/•" //. /. I rS TRA TIONS ru.i 305. Head of 11 shark - - - 281 3i>o 31a Heads of chick-embryos - s8a 311. Section of mouth of human embryo ----- 183 31a. Diagram of mouth-nose cavity - 284 313 314. Heads of human embryos - 285 315 ;,k>. Face of human embryo • 286 317. The human eye - ... 287 Eye of the chick embryo - - 288 319. Section of eye of a human embryo 288 320. The human ear .... 289 321. Tin- bony labyrinth - 289 322. Development of the labyrinth - 290 Primitive skull of human embryo 291 324. Rudimentary muscles of the ear 291 325-326. The human skeleton - - 293 327. The human vertebral column - 294 328. Piece of the dorsal cord - - 294 329-330. Dorsal vertebrae - - 295 331. Intervertebral disk - 296 332. Human skull - 296 333. Skull of new-born child - - 297 334. Head-skeleton of a primitive fish 297 335. Skulls of nine primates - - 298 336-338. Evolution of the fin - - 299 339. Skeleton of the fore-leg of an amphibian .... 300 340. Skeleton of gorilla's hand - - 300 341. Skeleton of human hand - - 300 342. Skeleton of hand of six mammals 301 343-345. Arm and hand of three anthropoids ... - 302 346. Section of fish's tail - - - 303 347. Human skeleton - - - 305 Skeleton of the giant gorilla - 305 The human stomach - 309 Section of the head of a rabbit- embryo ----- 310 Shark's teeth - - - - 311 Gut of a human embryo - - 312 3'3 348. 349- 35°- 35'- 352- 361. 362. 353-354. Gut of a dog embryo 355 .0'' Sections of head of lamprey 357. Viscera of a human embryo 358. Red blood-cells - 359- Vascular tissue .... Section of trunk of a chick* 1 mbryo - Merocytes - Vascular system of an annelid - 363. Head of a fish-embryo 364-370. The five arterial arches 371-372. Heart of a rabbit-embryo - 373-374- Heart of a dog-embryo' 375-377- Heart of a human embryo - 378. Heart of adult man - 379. Section of head of a chick- embryo 380. Section of a human embryo 381-382. Sections of a chick-embryo 383. Embryos of sagitta - 384. Kidneys of bdellostoma 385. Section of embryonic shield 386-387. Primitive kidneys 388. Pig-embryo . . . . 389. Human embryo 390-392. Rudimentary kidneys and sexual organs ... 393-394. Urinary and sexual organs of salamander ... 395. Primitive kidneys of human embryo 396-398. Urinary organs of ox-em- bryos - 399. Sexual organs of water-mole 400-401. Original position of sexual glands 402. Urogenital system of human embryo - - - - - 403. Section of ovary ... 404-406. Graafian follicles- 407. A ripe Graafian follicle 408. The human ovum PACE 3'4 • 3>5 3»9 3'9 320 321 32- 322 325 325 326 3-6 327 328 329 333 334 335 336 337 337 33* 338 339 34° 34' 342 342 343 343 344 345 GLOSSARY ACRANIA : animals Without skull (cranium) ANTHROPOGBJU : tin- evolution (gtntsis) ot man ( antliropos) ANTHROPOLOGY : the Science of man Archj- : (in compounds) the first of typical —as, archi-o tula, archi-gastrula, etc. Biookmv : the science of the genesis of life (bios) Blast- : (in compounds) pertaining to the early embryo (blasfos — a bud) ; hence : — Blastoderm : skin (derma) or enclos- ing layer of the embryo Blastosphere : the embryo in the hollow sphere stage Blastula : same as preceding Epiblast : The outer layer of (he embryo (ectoderm) Hypoblast : the inner layer of the embryo (entoderm) Branchial: pertaining to the gills ( branchia ) Caryo- : (in compounds) pertaining to the nucleus ( caryon) ; hence ; — Caryokinesis : 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 Cuclom or Cceloma : the body-cavity in the embryo ; hence : — Ccelenterata : animals without a body- cavity Coclomaria : animals with a body- cavity Cnplomation : 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 CryptorCHLSM : abnormal retention of the testicles in the body Deutoplasm : see Pi. asm 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 Eimderm : the outer layer of the skin Epigenesis : the theory of gradual develop- ment 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 primitive vertebrate Hyposoma-: see Soma GLOSSARY Lkii in- : pertaining to the yelk ( lecithus) ; hence i CentroSecHhal : eggs with the yelk in the centre Lecithoma : the yelk-sac Tclolecitbal : eggs with the yelk at one end Mi ROBLASTIC: cleaving in part ( mcron ) only Ml \ \- -. (in compounds) the "after" or secondary stage ; hence : — Metagastcr: the secondary or perma- nent gut (twister) Metaplasm : secondary or differentiated plasm Metastoma : the secondary or perma- nent mouth (stoma) Metazoa : the higher or later animals, made up of many cells Metovum : the mature or advanced oyum Mbtamera: the segments into which the embryo breaks up Mi i ami eusm : the segmentation of the em- brj o MONERA : the most primitive of the uni- cellular 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 NEPltRA : the kidneys ; hence : — Nephridia : the rudimentary kidney- organs Xephrotomes : 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 : cellsthatabsorb food (p/iagcin = to eat) Phylogf.ny: the science of the evolution of species f phyla ) Planocytls : cells that mov'e about ( plane in ) Plasm \ the colloid or jelly-like matter of which organisms are composed ; hence : — Caryoplasm : the matter of the nucleus (cat yon ) Cytoplasm : the matter of the body of the cell Deutoplasm : secondary or differen- tiated plasm Metaplasm • same as preceding Protoplasm : primitive or undifferen- tiated plasm PLASSON : the simplest form of plasm PLASTID! i. is : small particles ol 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 : — l'rochorion : the first Form of the chorion Progaster : the first or primitive stomach Pronephridia : the earlier form of the kidneys Prorenal : same as preceding Prostoma : the first or. primitive mouth Protists : the earliest or unicellular organisms Provertebrae : the earliest phase of the vertebrae Protophyta : the primitive or unicellular plants Protoplasm : undifferentiated plasm Protozoa : the primitive or unicellular animals Renal : pertaining to the kidneys (rents) 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 (cylos) 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 Teleqlogy : the belief in design and purpose ( ielos) in nature/ Telolecithal : see Lecith- UMBILICAL: pertaining to the navel (um- bilicus) VITELLINE : pertaining to the yelk (vi/ellits) P R E F A C E [By Joseph McCabb] 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 German)- 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 ot shame on t-he 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 illustra- tions. When it was issued in Germany, a few years ago, a distinguished biologist wrote in the Frankfurter Zeitang that it would secure immor- tality 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 ix * x PREFACE 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 o\' 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 foetus 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. Hacckel 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 rhus't 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: (i) 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 PREFACE xl 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 b\ 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 form, 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 his 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 greal American zoologist, President 1"). Starr Jordan : " It is, of course, true that the life-history of the individual is an epitome of the lite-histois 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. Hut the reproduction of ancestral forms in the course of the embryonic development is by no means always clear, or even always present. M iny of the embryonic phases t\o not recall ancestral stages at dl. 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. Vor the moment, I would impress on the reader the vital importance |>f fixing the distinction from the start. He must thoroughly familiarise limseli 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 J, or the science lescribing it. Roughly, ontogeny may be taken to mean embryology, and phytogeny what we generally call evolution. Further, the embryonic phenomena sometimes reproduce ancestral forms, and they are then jailed palingenetic (from palin - again) : sometimes they do not recall incest ral forms, but are later modifications due to adaptation, and they are .hen called cenogenetic (from kenos - new or foreign). These terms are low 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, PREFACE 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 oi 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 sub-divides 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 oi 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 oi' two layers oi' cells (two "germinal layers"). This is the gastrula (stomach) stage, and the process oi its formation is called ga.strulg.tion. A glance at the illustration on p. (>[ will make this perfectly clear. So much for the embryonic process iii 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 oi' ancestral forms. This is the work oi Chaps. 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 oi' 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 the}' all had a common ancestor ol' that nature. To this conjectural ancestor (it lived before the period oi' fossilisation begins) Haeckel gives the name of the Gastrata, and in the second volume we shall see a number of living animals of this type (" gast rapids "). The line oi' 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 Cwlomula. 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 PREFACE xiii middle skin) is formed, hv the growth inwards ol two ppuches or folds of the skin. The pouches blend together, and form a single cavity (the body cavity, or ccelom), 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 ( 'ce/onuea. The remaining task is to build up the complex frame of the higher animals always showing the identity ol the process (on which the evolutionary argument depends) in enormously different conditions of embryonic life out of the four "germinal layers." chap. IX. prepares US for the work by giving us a very clear account ol' the essential structure ol' the hack-honed (vertebrate) animal, and the probable common ancestor of all the vertebrates (a small fish ol' the lancelel type). Chaps. 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 he fashioned out ol the four simple layers that the reader must proceed carefully. In the second volume each oi' these organs will he dealt with separately, and the parallel will he worked out between its embryonic and its phylogenetic (evolutionary) development. The general reader may wait for this for a full understanding. Hut in the meantime the wonderful story ol' the construction ol' all our organs in the course ol a tew 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 ol' interest. It would he useless to attempt to summarise the process. The four chapters are themselves hut 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 lihrary 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 ho can consult the library edition. He must remember, too, that the present volume is only half the work. A second and longer volume, illustrated with equal generosity, will shortly be issued. This second volume will endeavour to trace the line of man's ancestry from the primeval microbe right up to the ape-man of Java, in a long series of chapters, and with illustrations ol' every step, and will also deal separately with the evolution ol' each set of organs in the body. A glossary will be found at the beginning of each volume, and an index to the two volumes will he printed at the und ol the second volume. Joseph McCabe. Cricklewood, March, hjub. HAECKEL'S CLASSIFICATION OK THE ANIMAL WORLD Unicellular animals (Protozoa) I. I'nnuclratod 2. Nucleated 3. Cell-colonies t Bacteria \ Protanuebse !a. Rhi/opoda b. Infusoria I Catallacta \ Blast seada J Moncra I Amoebina \ Radiolaria { Flagellate | Ciliata Multicellular animals (Metazoa) I. Coelenteria, Ccelenterata, or Zoophytes. Animals without body-cavity, blood or aims. II. Ccelomaria or Bilaterals. Animals with body- cavity and anus, and generally blood. a. Gastrajads b. Sponges r. Cnidaria (stinging animal d. Platodes ^ (flat-worms) a. \Termalia (worm-like) b. Molluscs c. Articulates d. Echinoderms c. Tunicatcs. 1 Gastrcmaria 1 Cyemai i.i I Protospongia t Metespongise l Hydrozoa A Polyps ' (Medusae fPlatodaria I Turbcllarici j Trematoda (.Cestoda (Rotatoria Strong-ylaria Prosopygia Frontonia /Cochlides I Conchades (Teuthodes /-Annelida -J Crustacea ( Trachea t a Monorchonia I Pentorchonia rCopelata '. Ascidia; whalidiae ,1. Acrania-Lancelet (without skull) II. Craniota (with skull) a. Cyclostomcs ("round-mouthed' /. Vertebrates < b. Fishes Amphibia Reptiles Buds f. MaminaJ fSelachii I danoids I Teleosts ' Dipncusts 'Monotremes Marsupials Placentals :- Rodents Edentates Ungulates Cetacea Sircnia [nsectivora Cheiroptera Cai nassia Primates * This classification is given for tlic purpose of explaining Haeckei'a use of terms in this volume. The general reader should hear in mind lli.it it differs very considerably from more rrrcnt schemed of damnification. IK should compare the scheme framed In Professor E. Ray Lankcsler. THE EVOLUTION OF MAN Chapter I. THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION The field of natural phenomena into which I would introduce m\ readers in the following chapters has a quite peculiar place in the broad realm of scientific inquiry. There is no object of investiga- tion that touches man more closely, and the knowledge of which should be more acceptable to him, than his own frame. Inn among all the various branches of the natural history of mankind, or aiithro- Pology, 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 impor- tant questions arc 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 astonish- ing 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 impor- tant truths and remarkable phenomena which anthropogeny teaches us. As an illustration of tin's 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 on<: simple cell, like any other plant or animal et^'. They arc equally ignorant that in the course of the development of this liny, round egg cell there is first formed a body that is totally different from the human frame, and lias not the remotest resemblance to it. Most of them have never seen such a human embryo in the earlier period of its develop- ment^ and do not know that it is quite indistinguishable from other animal em- bryos. 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 ^i^v;. 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 remark-, able changes through which man passes during his embryonic life should arouse considerable interest. But the mind will experience a far keener satisfaction when THE FUNDAMENTAL /..III' <)/■' Oh'CW/C EVOl f /VOX w< trace these curious facts to their -. and when we learn to behold in them natural phenomena which are of the highest importance throughout the whole held of human knowledge. They throw light first of all on the "natural history of creation," then on psychology, or "the science of the soul," ana through this on the whole of philosophy. And as t ht* 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 lc->s by the study of Hie evolution of man. But when 1 say that I propose to present here the most important features pf these phenomena and trace them to their causes, 1 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 1 must not give here this special description of the embryonic pro- cesses such as it has hitherto been given, as most of my readers have not studied anatomy, and are not likely to be en- trusted with the care of the adult organism. 1 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 arc found in the story of the development of the hum. m 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 held of inquiry as has been excited already in other brandies of ice ; though we shall meet more les 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 o\ the science of the development of the individual human organism. But this i- really only the first part of our task ..the first half of the story of the evolution of in that wider sense in which we understand it here. We must add as the six ond half as anothei and not less important and interesting branch oi the science of the evolution of the human stem phytogeny; 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 counties-, ages. Everybod) 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 publica- tion 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 " Phylogcny " 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 embry- ology, 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 morpholog r. These two branches of our science — on the one side ontogeny or embryology, and on the other phytogeny, 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 pro- found, 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 foetus i> a recapitulation of the history of the race "j or, in other words, "Ontogeny is a recapitulation of phylogcny." It may be more fully staled 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 THE FUND I MENTAL /..I II' OF ORGANIC EVOLUTION of the long series of forma which the animal ancestors of the said organism, or the ancestral forms of the species, have passed through from the earliest period of ink life down to the present day. The causal character of the relation which coQoocts embryology with stem- history i» due to the action of heredity and adaptation. When we have rightly understood these, and recognised their great importance in tin.- formation of organisms, we can go a step further and sa) : Phylogenesis is the mechanical cause of ontogenesis.' 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 t hanges which appear in a condensed form in the evolution of the foetUS. The chain of manifold animal forms which represent the ancestry of each higher organism, or even of man, accord- ing to the theory of descent, alw.u S form a connected whole. We may designate this uninterrupted series of forms with the letters of the alphabet : A, B, C, I), E, eu., 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 emhrvonic forms would run : A, B, I), F, H, K, M, etc.; or, in other cases, H, I), 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 oi\>: 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 I), we often have the Greek I'.and ^. In this case the text of the biogenetic law has been corrupted, jusl as it had been abbreviated ill the pre- ceding i But, in spite of all this, the serks of ancestral forms remains the same, and we are in a position to discover iginal complexion. In reality, there is always a certain parallel between the two evolutionary But it is obscured from the fad ■ The term ••genesis," which occurs throughout, TWt, "birth" or origin. From this we get: Hi origin of Km (t>i,^j: Anthro- tin origin ot in. m ( a nth whoa) ; Onto^env the origin of the individual ( mi J ; 1'hylogi ■ origin of ( - ( phulon ) ; and so on. fn each for to the process itsclt, or to the I I' CNS that in the embryonic succession much is wanting that Certainly existed in the earlier ancestral succession. If the parallel of the two series wc i e complete, itnd if this great fundamental law affirming the causal connection between ontogeny and phytogeny in the proper sense of the v were directly demonstrable, we should only have to determine, by means of the microscope and the dissecting knife, the ■m i ies of forms through which the ferti- lised 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 SUO sivel) assumed from the dawn of organic life down to the appearance of man. But such a repetition 01 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 distoi ted and falsified by causes which we will consider later. We are thus, for the most part, unable to deter- mine in cfetail, 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 huntan foetus — 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 emblyological 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 tew examples, we can infer from the fact that the human ovum is a simple cell that the first ancestor of our species was a tinv unicellular being, something like the amoeba. In the same way, we know, from the fact that the human foetus consists, at the first, of two simple cell-layers (the /rastrula), that the grutrtza, a form with two such layei was certainly in the line of our ancestry. A later hum. m embryonic form (the chordula) points just as clearly to a worm- like ancestor (the ptvchortUmia), the nearest living relation of which is found among the actual ascidia?. To this suc- ceeds a most important embryonic stage uu'a), in which our headless fcetus THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION presents, in the main, the structure of the Uvncelet. But we can only indirectly and approximately, with the aid of com- parative anatomy and ontogeny, conjec- ture what lower forms enter into the chain of oui ancestry between the gastraea and the chordula, and between this and the lancelet. In the course of the historical development many intermediate struc- tures nave 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 pur- pose of this work to prove the real har- mony and the original parallelism of the two. 1 hope to show, on a substantial basis of facts, that we can draw most important conclusions as to our genea- logical 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, palin- genetic (or ancestral) evolutionary pro- cesses and those due to cenogenesis." 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 embry- onic variations, to all those phenomena in the foetal development that cannot be traced to inheritance from earlier species, but are due to the adaptation of the fcetUS, or the infant-form, to certain conditions of its embryonic development. These ceno- genetic phenomena are foreign or later additions ; they allow us to draw no direct inference whatever as to corre- sponding processes in our ancestral • Palingenesis - new birth, or re-incarnation (/W/m — again, genesis or gttuu - development) j hence its application to the phenomena which are recapitulated by heredity troni earlier ancestral forms. Cenogenesis = foreign or negligible development ( benos andfAMfl I : hence, those phenomena which come later in the itoi v of life to disturb the inherited structure, by a fresh adaptation to environment.— Trans. history, but rather hinder us from doing so. This careful discrimination between the primary or palingenetic processes and the secondary or cenogenetic is of great importance for the purposes of the scien- tific 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 ofevolutioil as the careful distinc- tion between genuine and spurious texts in the works of an ancient writer, or the purging of the real text from interpola- tions 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 recapitu- lated 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 medul- lary tube and the gut, the temporary for- mation of the gill-clefts and arches, the primitive kidneys, and so on.' 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 indica- tions 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 foetal membranes and the cor- responding changes in the blood vessels. Further instances are : the dual structure of the heart cavity, the temporal'}- division of the plates of the primitive \ei tchr.e atitl ■ All (hotc, ruul the following structures, will be fully described in later chapters.— TRANS. THE FUNDAMENTAL /..III' OF ORGANIC EVOLUTION lateral plates, the secondary closing oflhe \ciitral and intestinal walls, the formation o\' the navel, and so on. All these and mam other phenomena arc certainly not traceable to similar structures in any earlier and completely-developed ancestral form, but have arisen simply by adaptation to tin. peculiai conditions of embryonic lift (within the foetal membranes). In view o\ these facts, we may now give the following more precise expression to our chief law of biogeny : The (.'volution i>f the toius (or ontogenesis) is a condensed and abbreviated recapitulation ol' the evo- lution o\ 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 4ess complete in proportion as a varying adaptation to new conditions. increases the disturbing factors in the development (or cenogenesis). The ccnogenetic alterations 01 distor- tions of the original palingenctic course of development lake the form, as a rule, of a gradual displacement ol the phenomena, which is slowly effected by adaptation to the changed conditions of embryonic existence during the course ol thousands o\ years. This displacement may take pi. ice as regards either the position or the time oi ,. phenomenon. The great importance and strict regu- larity of the lime-variations in embryology have been carefully studied recent I v by Ernest Mehnert, in his Biomeehanik (Jena, iS<>X). He contends that our biogenetic law has not been impaired by the attacks ol its opponents, and goes on to say : " Scarcely any piece oi knowledge has Contributed so much to the advance oi 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 o\' investigators, and they had accustomed themselves to see a reminiscence oi ancestral history in embryonic structures, that we witnessed the great progress which embryological research has made in the last two decades." The best proof oi the correctness ol' this opinion is that now the most fruitful work is done in all brant hes oi embryology with the aid oi 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 palingenctic, and when one takes careful account oi the changes which the latter may suffer from the former, that the radical importance oi the biogenetic law is recognised, and it is felt to be the mOSl illuminating principle in the scieno ,| evolution. In this task oi discrimination it is the siher 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 oi forms. Hence the brothers Sarasin, the zoologists, iould s.i\ with perfect justice, in their study of the evolution of the Fchthyophis, that " the great biogenetic law is just as important for the zoologist in tracing long-extiner processes as spectrum analysis 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 astonish- ment was felt at the remarkable similarity observed between the embryonic forms, or stages oi foetal 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 oi 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 ol' descent ; it is this that puts in their true light the action ol' heredity on the one hand and adaptation On the Other. It explains to us the vital importance ol' 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 cease- less struggle lor existence between living things, and to show how, under the influence oi this (by natural selection), new species were produced and maintained solely by the interaction ol' heredity and /'///•: FUNDA MENTAL LAW OF ORGANIC EVOLUTION adaptation. It was thus Darwinism thai tiiM opened our eyes to a true comprehen- sion 01 the supremely important relations between the two parts of the science oi organic evolution Ontogeny and Phy- togeny. I leredity and adaptation are, in fact, the t wo constructive pli\ siological functions ol living things ; unless we understand these propcrl) we can make no headway in the study of evolution. Hence, until the time oi Darwin no one had a clear idea of the real nature ana1 causes of embryonic development. It was impossible to explain tho curious series of forms through which the human embryo passed ; it was quite unintelligible why tins 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 tin' ovum, and that the development consisted only in an unfolding oi the various parts, a simple process oi growth. This is by no means the > On the contrary, the whole process of the development oi the individual presents to the observer a connected succession of different animal-forms; and these forms display a great variety oi external and internal structure. But why each indi- vidual human being should pass through this series oi forms in the course of his embryonic development it was quite im- possible 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, oi 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 In- efficient cause-. We -hall see, ill the Course oi our inquire, how the most wonderful and hitherto insoluble enigmas in the human and animal frame have proved amenable to a mechanical explana- tion, 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, purp" causes.' ■ The monistic or mechanical philosophy of nature holds that only unconscious, necessary, efficient cause* arc at work in the whole field of nature, in organic life If the new science of evolution had done no more than this, even thoughtful man would have to admit that it had accom- plished an immense advance in knowledge. It means th.it in the whole oi philosophy that tendency which we call monistic, in opposition to the dualistic, which has hitherto prevailed, must be accepted.' At this point the science oi human evolution has a direct and profound bearing i>\\ the foundations oi philosophy. Modern an- thropology has, by its astounding dis- cos cries during the second half oi the nineteenth century, compelled us to take a completely monistic view oi life. Our bodily structure and its life, our embry- onic 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, indispen- sable, 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. The facts of embryology have so great and obvious a significance in this connec- tion that even in recent years dualist and Ideological 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 cj;^ 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 oi the theological journals as a dishonest invention oi my own. The fact that the embryos oi man and the doi^ 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 oi its develop- ment, we find it to be quite different in shape and structure from the full-grown human being, but almost identical with that oi the ape, the dog, the rabbit, and as well as in inorganic changes. On tin- other hand, the dualist or vitalist philosophy o( 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. 1 Monism is neither purely materialistic nor purely spiritualistic, but a reconciliation of these two prin- ciples, since it regards the whole of nature as one, and srrs 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. Cl. The Riddle of the Universe, chap. xii. THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION other mammals, at the same Btage of ontogeny. We find a bean-shaped body of verj simple construction, with a t.iil below and a pair of tins at the 9ides, something like those of a t"l -1 1 , 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 withoul face, at the sides of which we find gill-clefts and arches as in the tish. ,\i this stage of its development the human embryo docs not differ in any essentia] 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, 1 sought to deny them There could not be a clearer proof of the profound importance of these embryo- logical tarts 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 .ire 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 -ays in his Mali's Place in Nature: "Though these facts are ignored by several well- known popular leaders, they are easy to prove, and ;fre accepted by all scientific men ; on the other hand, the'- 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 1 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 ivuuii'. This intimate connection of strui ture and function, or of the instrument and the work JioiM: by it, is .seen in the science of evolution and all its parts. Hence the story of the evolution ot structures, which is our immediate con- cern, is also the histoiv of the develop- ment of functions ; and this holds good of the human organism as nf any other. At the same time, I must admit that out- knowledge of the evolution of functions is very far from being as complete as our acquaintance with the evolution of struc- tures. One might say, in fact, that the whole science of evolution1 has almost confined itself to the study of struc- tures ; the evolution of Junctions hardly exists even in name. That is the fault of the physiologists, who have as yet con- cerned themselves very little about evolu- tion. 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 stud) of the evolution of functions with the same seal 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 ascidiae and Other low organisms ; with this is associated a very simple system of circu- lation 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 struc- ture-development, most important infor- mation 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 hodv the medium of sensa- tion, will, and even thought, the highest of the psychic functions; in a word, of THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION 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 nen ous system, or the internal texture of the brain and spinal cord. In those we find the elabo- rate cell-machinery, of which the psychic or soul-lite is the physiological function. It is so intricate that most men still look upon the mind as something supernatural that cannot he explained on mechanical principles. But embcyological research into the gradual appearance and the format ion of this import. mt system of organs yields the most astounding and significant results. The fust 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 hack, and from this first comes a simple spinal cord without brain, such as we hud to he 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 devclopes 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 distin- guishes the apes from the other verte- brates, and which attains its full develop- ment in man. Comparative physiology discovers a precisely similar growth. The function of the brain, the psychic activity, rises step by step with the advancing develop- ment of its structure. Thus we are enabled, by this story of the evolution of the nervous system, to understand at length the natural develop- ment 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. Ilappih our embi \ ologii al knowledge of man's central nervous System is now so adequate, and agrees SO thorough!) vi ith the complementary results of Comparative anatomy and physiology, that we arc thus enabled to obtain a clear insight into oiw ot 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 embryo- logical study of it, or the ontogeny of the soul. This important section of psycho- logy owes its origin especially to W. Preyer, in his interesting works, such as The Mind of the Child, /'he Biography of a Baby (1900), of Milicent Washburn Shinn, also deserves mention. [See also Preyer's Mental Development in tin' Child (translation), and Sully S Studies of Child- hood and Children 'x 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 gastraa-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 THE OLDER EMBRYOLOGY (act is thai these embryological pheno- mena 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 physio- logical functions Of heredity and adapta- tion. Chapter II. THK OLDF.R K.MIiKYOUX'.V I r is in many ways useful, on entering upon the stud) of .\n\ science, to cast a glance at its historical development. The saying that "everything is Ivst under- stood in its growth " lias a distinct appli- cation to science. While we follow its gradual development we get a clearer in- sight into its aims and objects. Mou over, we shall see that the present condition of the science of human evolution, with all its characteristics, can only be rightl) understood when we examine its historical growth. This task will, however, not detain us long. The study of man's evo- lution is one of the latest branches of natural science, whether you consider the embryological or the phylogenctic section o( it. " Apart from the few germs of our science which we find in classical antiquity, and which WC shall notice presently, we may that it takes its definite rise, as a science, in the year 1750. when one of the greatest German scientists, Caspar Friedrich Wolff, published his Theoria genertltionis. That was the foundation- stone of the science of animal embi \ ology. It was not until fifty years later, in 1809, that Jean Lamarck published his Philo- sophie Zoologique the first effort to pro- vide a base for the theory of evolution ; and it was another lialf-ccntiirv before Darwin's work appeared (in 1859), which w< may regard as the first scientific attainment of this aim. Bu1 before we go further into this solid establishment o( 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 2,000 years : the "father of natural histor) ," Aristotle. The extant scientific works of Aristotle dial with many different sides of bio- logical research ; the most comprehensive of them is his famous History of Animals. Hut not less interesting is the smaller work, On the Generation of . \nimah (Pen toon geneseos) This work treats espe- cially of embryonic development, and it is of great interest .is being the earliest ol its kind and the only o\\o that has come down to us in any completeness from classical antiquity. Aristotle studied embryological ques- tions ill various classes of animals, and among the lower groups he learned many most remarkable facts which we only re- discovered between 1830 and i860. It is certain, for instance, that he was acquainted with the very peculiar mode of propagation of the cuttle-fishes, or CCphalopods, in which a yelk-sac hangs out of the mouthof the fcetUS. 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 distin- guished 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 tisbes of the shark family is attached to the mother's body by a sort of placenta, 01 nutriti\e organ very rich in blood ; apart from these, such an arrangement is on!) found among the higher mammals and IO THE OLDER EMBRYOLOGY man. This placenta of the shark was looked upon as legendary Cor a longtime, until Johannes Mullet- proved it to be a Cut in 1839. Thus a number of remark- able discoveries were found in Aristotle's embryological work, proving a very good acquaintance of the great scientist pos- sibly helped by his predecessors with the Cuts of ontogeny, and a great advance upon succeeding generations in this respt 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 remai Us arc of particular interest, because they show a correct appreciation of the nature of the embryonic processes, lie conceives thedevelopment 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, ov separately in an et,rt,r, the heart -which he regards as the starting-point and centre of theorganism must appear first. Once the heart is Conned the other organs arise, the internal ones before the external, the upper (those above the diaphragm) before the lowcr(orthose beneath thediaphragm). The brain is formed at an early stage, and the eyes grow out of it. These observa- tions are quite correct. And, if we try to Conn some idea from these data of Aristotle's general conception oC the em- bryonic process, we find a dim prevision of the theory which Wolff showed 2,000 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 01 genus, the group of similar individuals, might be eternal, but the individual itself is temporary. Ft comes into being in the act of procreation, and passes away at death. During the 2,cx/> years after Aristotle no progress whatever was made in general zoology, or in embryology in particular. People were content to read, copy, trans- late, 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 OlStrong religious beliefs put formidable obstacles in the way of independent scientific inves- ition. There was no question of resuming the advance oC 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 em- bryo, and its development. There were many reasons lor the prevailing horror of such studies. It is natural enough, when we remember that a Hull 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 anato- mist, Fabricius ab Aquapendente, a pro- fessor at Padua, opened the advance. In his two books (/)c formato fcetu, 1600, and De formatione foetus, 1604} he pub- lished the older illustrations and descrip- tions of the embryos of man and othei mammals, and of the hen. Similar imperfect illustrations were given by Spigelius (De forrtato fectu, 1 C> 3 1 > , 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 lite 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. Hut the most important embryological studies in the sixteenth century were those of the famous Italian, Marccllo Malpighi, of Bologna, who led the way both in zoology and botany. His treatises, De format ione ptilli and De 0V0 in< ubato (1687), contain the first consistent description of the development of the chick in the fertilised egg- THE OLDER EMBRYOLOGY Hire 1 ought to sa\ .1 word about the important part played b) the chick in the growth of our science. The development of the chick, like that of the young of all other birds, agrees in .ill it-- 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, tortoise--, i 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 impos- sible 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 there- of man). As soon as scientists began ituJy the human embryo, or the mammal-embryo generally, in its earlier Stages about the middle and end of the tlteenth 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. Hut for the* practical purposes of embryo- logical research the discovery is invalu- able, because we can fill up the gaps in our imperfect knowledge of the embryo- logy 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 et,ri,r, 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 .1- far .is it possible to do with the imperfect micro- scope of his time in the embryological study of the (hick. 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 M.ii;is that you cannot go very far into the study of them without a good microscope and other technical aid. But this substantial im- provement of the mil row Ope and the other apparatus did not take place until the beginning of the nineteenth century. embryology made scarcely any advance in the fust half of the eighteenth century, when the Systematic natural histor plants and animals received so great an impulse through the publication of Linne's famous Syslema Naturae. 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 empi- rical 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 evolu- tion theory "; it is better to describe it as " the preformation theory.'" Its chief point is this : There is no new formation of structures in the embryonic develop- ment of any organism, animal or plant, or even of man ; there is only a growth, or unfolding, of parts which have been con- structed 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 ci^^; not merely a simple cell, that divides and sub- divides and forms germinal layers, and at last, after all kinds of variation and cleavage and reconstruction, brings forth ' This theory is usually known as the "evolution theory" in Germany, in contradistinction to the" " cpi- ecnesis theory." Hut as it is the latter that is called the " evolution theory " in England, France, and Italy, and "evolution" and "epigencsia are taken to be synonymous, it set tns better to call the fint the " pre- formation theory." II THE OLDER EMBRYOLOGY iIk> body of the chick ; bul there is in egg from the first ■ complete chicken, with all its parts made and neatly packed. These parts arc so sm ill or so transparent that the microscope cannot detect them. In the hatching, these parts merely grow larger, and spread out in the not in il w a\ . When this theory is consistently deve- loped it becomes a " scatulation theory."1 According to its teaching, there was made in the beginning one couple or one indi- vidual of each species of animal or plant ; hut this one individual contained the germs of all die 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 he fixed by the Bible at 5,000 or 6,000 years, it seemed possible to calculate how many individuals of c uli 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. 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, Leeuwen- boek, discovered the male spermatozoa in 1690, and showed that an immense num- ber of these extremely fine and mobile thread-like beings exist in the male sperm (this will be explained in the seventh chapter). 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 sup- posed to penetrate into the fertile body of the ovum and begin to develop there, as the plant seed docs 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 ' " Packing theory " would be the literal translation, Scatula is the Latin for a case or box.— Trans. ovum. This theory, also, was consistently developed in the sense that in each of these thread-like bodies the whole ofits posterity was supposed to he present in the minutest form. Adam's se\ual 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 owcu 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 sperma- tozoa 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 con- ception 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 Gottingen, who is often called the r * her, of physiology, was a man of wide a> 1 rjed learning, but he docs not occupy a e; v high position in regard to insight into natural pheno- mena. He made a vigorous defence of the "evolution theory "in his famous work, Elementa ph vsiojogtae, affi rming : ' ' There is no such thing as formation (nulla est epigenesis). No part of the animal frame is made he Tore 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 6,000 years, the average age of a human being to be thirty years, and the population of the world at THE OLDER EMBRYOLOGY 13 that time to be 1,000 millions. And the famous Haller maintained .ill this non- sense, in spite of its ridiculous conse- quences, even after Wolff had discovered tli.- re d 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," an J by his authority and literary prestige won many adherents to if. Supported by his system ofmon ids, according to which body and soul are united in inseparable association and by their union form the individual, or the " mon id," Leibnitz con- sistently extended the " scatulation theory" to the soul, and held that this was no mon- evolved than the body. He says, for instance, in his Thiodicie: "I mean tli it these souU, 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 hack as Adam, or from the beginning of the world, in the forms of organised bodies. " The theory seemed to receive consider- able supporl from the observations of one of its most ze dous supporters, Bonnet. In 1745 he discovered, in the plant-louse, a case of parthenogenesis, or virgin-birth, an interesting form of reproduction that has latch been found by Siebold and others among various classes of the articulata, especially cr 'tacefl arid insects. Among these and ot al inals 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 rigidlv 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 d lys ninety- four other daughters ; and that all of them went on to reproduce in the same wav without any contact with males. It seemed as if this furnished an irrefutable proof of the tr-uth 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 vv.es the condition of things when suddenly, in ij$q, Caspar Friedrich Wolff appeared, and dealt a fatal blow at the whale 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 studios, 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 disserta- tion (November 2X1 h, 1750), the Theoria generationis, expounded the new theory of a real development on a basis of epi- genesis. This treatise is, in spite of its smallness and Its obscure phraseology, oiw 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-re u hing and pregnant ideas which the author e\ ei \ wh ■, c extracts from his obser- vations and builds into a luminous and accurate theory of generation. Neverthe- less, it met with no success at the time. Although scientific studies were then assiduously cultivated owing- to the im- pulse given by Linnd— although botanists and zoologists were no longer counted by dozens, but by hundreds, hardly any notice was taken of Wolffs theory. Even when he established the truth of epigenesis by the most rigorous observations, and de- molished the airy structure of the prefor- mation 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 physiolo- gists 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 inno- vation. 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 Funnation of the Alimentary ("anal (written in 1768), and called attention to its great importance, did people begin to think of him once more ; vet this obscure writer had evinced a profou -der 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 M THE OLDER EMBRYOLOGY century. He opened up the true path for irch in many directions. In the first place, his th« >i v of epigenesis gave us our first real insight into the nature of embryonic development. He showed con- vincingly that the development of every organism consists of a series ol 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 aUo quite different, in its internal arrange- ment 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 eon- Vinced it is a fact, and can demonstrate it at any moment with the aid of the micro- m rope. Wolff furnished the conclusive empirical proof of his theory in his classic disserta- tion on The Formation of the Alimentary Canal (1768). In its complete state the alimentary canal of the hen is a long and j 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 what- ever 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 o\' a flat, oval-shaped leaf. When we remember how difficult the exact obser- vation of sci tine and delicate a structure as the early leaf-shaped body of the chick must have been with the poor micro- scopes then in use, we must admire the rare faculty for observation which enabled Wolff to make the most important dis- coveries in tills most difficult part of embryology. By this laborious research he reached the correct opinion that the embryonic body of all the higher animals, uch as the birds, is for some time merely a ll.it, 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 told together and form a closed canal, and at length the two external openings ot the tube (the mouth and anus) appear. Moreover, the important fact that the cither systems of organs are developed in' the same way, from tubes formed out of simple layers, did not escape Wolff. The nervous system, muscular system, and vascular (blood-vessel) svstem, 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 elemen- tary parts out of which the germinal layers arose. Finally, I must invite special attention to the mechanical character of the pro- found philosophic reflections which Wolff always added to his remarkable observa- tions. He was a great monistic philo- sopher, in the best meaning of the word. It is unfortunate that his philosophic dis- coveries 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. MODERN EMBRYOLOGY 'S Cll AIM ER HI. MODERN EMBRYOLOGY Wi may distinguish three chief periods in the growth of our science of human embryology. The first has been con- sidered 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 gencrationis 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 i which brought about a change in the very foundations of biology, and, in particular, of embryology, The third period begins with Darwin. When >u' say that the second period lasted a lull century, we must remember that Wolffs work had remained almost unnoticed during half the time namely; until the war iNij. During the whole of these fifty-three years not a single hook 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 of\ extensive observations of fact men- tioned lure 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 AJbrecht von Haller; it is One of tile most astonishing instances of a great authority, as such, preventing for J a long lime 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 alimentar) canal of the chick, and hit upon the right clue to the embryonic puzzle, without knowing a word about Wolff's important treatise m\ the same subject. They were treading in his very footsteps without suspecting it. This can he easily proved from the fact that they did not travel as far as Wolff. It was not until Meckel translated into German Wolff's hook on the alimentary system, and pointed out its great importance, that the eyes of anatomists and physiologists were suddenly opened. At once a number ot biologists instil uted fresh embryological inquiries, and began to confirm Wolff's theory <^f epigenesis. This resuscitation of embryology and development of the epigenesis-thcory was chiefly connected with the university of Wurtzburg. One of the professors there at that time was Ddllinger, an eminent biologist, and father of the famous Catholic historian who later distinguished himself by his opposition to the new dogma of papal infallibility. Ddllinger was both a profound thinkerand an accurate obsen er. He took the keenest interest in embry- ology, and worked at it a good deal. However, he is not himself responsible tor 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 \on Baer, came to Wurtzburg. Baer's conversations with Ddllinger on embryology led to a fresh series of most extensive investigations. Ddllinger had expressed a wish that some yount; scientist should begin again under his guidance an independent inquiry into the development of the chick during the hatching of the Cgg. 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 b) Baer to come to Wurtzburg. 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 fat- 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 1S17. He clearly enunciated the theory of germinal layers which Wolff II. MODERN /• \f BRYOLOGY had anticipated, and established the truth lit" 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 lien's egg divides, before the incubation has proceeded twelve hours, into two different Livers, an external serous layerand an internal mucous layer ; between the two there developes later a third layer, the vascular (blood-vessel) layer.' Kail Ernst von Baer, who had set afoot Pander's investigation, and had shown the liveliest interest in it after Pander's departure from VYurt/burg, began his own much more comprehensive research in iSu). 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 18281, the second in 1837. The hook proved to he the foundation o\\ which the whole science of embryology lias 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 ol~ embryological research ever since. He taught that in all the vertebrates Inst 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 he seen in the globular yelk of the fertilised egg, as an oval disk which fust divides into two layers. Prom 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. * The technical term* which arc bound to creep into thit chapter will l>e fully understood later on.— Trans. Bach of these original layers divides, according to Baer, into two thinner and superimposed layers or plates, lie calls the two plates of the animal layer, the skin-stratum and muscle-stratum. From the upper Ol 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, o\- 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. Prom 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 line, 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, fiat, leaf-shaped, em- bryonic disk into the complete vertebrate bodv, through the conversion of the layers ov plates into tubes. The fiat leaves bend themselves in obedience to certain laws oi growth ; the borders of the curling plates approach nearer and nearer; until at last thev come into actual contact. Thus out of the fiat gut-plate is formed a hollow gut-tube, out of the fiat spinal plate a hollow nerve-tube, from the skin- plate a skin-tube, and so on. Among the main 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 i'« of an inch in diameter; it can be seen under very favourable circumstances with the naked eye as a tiny particle, hut is other- wise quite invisible. This particle is formed in the ovary inside a much larger MODERN EMBRYOLOGY »r globule, which takes the name of the Graafian follicle, From its discoverer, Graaf, and had previously been regarded .is the true ovum. However, in 1827 Baer proved that it was not tlu real ovum, which is m>ich smaller, and is con- tained within the follicle. (Compare the end of the t wiiitx -ninth chapter.) Baer was also the first to observe what is known as the segmentation sphere of the vertebrate ; thai is to say, the round vesicle which first developes 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 the twelfth chapter). Another discovery of his that was ol great importance in constructing the vertebrate stem and the characteristic organisation of (his extensive group (to which man belongs) was the detection of the axial rod, or the chorda dorsaJis, This i-> 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 fust sketch of the spinal column, the solid skeletal axis of the ver- tebrate. In the lowest of the vertebrates, the amphioxus, the internal skeleton con- sists only of this cord throughout lite. Hut 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 verte- brate embryology, his researches were even more influential, from the circum- stance that he was the fust to emplov the comparative method in studying the development of the animal frame. Baer occupied himself chiefly with the embryo- logy 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 \crtehrata ; 2, the articulata ; 3, the molhisca ; and 4, all the lower groups which were then wrongly comprehended under the general name of the radiata. Georges luvier had been the first to formulate this distinction, in 181 2. He showed that these groups present specific differences in their w hole internal structure, and the connection and disposal of their systemsof oi^.ms; and that, on the other hand, all the animals of the same type say, the vertebrate! essentially agreed in their inner structure, in spite of the greatest superficial dif- ferences. But Baer proved that these four groups are also quite differently developed from the ovum ; and that the series of embryonic tonus is the same throughout toi animals of the same type, but different in the case of other animals. I'p to that time the chief aim in the classification ol the animal kingdom was to arrange all the animals from lowest to highest, from the infusorium to man, in i^ne long and Continuous series. The erroneous idea prevailed nearly everywhere that there was one uninterrupted chain of evolution from the lowest animal to the I ' liest, 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 nc'wly opened field, enlarging it in a very short time with great energy by their various dis- coveries in detail. Next to Baer's comes the admirable work of Heinrich Rathke, of Konigsberg (died i860); he madean ex- tensive 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 com- prehensive studies of mammal embryology to the careful research of Wilhelra Bis< hoff, of .Munich ; his embryology of the rabbit (1840), the doi,r (1X42), the guinea-pig (1852), and the doe (1854), still form Classical studies. About the same time a great impetus was given to the embryo- logy of the invertebrates. The way wa£ opened through this obscure province by the studies of the famous Berlin zoologist, Johannes .Miiller, on the echinoderms. He was followed by Albeit Kolliker, of Wiirtzburg, writing on the cuttle-fish (or the cephalopods), Siebold and Huxley on worms and zoophytes, Fritz Miiller ( Desterro) on the Crustacea, Weismami on insects, and soon. The number of workers in this field has greatly increased of late, and a quantity of new and astonishing discov cries have been made. One notices, in several of these recent works oa iR \fODERN EMBRYOLOGV embryology, thai theii authors are too little acquainted with comparative anatomy and classification. Paleontology is, un- fortunately, altogether neglected bj man) of these now workers, although this in- teresting science furnishes most important facts for phytogeny, and thus often proves ol verj great service in ontogeny. A yer) important advance was made in our science in 1839, when the cellular theory was established, and a new field ol inquiry bearing ot\ embryologj 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 pruts, which we call cells, a pupil of Johannes Muller 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 micro- scope, 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 b ings ; the} 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 question's. What is the relation of the cells to the germinal layers? .Are tliegerminal layers composed of cells, and what is their rela- tion 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 impor- tant questions were now imposed on the embryologisl 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 Veitebrates (not trans- lated) of Robert Rcmak, of Berlin (1S51). •This gifted scientist succeeded in master- ing, 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 BoguslausReichert, had ahead vat tempted 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 struc- ture and development of the tissue in par inula.. Rcmak at length brought order into the dreadful confusion that Reichert had caused ; he gave a perfectly simple explanation ol the origin of the' tissues In Ins opinion the animal ovum is always astmple cell; the germinal layers which develop out ol it are always composed ol cells 'K and these cells that constitute the germinal layers arise simply from the continuous and repeated cleaving (seg- mentation) of the original solitary cell. It Inst 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 ol all out of the simple egg cell, by a repealed sub-di\ ision, a cluster of cells, as Kolliker had already stated in connection with the cephalopods in 1844. The cells of (his 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 dif- ferent shapes, increase, and differentiate; and in the end (here 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 (histaj, as it was established by Rcmak and Kolliker. 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 srmple leaf- shaped structure of the vertebrate body (oi- 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 depen- dencies (hairs, nails, etc.) -that is to say, the entire1 outer envelope of the body are developed out of the outer or upper layer; but there are dse) developed in a curious way out of the same layer the cells which form the central nervous system, the MODERN EMliRYOLOGY in brain and the spinal cord. In the second place, the inner or lower germinal layer skives 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 thai 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 thai 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 Baerhad 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 tonus the outer envelope oi the alimentary canal, with the mesentery, the heart, the blood-vessels, etc. On this firm foundation provided by Remak for histogeny, or the science o\ the formation o\ 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 anato- mists, Reichert {of Berlin) and Wilhelm His [of Leipiic), especially, have endea- voured in their works to introduce a new conception of the embryonic development of the vert -hrate, according to which the two primary germin/J layer-, would not he the sole sources of formation. But these efforts were SO seriously marred by igno- rance of comparative anatomy, an imper- fect 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 bj tlu general lack of discrimination and of grasp of the true object of embry- ology. Wilhelm His published, in 1868, his extensive Researches into the Earliest H of the Vertebrate Body,1 one of the curiosities of embryological literature. The author imagines that he can build ■ llis\ \wrk-. haw been translated into LiikIisIi. 1 "mechanical theory i^f embryonic development " by merely giving an exact description of the embryology of the chick, without any regard to comparative anatomy and phytogeny, and thus falls into an error that is almost without parallel in the history of biological litera- ture. As the fmal resull of his laborious investigations, His tells us "that a com- paratively simple law of growth is the one essential thing in the firsl develop- ment. Ever) formation, whether it con- sist in cleavage of layers, or Folding, or complete division, is a consequent e 01 this fundamental law." Unfortunately, he doe> not explain what this " law of growth " is ; |USt 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 he a sort o\' skilful tailor. The ingenious operator succeeds in brin^in^ into existence, by "evolution," all the various forms of living things by cutting up in different vvavs the germinal layeis, bending and folding, tugging and split- ting, and so on. llis'> embryological theories excited a good deal oi' interest at the time oi publi- cation, 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 distri- bution of strain in an elastic plate). It is quite true that a mechanical or monistic explanation (or a reduction of natural phenomena to physical and chemical processes) is the ideal of modern science, and this ideal would he realised if we could succeed in expressing these forma- tive processes in mathematical formula-. His has, therefore, inserted plenty of numbers and measurements in his em- bryological works, and given them an air of "exact" scholarship by putting in ;i quantity of mathematical tables. Unfor- tunately, they are of no value, and do not help us in the least in forming an " exact " acquaintance with the embryonic pheno- mena. Indeed, they wander from the true path altogether by neglecting the phylogenetic method ; this, lie thinks, is " a mere by-path," and is " not necessai \ at all for the explanation of the facts of 20 MODF.RX EMBRYOLOGY embryology," which arc the direct conse- quence 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 dired 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 heredi- tary repetition of countless phylogenetic changes which have taken place for thousands of years in the race-history of the said ancestors. Each of these his- torical 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 evolu- tion 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 Bacr 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 inverte- brates, 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 zoo- phyte is developed correspond, both mor- phologically and physiologically, to the two original germinal layers of the verte- brate. 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 wars 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 (\Kj>) 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 elasses, in identical form. This "homology of the tu o primary germinal layers " extends through the whole of the metazoa, or tissue-forming animals; that is to say, through tlii' 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 amdebae and infusoria), or of a loose aggregation of only slightly differentiated cells, though it may not even .each the full structure of a single cell (as with the monera). Hut 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 ento- derm, 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 pro- gaster) and its opening, the primitive mouth (prostoma). The typical em- bryonic form of the metazoa, as it is presented for a time by this simple struc- ture 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 gastr&a. This applies to the sponges and other zoophyta, and to the worms, the mollusca, echinoderma, arti- culata, and vertebrala. All these animals may be comprised under the general heading of "gut animals," or metazoa, in contradistinction to the gutless pro- tozoa. I have pointed out in my Study of the Gastrcea Theory [not translated] (1873) the important consequences of this concep- tion in the morphology and classification of the animal world. 1 also divided the realm of metazoa into two great groups, the lower and higher metazoa. In the first are comprised the ccelenterata (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 dif- ferentiated. But with the higher forms of the ccelenterata (the corals, higher medusa?, ctenophora?, and platodes) a middle layer, or mesoderm, often of con- siderable size, is developed between thu MODERN EMBRYOLOGY 2\ Other two layers ; but blood and an internal cavity arc -till lacking. To t he second great group of the meta- zoa I gave the name of the ceelomaria, or bilateral* (or the bilateral higher forms). They all have a cavity within the body (cceloma), and most of them have blood and blood-vessels. In this are comprised the si\ higher stems of the animal king- dom, 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 tour secondary germinal layers, of which the inner two construct the wall of the alimentary canal, and the OUtei two the wall of the body. Between the two pairs of layers lies the cavity u a loma). Although I laid special stress on the great morphological importance of this cavit v in my Study of the (iastnra Theory, and endeavoured to prove the significance of tlie four secondary germinal layers in the organisation of the coelomaria, I was unable to deal satisfactorily with the difficult question of the mode of their origin. This was o\onc eight years after- wards by the brothers Oscar and Richard Rertwig in. their careful and extensive Comparative studies. In their masterly Coelom Theory: An Attempt to Explain the Middle Germinal LayerVaaX. translated! 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 ca loin-pouches proceed from the rudimentary mouth of the gas- trula, between the two primary layers. The inner plate of the two-layered coelom- poueh (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 coelom, by the blending of the right and left ccefom-sacs. We shall -ee this more fully in Chap. X. The many new points of view and fresh suggested by my gastraea theory and Hertwig's CCefom theory led to the publication of a number of Writings on the theory of germinal layers. Most <■>[' 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 compe- tent 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 Con- troveisv is that it brought with it a recog- nition of the n^co\ tor phylogenetic study .i\)J explanation. Interest and practice in embi yologM al research have been remarkably stimulated during the past thirty years bv this appre- ciation of phylogenetic methods. Hun- dreds of assiduous and able observers are now engaged in the development of com- parative 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 emhrv ological literature since that time. References to them will be found in the Litest manuals of embryology of Kolliker, Balfour, Eiertwig, Kqllman, Korschelt, and Heider. Kolliker's Entwickelungsgeschichte des Menschen und der hbherer J'hiere, 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 Wiirtzburg 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 Compara- tive Embryology (1880), the very scatteied and extensive literature of the subject ; he has also widened the basis of the gastra a 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 witii theexi client Text-Hook of the Embryo- logy of the Veitebrutes (1890) [translation 1895] of Korschelt and Heider shows what astonishing progress has been made in the science in (he 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 bv these chapters on human THE OLDER PHYLOGENY embryology. Kollmann's work is i om« mcndable for its clear treatment of the subject and very fine original illustrations ; its .mt hoi adheres firmly to the biogenetii law, and uses it throughout with consider- able profit. That is not the case in Oscar Hcrtwig's recent Text-book of the Embryo- wan and the Mammals [translations 1S9J and 1899] (seventh edition, 1902), This able anatomist lias of late often been quoted as ,u\ opponent of the biogenetic law, although he himself had demon- strated its great value thirty years ago. Ili^ recent vacillation is partly duo to the timidity which our "exact" scientists have with regard to hypotheses; though it is quite impossible to make any head- \v iy in the explanation of facts without them. However, the purely descriptive part of embryology in Hertwig's Text-book i^ verj 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, Lan/erote, in 1866. I there made a thorough investigation of the almost unknown embryology of the siphonpphorae. I cut a number of the embryo-- of these animals (which develop freel) in the water, and pass through a very curious transformation), at an early stage, into seVeral pieces, and found that a fresh organism (more oi~ less complete, according to the size of the piece) was developed from each particle. More" recent!) some of my pupils have made similar experiments with the embryos of vertebrates (especially tin' frog) and some of the invertebrates. Wilhelm Koux, in particular, has made extensive experi- ments, anil based on them a special "mechanical embryology," which has given rise to a good deal of discussion and controversy. Rou.x has published a special journal fov these subjects since 1895, the Archiv ftir Entwickelungs- mecnanik. 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 lias for a long time derived assistance from the pathology of the diseased organism. Other of these mcchanical-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 foun- dation— 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 embry- ology. Impartial reflection and a due attention to paleontology and compara- tive 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. Till- OLOKR PHYLOGENY The embryology of man and the animals, chiefly directed to the discovery, by careful the history of which we have reviewed in observation, of the wonderful facts of the the last two chapters, was mainly a : embryonic development of the animal descriptive science forty years ago. The body from the ovum. Forty years ago earlier investigations in this province were no one dared attack the question of the THE OLDER PHYLOGENY 23 causes of these phenomena. For fully a century, from the year 1759, when Wolff's solid Thcoria generuHonis 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 tliat Darwin occupied himself very little with direct embryological irch, and even in his chief work he only touches incidentally on the embrv onic 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 -<. ience of evolution. When We turn our attention to this latest period of embryological research, we pas> into the second division of organic evolution stem-evolution, or phytogeny, I have already indicated in the first chapter the important and intimate causal connec- tion between these two sections of the science of evolution- -bet ween the evolu- tion 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 essen- tial points relating to the causes of evolu- tion ; and we shall seek throughout this work to confirm this principle and lend it the support o( 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 ( phvlon ) shows us, in the physiological functions of heredity and adaptation, the conditioning .causes on which the evolution of the individual depends"; or, more briefly: "Phylo- genesis is the mechanical cause of onto- sis." 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 nanus to consider here. At the head of them we find the great brcin h 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, lioethe, 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 tbe beginning of the nineteenth century. In the whole of the time before this no one had ven- tured 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 cither 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 tbe name of species. Thus the definition of the species becomes important. It is well known that this definition was given by Linne, who, in his famous Systema Natural (17^5), 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 indispens- able idea in descriptive natural history, in zoological and botanical classification ; although there have been endless contro- versies as to its real meaning. What, then, is this "organic species"? Linne* himself appealed directly to the Mosaic narrative ; he believed that, as it is stated in Genesis, one pair of eat h species of animals and plants was created in the beginning, and that all the indi- viduals of each specie- 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, Linne 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 24 THE ( >/. I )/•:/>' /»// 1 'LOGENY and topographical distribution oJ organ- isms. 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 Linne* particularly suitable for the landing, as it reaches a height of more thaii n>,,xx> feet, and thus provides in it > higher zones the several climates de- manded by the various species of animals and plants : the animals that were accustomed to .1 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 halt-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 Linne's time, as one of the chief sources of information, paleontology, was still wholly unknown. This science of the fossil rem. li lis of extinct animals and plants is very closely bound up with the whole question of evolution. It is im- possible to explain the origin of living organisms without appealing to it. But this science did not rise until a much later dale. The real founder of scientific paleontology was Georges Cuvier, the most distinguished zoologist who, after 'Linne, 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 Linne and the Mosaic story of creation, though this was more difficult for him with his acquain- tance 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 indepen- dent 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 limes. 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 revolu- tions or cataclysms — something like the legendary deluge must have taken place repeatedly. Cuvier was all. the more interested in these catastrophes or cata- clysms 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 paleon- tological 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. Accord- ing 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 interven- tion on the part of the Creator. This catastrophic theory, which Cuvier des- cribed in a special work, was soon generally accepted, and retained its posi- tion in biology for half a century. However, Cuv ier's theory was com- pletely 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 *alse, 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 arc at work to-day in altering and recon- structing the surface of the earth. These causes are- the action of the atmosphere and water in its various forms (snow , ice, fot^, rain, the wear of the river, and the stormy ocean), and the volcanic action which is exerted by the moben central the or.nr.R riivioci \v 25 m.is^. L\cll convincingly proved ili.it these natural causes are quite adequate to explain every feature in the bund 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 un- disputed authority in biology. All (lie 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 < rcations. In order to illustrate the complete stag- nancy 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 1 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. No't one <.•*{ them ever mentioned this question of the origin of species. N'ot a word was ever said about the earlier efforts to understand the formation of liying things, nor about Lamarck's /'////<>- sophte ZooTogique which had made a fresh attack on the problem in 1809. Hence it is easy to understand the enormous oppo- sition th.it Darwin encountered when lie took up the question tor the first time. His views seemed to float in the air, with- out a single previous effort to support them. The whole question of the forma- tion of living things was considered by biologists, until 1859, as pertaining to the provinceof 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 super- naturalisl system as regards the origin of living things. He even went so far as to " It is quite certain that we cannot even satisfactorily understand, much less explain, the nature of an organism and its intern. il forces on purely mechanical prin- ciples; it i-. so certain, indeed, that we may confidently say: ' It is absurd fov 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 scien< e. Nevertheless, Kant deserted this point of view at times, particularly in several remarkable passages which 1 have dealt with at length in my Natural History reation (chap, v.), where he expresses himself in the opposite, or monistic, sense. In fact, these passages would justifj one, as I showed, in claiming his support for the theory of evolution. However, these monistic passages .n\- 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-dav most of our philosophers still regarding these two provinces as totally distinct. They put, on the one side, the inorganic oi~ "lifeless" world, in which there are at work only median cal 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, e*( ept in the light of preconceived design, tarried 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 origio o\' man, from being uded by the bulk of people as a scientific question at all until 1859. Never- theless, 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 oi 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, BufTon, (ieoffrov St. Hilaire, and Blainville in France ; Wolf- gang Goethe, Rcinhold Treviranus, Schel- ling, and Lorcntz 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 tsti 1744; he was the son of a clergyman, and was destined for the Church. Hut he turned to seek glory in the army, and eventually devoted himself to science. His PhUosophie Zoologiqne was the THE oi.nr.R phyi.ocksy Tiim scientific attempt to sketch the real course of the origin o\ species, the Brs1 "natural history of creation" of plants, animals, and men. But, as in the case of WolfTs book, this remarkably able work had no infiaence whatever ; neithei on^- 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 o( the most im- portant 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, lias not thought fit tii 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 Philosophic Zoologiquc shared the fate of Wolffs theoryof develop- ment, 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 theoryof evolution much farther than they found it possible to do. To give an idea of the great importance j o\ the Philosophic /.oologique, 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 tobe 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 crea- tion, or repeated creations (as in Cuvier's theory), to explain this, but a natural, continuous, and necessary evolution. The whole evolutionary process has been un- interrupted. 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 spon- taneous 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 impor- tant 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 develop- ment, 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 accu- mulation 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 THE OLDER PHYLOGENY 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, espe< ially Reinhold Treviranus, in li i s Btologu (1802), and Lorenti Oken, in his Naturphilosophie (1800), turned its atten- tion to the problem of evolution about the end of the eighteenth and beginning of the nineteenth century. I have described it- woik in mj History 0/ Creation (chap. IV.). Hero 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 earl) 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. Hut the most valuable of his scientific studies are those which relate to that " living, glorious, precious thing," theorganism. He made profound research into the science of structures or morpho- logy (morplue — 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 discovers of the pineal gland in man. his system of the metamorphosis of plants, etc. These morphological studies led Goethe o\i to research- into the forma- tion 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 Hue 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 morpho- logy. 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. Hut we mav say that the plants and animals, beginning with an almost inseparable closeness, gradually advaiue 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, hut in a literal geiiealogit.il, sense of this close atlinit\ of Organic lot ins is clear from oilier remarkable passages in which he treats of their variety in out- ward form and unity in internal structure. IK- believes that every living thing has arisen by the interaction of two opposing formative forces or impulses. Tin.- inter- nal 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 01 "centri- fugal" force, the element of variation or "impulse to metamorphosis," is con- tinually 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 concept ion of nature with his pantheistic philosophy. The warm and keen interest with which he followed, in his last years, the controver- sies 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 he 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. 28 THE MODERN SCIENCE OF EVOLUTION Chapter V. THE MODERN SCIENCE OF EVOLUTION' Wi 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, nol only on the side of aca- demic zoology and botany, the sciences which were chiefly affected by Darwin's theory, but in a Tar wider circle, so that We find Darwinism discussed in popular literature with a vigour and /est 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 evolu- tion. 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 scien- tific proofs to allege than Lamarck, or St. Hilaire, or Goethe, or Treviranus had had. Hut, in the second place, we must acknow- ledge that Darwin had the special distinc- tion 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 vf 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 stale, 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 struc- tures. * 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 pie-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 hepublished his theory. His grandfather, Erasmus Darwin, was an able scientist of the older school of natural philosophy, who pub- lished a number of natural-philosophic works about the end of the eighteenth century. The most important pi 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. THE MODERN SCIENCE OF EVOLUTION ?0 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 noi 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 v ear appeared the Origin of Species, in which lie developes it at length and Supports it with a mass of proof. Wallace had reached the same conclusion, hut he had not so clear a per- ception as Darwin of the effectiveness of natural selection in forming species, and did not develop the theory so fully. Nevertheless, Wallace's writings, espe- cially those on mimicry, etc., ami an admirable work on The Geographical Distribution of Animals, contain many fme original contributions to the theory of selection. Unfortunately, this gifted Scientist has since devoted himself to spiritism.1 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 yen 9 to recover from the astonishment into which they had been thrown through the revolutionary idea of the work. Hut 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 con- cerns us here— the extension of the theory to man — was not touched at all in Darwin's first work in 1 859. 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 lavmen (especially several theologians), but also a number of men of science, said very naively that Darwinism in it -elf was not tn be opposed ; that it was quite right to use it to explain the origin of the various 1 Darwin and Wallace arrived at the theory quite independently. Vide Wallace'! Contributions to the Theory of Natural Selection (1870) and DorwmitM species, of plants and animals, but that it was lot. illy inapplicable to man. In the meantime, however, it Seemed to a good many thoughtful people, lavmen as well .is 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. Manv of the acuter opponents of the theory saw at once the justice of this position, and, as this con- sequence was intolerable, they wanted to gel rid of the whole theory. fhe Rrst scientific application of the Darwinian theory to man was math' by I iuxley, the greatest zoologist 'm England. This able and learned scientist, to whom zoology owes much of its progress, pub- lished in i stage, and with proportionate confidence, from the accumulation of detailed observations. These inductive conclusions " cannot command absolute confidence, like mathematical axioms ; bul 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 thes* inductive laws is not diminished from the circumstance that they are looked upon merely as temporary acquisi- tions of science, and may be improved to an) extent in the progress of scientific knowledge. The same may be said of the attainments of many other sciences, such as geology or archeology. I [owe^ er much thej ma\ be altered and improved in detail in the course of time, these inductive truths may retain their sub- Btance 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 fust 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 tos.ils 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 lias been gradual, and unbroken by any violent revolutions. And when we com- pare 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 I 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 highei classes of vertebrates the reptiles birds, and mammals, for the first time; onl) the lowest and leasl perfect forms of the mammals are found at first ; and it is onl) ■'! a \er\ htte period that placental mammals appear, and man belongs to the latest and youngest branch of th< 1 hus perfection i^f form increases as well as vanet) from the earliest to the latest sta^e. That is a fait of the greatest importance. Ii can onl) be explained by the theor) of evolution, with which it is in perfect harmony. If (he different groups of plants and animals do really descend from each other, we must ex] 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 sa ond series of fads which are of gieat impor- tance for the forming of our inductive law, I his branch of morphology compares the adult structures of living things, and seeks in the great variety of organic forms Un- stable and simple law of organisation, or the common type or structure. Since Cuvier founded this science at the begin- ning of the nineteenth century it has been a favourite study of the most distinguished scientists. Even before Cuviers time Goethe had been greatly stimulated by it, and induced to take up the study of mor- phology. _ 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 Mem and the plants of each class is the same in its essential features, how- much they differ in external appear- ance. Thus man has SO great a resem- blance 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 Ol 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 dif- ferences are of no account whatever. We kirn further from comparative anatomy that the chief features of animal structure THE MODERN SCIENCE OF EVOLUTION are bo similar in the various classes (fifty to sixty in number altogether) thai the) may all lie comprised in from eight to twelve great groups. Bui even in these groups, the stem-forms or animal typos, certain organs (especially the alimentary canal) can be proved to have been origi- nally 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 ana- tomy 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 touchstone in the science. " Up to now there is no fact in comparative anatomy that is inconsis- tent 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 won- derful 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 environ- ment. 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 rudi- mentary 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 eai, so as to catch the waves of sound as much as possible. Hut in the case of man and other short-eared mammals these muscles are. useless, though they arc 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 list relic of a third inner eve-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 intes- tinal 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 diges- tion, 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 vegetrtrian 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 dis- appears. There is no other way of. explaining rudimentary organs. Hence they are also of great interest in philo- sophy ; they show clearly that the monistic or mechanical v ievv of the orga- nism is the only correct one, and thai the dualistic or teleological conception is wrong. The ancient legend of the direct creation of man according to a pre-con- ceived plan and the empty phrases about THE MODERN SCIENCE OF EVOLUTION 33 "design" in the organism are completely shattered by them. It would be difficult to conceive a more thorough refutation of tejeolog) than is furnished by the lad that all the higher animals have these rHidimentar) organs. The theor) of evolution finds its broadesl inductive foundation in the natural classi- fication of living things, which arranges all the various forms in larger and smaller groups, according to their degree of affinity. These groupings or cate- gories of classification the varieties, species, genera, families, orders, classes, etc. — show such constant features of >>>- ordination and subordination that we are hound to look on them as geneed 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 vvav the natural tree-like form of the system of organisms, we must regard it at once as a weight] proof Of the truth of evolution. The careful construction of these genea- logical trees is, therefore, not an amuse- ment, hut the chief task of modern classification. Among the chief phenomena that hear 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 fea- tures the "science of distribution," or chorology (chum a place) has been put sued with lively interest since the dis- coveries made by Alexander xon Hum- boldt. Until Darwin's time the work was confined to the determination oi' the facts of the science, and chiefly aimed at settling the spheres ot distribution of the existing large and small groups oi 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 lias 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 stein-forms, whose ever-branch- ing offspring have gradually spread themselves by migration over the earth. For eai h group oi species we must admit a "centre of production," or common home ; this is the original habitat in which the ancestral form -vas 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 spei and these also Scattered themselves by active and passive inigraticii, and so on. A each migrating organism found a dit- ferent environment m its new home, and adapted itself tO it, it was modified, and gave i ise tO new forms. This very important branch of science that deals with active and passive initia- tion was founded by Dai win, with the aid oi the theory of evolution ; and at the same time he advanced the true explana- tion of the remarkable relation OT simi- larity 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 lie 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 stein- form of a new species is isolated) is really only a special case of selection. The striking and interesting facts of choiology 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, domes- ticity, 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 bene- ficent 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 34 THE MODERN SCIENCE OF EVOLUTION theorj of evolution the foetal develop- ment of the individual organism, the whole science of embryology or ontogeny. But a> the later chapters will deal with this in detail, 1 need say nothing further here, 1 shall endeavour in the following pages to show, step by stop, how the whole oi the embryonic phenomena form a massive chain of proof for the theon of evolution; for they can be explained in no other way. In thus appealing to the close causal connection between onto- genesis and phylogenesis, and taking our stand throughout on the biogenetic law, we shall be able to prove, stage by stage, from the facts oi embryology, the evolu- tion of man from the lower animals. The genera] adoption oi the theon tl' evolution has definitely closed the con- troversy as to the nature or definition oi the species. The word has no absolute meaning whatever, but is only a group- name, or category oi 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 oi classification." He did this in his Essay on Classification, in which he turns upside down the pheno- mena 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 oi 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. 1 gave an analytic proof of this in my mono- graph 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. Accord- ing as the classifier takes his ideas of genus, species,, and variety in a broader or in a nnirouci sense, he will find in the small group oi the sponges either one genus with three species, or three genera with 238 species, or 113 genera with 501 species. Moreover, all these forms arc so connected by intermediate tonus that we can convincingly prove the descent oi all the sponges from a common stem- fdi m, the olynthus. Here, 1 think, I have given an analytic solution oi the problem oi the origin of species, and so met the demand of certain opponents of evolution for an actual instance oi descent from a stem-form. Those who are not satisfied with the Synthetic proofs oi the theory oi evolu- tion which are provided by compara- tive anatomy, embryology, paleontology, dysteleology, chorology, and classifica- tion, may try to refute the analytic proof given in my treatise on the sponge, the outcome of l\\c years of assiduous study. 1 repeat : It is now impossible to oppose evolution on the ground that we have no convincing example of the descent of all the species oi 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 eves 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 arc fur- nished 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 THE MODERN SCIENCE OF EVOLUTION 35 and understanding the proofs we ahead; have I u.i> almost alone thirty-six yeai when 1 made the first attempt, in my General Morphology, to put organic science on a mechanical foundation through Darwin's theorj of descent. The association of Ontogeny and phvlo- geny and the proof of the intimate causal connection between these two sections of the science o\ evolution, which I expounded in my work, nut with the most spirited opposition on nearly all sides. The next ten years were a terrible "struggle for life" foi the new theory. But for the last twenty-five years the tables have been turned. The phvlo- genetic method has met with so general a reception, and found so prolific a use in ever} branch o\ biology, that it seems Superfluous to treat any further lure o\ its validity and results. The ptoo\ o( it lies in the whole morphological literature o( the last three decades. Hut 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 1 am now seeking to establish— monistic anthropogeny. This statement may seem to be rather audacious, since the wry next branch of biology, anthropology in the stricter sense,' makes very little use of these results o( anthropogeny, and sometimes expressly opposes them.' This applies especially to the attitude which has characterised the German Anthropological Society (the Deutsche Gesellschajl fur Anthropologic) for some thirty years. Its powerful president, the famous patho- logist, Rudolph Yirchow, is chiefly ' This docs not apply to English anthropologists, who arc almost all evolutionists. responsible for this. Until his death (September 5th, 1903) he never ceased to reject the theon o\ descent as unproven, and to ridicule its chief consequence the descent o\ man from a seiies iA mammal ancestors as a fantastic dream. 1 need Only recall his well-known expression at the Anthropological Congress at Vienna in [894, that " it would be just as well to s.i\ man came frpm the sheep or the elephant as from the ape." Virchow's assistant, the -.> i«. tar) oi the German Anthropological Society, Pro- fessor Johannes Rahkc o\ Munich, has also indefatigably opposed trahsformisoi : he has succeeded in writing a work in two volumes (Det Mcnsch), in which all the facts relating to his organi it ion are explained in a sense hostile tp evolution. This work has had a wide circulation, owing to its admirable illustrations and its able treatment o\~ the most interesting facts o\ anatomy and physiology exclu- sive oi the sexual organs ! Hut, as it has done a great deal to spread erroneous views among the general public, I have included a criticism oi it in my History of ( 'nation, as well as met Virchow's attacks on anthropogeny. Neither Yirchow, 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 transcen- dental 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 36 TIIK OVUM AND THE AMCEBA Chapter VI. THE OVUM AND THE AMCEBA In order to understand clearly the course of human embryology, we must selecl the more important of its wonderful and manifold processes for killer explanation, and then proceed from these to the in- numerable features oi 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. . This fact is so important that few should be unaware o( its extreme significance ; yet it .was quite unknown in the first Fig. i.— The human ovum, magnified iob times. The globular mass of yelk (b) is enclosed by a trans- parent membrane (the ovolemma or zona pellucida [a]), and contains a non-central nucleus (the germinal vesicle, c). Cf. Fig. 14. 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 anim lis, 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 con- dition of «the theory and the significance o\ the views it has suggested. In order properly to appreciate the Cel- lular 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 struc- ture of these organs with the microscope, we arc 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 docs 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 inde- pendent living unit. It is, in the words of Briicke, "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 organ- isms, a colony or commonwealth, made up of innumerable independent units, or very different tissue-cells. THE OVUM AND THE. AMOEBA 37 In reality, the term "celJ,."whrtri 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 tlu- 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. Hut some cells, especially young ones, are entirely without the enveloping membrane, or stiff wall. Hence we now generall) describe the cell as a living, viscous particle o\ protoplasm, enclosing a firmer nucleus in its albuminoid body. There may be an enclosing meinhrane, as there actually is in the ease iA~ most of the plants ; hut it maj be wholly lacking, as is the case with most oi the animals. There is no memhrane at all in the first stage. The young cells are usuallj round, but they vary much in shape later on. Illustrations of this will he found in the cells o\ 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 cary on or cytoblastur, Tig. \c and Fig. ik). The outer and larger part, which encloses the other, is the body of the cell fcelleus, cytos, or cytosoma J. 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 caryo- plasm) ; that of the cell body is called plastin (or cytoplasm). In the most rudi- mentary 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 lind a certain structure in the protoplasm. The chief and most common form oi~ this is the fibrous or net-like "thready Structure" (Frommann) and the frothy " honeycomb structure " ( Butschli). The shape or outer form of the cell is infinitely varied, in accordance with its endless power o\' 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 con- struction, and those that aie developed in a free tluid without anv external pressure. In siu h cases the noCleua also is not infrequently round, and located in tho centre o\ the cell-body ( Fig. ai). In other cases, (he tells have no definite shape; they are constantly changing their form owing to their automatic movements. This is the case with the amu.hu- (Figs. 15 and 16) and the amoeboid travelling cells (Fig. 11), and aUo 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 multi- cellular organism, in which a number of similar cells are bound together in virtue ol" certain laws o\ heredity, the shape is determined partly by the form o\' their connection and partly by their special F10. 2— Stem-cell of one of the echinoderms (cvtula. or "first segmentation-cell 'fertilised ovum),' after Herfarig, k is the nucleus v*t caryon. functions. Thus, for instance, we find in the mucous lining of our tongue very thin and delicate Hat cells of roundish shape (Fig. 3).. In the outer skin we lind simi- lar, 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 l \ pe, the group of the " covering-tissues," or epithelia. In these " primary tissues " (to which the germinal layers belong) simple cells ol" the same kind are arranged in layers. The arrangement am' shape- are more complicated in the " secondary tissues," which are gradually developed out of the primary, as in the tissues o\ the muscles, nerves, bones, etc. In the bones, for instance, which belong to the group o( supporting or connecting organs, 3* THE OVCM AND THE AMCEBA the cells (Fig. (>) are star-shaped, and are joined together by numbers of oet-fike interlacing processes; so, also, in the tissues ol the teeth (.Fig. 7). and in other forms of supporting-tissue, in which n soft 01 hard suhsta. cc (intercellular mattei , 01 is inserted between the cells. The colls also differ wry much in size. The great majority of them are invisible to the naked eye, and ran be seen only through the microscope (being as a rule between ,' and ' ,, inch in diameter). There are many of the smaller plastids such as the famous bacteria which only come into view with a very high magni- fying power. On the other hand, main' cclK attain a considerable si/e, and run occasionally to several inches in diameter, as do certain kinds of rhizopods among l he passive portions come third ; these arc subsequenth formed from the others, .u\<.\ 1 have given them the name of " plasma- products." They are parti) external (cell-membranes and intercellular m alter) and partly internal (cell-sap and i ell-contents). The nucleus for canon), which is usually e^' a simple roundish form, is quite struc- tureless at first (especially in very young cells), and composed o\ 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 bast' ( caryobasis ) and a softer or fluid nuclear sup ( caryolyinpli ). 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 Fie Fig. Fig. Fig. ;,.— Three epithelial cells from the mucous lining of the tongue. FlG. 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 ot them (b) has two nuclei. the unicellular protists (such as the radio- laria 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-tilled ova; as, tor instance, the yellow " yolk " in the hen's a^, which we shall describe later (Fig. 15). Cells also vary considerably in structure. In this connection we must first distin- guish between the active and passive com- ponents of the cell. It is only the former, or active parts of the cell, that really live, and effect that marvellous world of pheno- mena 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). opaque, solid body, called the tu/c/eo/us. 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 centra/ body (cen- trosoma) — 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 Jiomogene* ous viscid plasmic matter. But, as a rnte, THE OVl.M .WD THE A MCE B A 39 onl) the smallei part o\' it is formed of the living act i\ e cell-su bstan< e (protoplasm) ; the greater pari consists of dead, passive plasma- products (metaplasm). It is useful to distinguish between the inner and outer i>t these. External plasma-products (which are thrust out from the protoplasm as solid " st ruct ural mat ter ") aio 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 s>> arranged that the proto- plasm (like tlie car\o- plasm in the round nucleus) forms a sort o\ skeleton or frame-work. Fig. 6.— Nine star-shaped bone-cells, with interlaced branches. The spaces of itself the whole multicellular body. It is this network are filled partly with the fluid the common parent of all the countless cell-sap and partly by hard structural products The simple round ovum, which we take as the starting-point of our study (Figs. I and _>), has in many cases the vague, in- different features oi 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 generations of cells which form the dif- ferent tissues of the body ; it unites all their powers in itself, though only poten- tially or in germ. In complete contrast to this, the neural cell in the brain (Fig. .— A large branching nerve-cell, or "soul-cell," from the brain of .in electric Rah (Torpedo J, magnified 600 times. In the middle ot the cell 1- the large transparent round >/«<■/< us, one nucleolus, and, within the I. liter again, a uucU'u/hius. The protoplasm ot the cell is split into innumerable line threads (or fibrils), \\ hicll 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 Mux Sckuti •r THE OVUM AND THE AMCEBA cleavage is much more frequent; in this the caryoplasm of the nucleus and the cytoplasm o\ the cell-body act upon each other in a peculiai way, with a partial dissolution (caryo/vsis), the Formation of knots and loops ( mitosis J, and a move- ment of the halved plasma-particles towards two mutually repulsive poles o\ attraction (caryokinesii, Fig. 1 1 ). rhe intricate physiological processes which accompan) this "mitosis" have been very closely studied of late years, rhe inquiry has led u> the detection of certain laws of evolution which are oi extreme importance in connection with heredity. As a rule, two very different paiis of the nucleus play an important part in these changes. They are : the chromatin, or coloured nuclear substance, Fig. lo.-Blood-cells, multiplying by direct division. From the blood >>i the embryo of a stag. Originally, each blood-cell lias a nucleus and is round ( a >. When it is going to multiply, the nucleus divides into two f b, r, d ). Then the protoplasmic body is con- stricted between the two nuclei, and these move away from each other (e). Finally, the constriction is com- plete, and the cell splits into two daughter-cells (f). (From Frey.) which has a peculiar property of tinging itself deeply with certain colouring matters (carmine, hematoxylin, etc.), and the achromia (or linin, or achromatin), a colourless nuclear substance that lacks this property. The latter generally forms hi the dividing cell a sort of spindle, at the poles oi which there is a very small particle, also colourless, called the "central body" (centrosotna). 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 tun central bodies, stand- ing opposed to each Other at the poles of the nuclear spindle, form "the double- star " (or ampniastcr. Fig. 1 1, B, C). The 1 chromatin often forms a long, irregulai l\- WOUnd tine. id " the coil " Upirema, Fig. A). At the commencement Of the cleavage it gathers at the eciuat or ofthecell, between the stellar poles, and forms a crown of U-shaped loops (generally four or eight, or some other definite numbei ). 1 lie loops split lengthwise into two halves (B), and these back away from ea< li other towards the poles o\ the spindle (C). I leu each group forms a crown once more, And this, with the corresponding half of the divided spindle, forms a fresh nucleus ( I )). Then the protoplasm ol' 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 in- direct ceH-division which is the normal cleavage-process in most cells o( the higher animals and plants and thesimple direct division (Fig. 10) we find every grade ol segmentation ; in some circum- stances even one kind ol' 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 com- prehend 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 1 important base of our physiological con- ception 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 I the application of the cell theory to the ovum. Here comparative research yields the important result that every ovum is at first a si////>/f cell. 1 say this is very important, because our whole science ol , embryology npw resolves itself into the problem: "How does the multicellular THE OVUM AND THE AM(EB I 43 organism arise from the unicellular?" Every organic individual is at firs! a simple t ell, and as such an elementarj organism, or a unit of individuality. This cell produces .1 duster o\ cells by segmenta- tion, and from these developes the multi- cellular organism, or individual o( higher rank. internal constitution. • Later, though the remain unicellular, they differ in size and shape, enclose various kinds ol yelk- particles, have different envelopes, and so ow. I>ut when wt examine them at their birth, in the ovarj pf the female animal, we find them to be always of the same form in the first stages of then* Kfe. In A Mother-cell Cytoaoma (Knot. spiri.in.il Protoplasm of the cell-body 1?. Mother-star. the loops beginning to split lengthways (nuclear membrane gone) C. The two daughter-stars, produced by the breaking oi the loops of the mother-star (moving awa\ ) P. The two daughter-cells, produced by the complete division of the two nuclear halves (cytosomata still connected at the equator) (Double-knot, Dispirema) ^ Nuclear threads (chromo* (coloured nuclear matter, chromatin) N . ai membrane .Nuclear sap Star-like appearance in cytoplasm Centrosoma (sphere of attraction) (' C~ * V ' Nuclear spindle (achromin, colourle r**.*M J ' matter) -m Nuclear loops (chromatin, coloured matter) — Upper daughter-crown — Connecting threads of the two crow US (achromin) — Lower daughter-crown Double-star (amphiaster) •Jfj»Jvv':>' Upper daughter-nucleus Equatorial constriction of the cell-body I — Lower daughter-nucleus Fig. i i.— Indirect or mitotic cell-division (with caryolysis and caryokinesis) from the skin of the larva of a salamander. (From Rabl.) When wc examine a "little closer the l 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 ( of man and all the animals (Fig. 13). We are unable to detect any material difference between them, either in outer .shape or [ 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). Spci ial names have been given to these parts of the ovum ; the cell-body is called the yelk (vitcllus), and the cell-nucleus the germinal vesicle. As a rule, the 44 THE OI'C.W AND THE A MCE B A nucleus of the ovum is soft, and looks like i small pimple or vesicle. Inside it. as in many other cells, there is \ nuclear skeleton or frame and a third, hard nuclear body (the nucleolus}. In the ovum this is called the germinal spot. Finally, wo find in many ova (bul not in all) a still further point within the ger- minal spot, a "nucleolin," which goes by the name oi the germinal point. The latter parts (germinal spot and germinal point) haw-, apparently, a minor impor- tance, in comparison with the other two (the yelk and germinal vesicle). In the yolk we must distinguish the active for- mative yelk ipx protoplasm firsl plasm) from the passive nutritive yelk (or deuto- plasm = second plasm). !§&* wk Fig. i2.— Mobl'-s cells from the inflamed eye of a frog (from the watery fluid o( the eye, the humor aqiteus). The naked cells creep freely about, by (like the amoeba or rhizopod) protruding fine processes trom the uncovered protoplasmic body. These bodies vary continually in number, shape, and size. The nucleus of these amoeboid lymph-cells ("travelling cells," or planocytes) is invisible, because concealed by the numbers of fine granules which arc scattered in the protoplasm. (From Frey.) In many of the lower animals (such as sponges, polyps, and medusa?) the naked ova retain their original simple appear- ance 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 rjourish the ovum, and partly of external membranes for their protection (the ovolemma, or prochorion). A mem- brane of this sort is formed in all the mammals in the course of the embryonic Erocess. The little globule is surrounded y a thick capsule of glass-like trans- parency, the zona pcllucida, or ovolemma pel/ueiilum (Fig. 14). When we examine it closely under the microscope, we see verj line radial streaks in it, piercing (he ona, which are really verj narrow canals. The human ovum, whether fertilised o\- not, cannot be distinguished from that of mosl of the other mammals. It is nearly the same everywhere in form, si/e, and composition. When it is fully formed, it has a diameter o( (on an average) about ,'„ of an inch. When the mammal ovum has been carefully isolated, and held against the Kghl on a glass-plate, it may be seen as a line point even with the naked eye. The ova oi' most o\ the higher mammals are about the same si/v. The diameter of the ovum is almost alw a\ 5 between . _,',., , ] , 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 arc 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 the twenty- ninth chapter). The fertilised bird-ovu'rrj (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 nourish- ment and works (his into the familiar large yellow yelk. When we examine a very young ovum in the hen's oviduct, we THE OYCU AXD THE AMlEliA 45 find ii to be a simple, small, naked, amoeboid cell, just like the young ova of othei animals (Fig. 13). l>ut it then grows to il>*-' siae we are familiar with in the round yelk of the egg. The nucleus of the ovum, or the germinal vesicle, is thus pressed right u> the surface of the globular ovum, and i> 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 " (ckatricula) (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 theyelk-* a> ity, or latebra, Fig. 15^). The yellow yelk-matter which surrounds this white yelk has ihe appearance in the egg (when boiled hard) of concentric layers (<)• The yellow yelk is also enclosed in a delicate structureless mem- brane (the membtana vitellina, a). As the large yellow ovum ol' the bird (gy F10. ij.-Ova of various animals, executing amoeboid movements, highly magnified. All the ova are naked cells ol varying shape. In the dark fine-grained protoplasm (yelk) is a large vesicular nucleus (the germinal vesicle), and in this is seen a nuclear bod) (the germinal spot;, in which again we often tee a germinal point. Ptgi .// .Ij represent the ovum a( ■ sponge (Leuculmii echinus) in lour successive movements. Hi Hs are the ovum ol a parasitic cnb(Chomiractmthus conttttus), in eight successive movements. (From J Jward x-un Ji,i,,-il times I he whole ovum is a simple round cell. The chief part of the globular mass n formed by the nuclear velk (deuiofilasm), 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, winch corresponds to the nucleus. This encloses a darker granule the k.,r,m„al spot, which shows a nucleolus. The globular velk is surrounded V.YA »<••. thick transparent germinal membrane (ovolemma, or zona iellucida) 1 his 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 velk at impregnation. ' ever 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 germinal disc. We shall return to this discogastnda in the ninth chapter. When the mature hird-ovum lias left the ovary and been fertilised in the ovi- duct, it covers itself with various mem- branes 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 tine inner skin. All these gradually forming en- velopes 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 en- velopes 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 arc formed round the vgg. The ovum of many other animals has the same internal and external features. They have, however, only a physiological, not a mor- phological, importance ; they have uo direct in- fluence On the formation of the foetus. They are partly consumed as food by the embryo, and partly serve as protec- tive envelopes. Hence we may leave them out of consideration altogether here, and restrict ourselves to material points— to the Substantial identity oj 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 'uiman ovum. We reach a very simple, cells, and is now commonly called the I but very important, conclusion. From THE OVUM AND THE A MCE B A 47 the fact that the human ovum and that of all other animals consists of a single cell, it follows immediately, according to. the bio- genetic law, that all the animals, including man, descend from a unicellular organism. Fio. 15. -A fertilised ovum from the oviduct of a hen. The yellow ydk (c) cotwirti til •even] con- uentric lasers ( J ). .uul is enaoaed in ■ thin yclk-mcm- branef'u). The nucleus or germinal VCtiCtC 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 Jo not differ very much. 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 (art that all the ova are at first simple cells, that aJl 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 Btem-ibrm 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. 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 multi- cellular 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 inde- pendent 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 gregarina\ 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 Am.iba. For a long time now we have bom- prised under the general name of amaba- a number of microscopic unicellular organisms, which are very widelv distri- buted, especially in fresh water, but also in the Ocean ; in fact, they have lately been discovered in damp soil. There are also parasitic amu-ha- which live inside Other animals. When we place one of these amoebae in a drop of water under the microscope and examine it with a high power, it generally appeals as a roundish particle of a very irregular and varying shape (Figs. 16 and 17). In its soft, slimy, semi-fluid substance, which con- sists of protoplasm, we see only the solid globular particle it contains, the nucleus. This unicellular body moves about con- tinually, creeping iii 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. Alter a time, perhaps, the action changes. Fie. 16— A creeping amoeba (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. The amoeba suddenly stands still, with- draws its projections, and assumes a globular shape. In -a little while, how- ever, the round body begins to expand again, thrusts out arms in another 4» THE OVl '.!/ . I ND THE . I McEH. I direction, and movea on once more. These changeable processes are called "false feet," or pseudopodia, because they act physiologically as feet, yel are not special organs in the anatomic sense. They disappear as quickly as they come, and are nothing more than temporary projec- tions of the semi-fluid and structureless body. If you touch one of these creeping amoeba- with a needle, or put a drop of acid in the water, the whole body at once contracts inconsequenceof this mechanical Fig. 17.— Division of a unicellular amoeba (Amceba polypodia )\n six stages. (From F. E. Schultze.) The dark spot is the nucleus, the Jighter spot a contractile vacuole in the protoplasm. The latter re-forms in one ot the daughter( the sponge. We also find this remarkable phenomenon among other animals, SUCll as the graceful, bell-shaped zoophytes, which we 1 all polyps and medusa-. Their ova remain naked cells, which thrust out amoeboid projections, nourish themselves, and move about. When they have been fertilised, the multicellular organism is formed from them by repealed segmen- tation. It is, therefore, no audacious hypothesis, but a perfectly sound conclusion, to regard the amoeba as the particular unicellular organism which offers us an approximate illustration of the ancient common unicel- lular ancestor of all the metazoa, or multi- cellular animals. The simple naked amoeba has a less definite and more original character than any other cell. Moreover, there is the fact that recent research has discovered such amoeba-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 amoebae, take up solid particles, or " cat " (whence they are called phagocytes__ - "eating-cells," Fig. 19). Lately, it has been discovered that marly different cells may, if they have room erough, execute the same movements, creeping about and eating. They behave just like amoeba (Fig. 12). It has also been shown that these "travelling-cells," or p/anocytes, play an important part in man's physio- logy and pathology (as means of transport for food, infectious matter, bacteria, etc.). The power of the naked cell to execute these characteristic amoeba-like move- ments conies from the contractility (or automatic mobility) of its protoplasjrf. This, seems to be a universal piopertyof young cells. When they are not enclosed by a tinn membrane, or confined in a " cellular prison," I In \ 1 an always accom- plish these amoeboid movements. This is true o\ the naked ova as well as o[ any other naked 1 ells, of the " travelling-cells," iA' various kinds in connective1 tissue, lymph-cell^, mucus-cells, etc. We have now, by our study of the ovum and the comparison o( it with the amoeba, provided a perfectly sound and most valuable foundation for both the embryology and the evolution oi man. We have learned that the human ovum is a simple cell, that this ovum is not materially different from that of other Fig. 18.— Ovum of a sponge (OJynthua). The ovum creeps about in the body of the sponge by thrusting out ever-changing processes. It is indistin- guishable from the common amoeba. mammals, and that we may infer from it the existence of a primitive unicellular ancestral form, with a substantial resem- blance to the amoeba. The statement that the earliest pro- genitors of the human race were, simple cells of this kind, and led an independent unicellular life like the amoeba, has not only been ridiculed as the dream of a natural philosopher, but also been violently censured, in theological journals as " shatneful 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 im- moral" 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 amoeba. So THE OVUM AND THE AMCEBA We can show this to be a fact an] daj with the microscope, and it is little use to dose one's eyes to " immoral " facts of this kind. It is as indisputable as I he momentous conclusions we draw from il and a-, the vertebrate character of man (sec Chapter XL). \\ .■ now see very clearly how extremely important the cell theory has been for our whole conception o\ organic nature. " Man's place in nature " is settled beyond I u; rg.-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 ol the invertebrates arc unprotected pieces ot plasm and take in food, by means of their peculiar move- ments, like the amoebae." I had (in Naples, on May 10th, i8«) injected into the blood-vessels of one of these snails' an infusion ol water and ground indigo, and was greatly astonished to find the blood-cells themselves more or less filled with the particles ot indigo after a few hours. After repeated injections I Succeeded in "Obseryinsr the very entrance of the coloured particles m the blood-cells, which took place just in the same way as with the amoeba." I have. /given further particulars about this in my Monograph on the Radiolaria. 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 under- stood in the light of the'cell-soul, and we have the simplest form of this in the amoeba. 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 ot man, was gradually evolved from them'. I he academic psychologists who lack this zoological equipment are unable to do so. This naturalistic and realistic con- ception 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 ils foundation. An instructive instance of this was seen a few years ago, in the academic discourse delivered by a distinguished theologian, Wil- libald Beyschlag, at Halle, January 12th, 1900, on the occa- sion of the centenary festival. The theologian protested violently against the "materialistic dust- men 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." Never- theless, not a single informed and im- partial scientist doubts the fact that these greatest men were, like all other men— and all othervertebrates — developed from an impregnated ovum, and that this simple nucleated globule of protoplasm' has the same chemical constitution in all the mammals. CONCEPTION 5' Chaptkr yi r. CONCEPTION Thk recognition of the fad that every man begins his individual existence .is .1 simple cell is the solid foundation of .ill research into the genesis of man. From this tact 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 amoeba ,h particularly impor- tant (if. Chapter VI.). That these unicel- lular ancestral forms did once exist follows directly from the phenomena which we perceive ever) day in the fertilised ovum. The development of the multicellular organism from the ovum, and the forma- tion o( the germinal layers and the tissues, follow the same laws in man and all the higher animals. It will, therefore, he our next task to consider more closely the impregnated ovum and the process of conception which produces it. The process o\ impregnation or sexual conception is one of those phenomena that people love to conceal behind the mystic veil o\ supernatural power. We shall soon see, however, that it is a purely mechanical process, and can he reduced to familiar physiological functions. More- over, 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 tor its main purpose the introduction oi 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 impor- tant 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 monogony ; these are especially the sexless monera (chromacea, bacteria, etc.), but also many other protists, such as the amoebae, foraminifera, radiolaria, myxomycet.e, etc. In these the multipli- cation of individuals takes place by unsexual reproduction, which lakes the form of cleavage, budding, or spore- formation. The copulation of two coales- cing 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 c. isis of parthenogenesis (virginal conception) among the verte- brates. Sexual reproduction offers an infinite variety ot interesting forms in the dif- ferent classes of animals and plants, especially as regards the mode of concep- tion, and the conveyance of the spermato- zoon 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 o\ Charles Darwin and Hermann Muller on the fertilisation of flowers by insects have given us very interesting particulars of this.' 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 them- selves, 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. ' Sec Darwin's work, On the Various Cont NMMH by which Orchitis are Fertilised (i86j). D 5-^ CONCEPTION 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 arc known as the sperm or seed-cells, or the sperma- tozoa (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 rase with hirds and reptiles and manv 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 Firstly, they are extraordinarily small, being usually the smallest ceHs 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 (hut 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- M Fig. 20— Spermia or spermatozoa of various mammals. The pear-shaped flattened nucleus is seen from the trout in /. and sideways in //. k is the nucleus, "/ its middle part (protoplasm), .« the mobile, serpent-like tail (or whip); M four human spermatozoa, A four spermatozoa from the ape; K from the rabbit ; // irom the mouse ; C from the dog ; .S" from the pig. have to consider in the process of concep- tion, 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 it it, that cause conception. The spermatozoa of the great majority of animals ha vet wo characteristic features. zoidia) ; they are now scientifically known as spermia or spetmidia, or as speimato- soniata (seed-bodies) or spermatofifa (seed threads). It took a good deal of com- parative reasearch 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 (Ji/anaJ; in this case they are sometimes CONCEPTION si amoeboid and like very small >ua [Fig. 21 c-e). But in most of the lower animals (such as the sponges and polyps) the) have the same pine-cone shape a> in man and the Other mammals (I a. A). Fig. ii. -Spermatozoa or spermidia of various animals. (From /.)■ In the central piece ( in ) we can distin- guish a short neck anil a longer connective piece (with central hod;. ). The tail consists of a long main section (h) and a short, yen tine tail ft J. The process of fertilisation by sexual conception consists, therefore, essentially in the coalescence and fusing together of two different cells. The lively sperma- tozoon travels towards the ovum by its Serpentine movements, and bores its vv.iv into the female cell ( big. -'.;). The nuclei of both sexual cells, attracted by a certain " affinity," approach each other and melt into one. The fertilised cell is quite another tiling from the unfertilised cell. For if we must regard the spermia as real cells no less than the ova, and the process ^\ concep- tion as a coalescence of the two, we must Consider the resultant cell as a quite new and independent organism. It hears in the cell and nuckar matter o( the pene- trating spermatozoon a part of the father's body, and in the pro- toplasm and caryo- plasm o\ the ovum a part o( the mother's bodv. This is clear from the fact that the child inherits many features from both parents. It inherits from the father by means oi the sperma- tOZOOn, and from the mother by means of the ovum. The Pn, -j -A single human spermatozoon magni- fied 2.000 times; a shows it Irom the broad, r and h from the narrower side, i head (with nu, middle-stem, h lonjj-stem, and e tail. (From A 54 CONCEPTION actual blending ol the two colls produces! a third cell, which is the germ of the child, or the new organism conceived. Oik- maj also say of this sexual coales- cence that the stem-cell is a simple herma- phrodite; it unites both sexual substances in itself. 1 think ii necessary to emphasise the Fundamental importance of this simple, but often unappreciated, feature in order to have a correct and clear idea of concep- tion. With that end, I have given a special name to the new cell from which the child developes, and which is gene- rally loosely called "the fertilised ovum," or " the lust segmentation sphere." I call i( "the Mem-cell " (eytula). The name " stem-cell " seems to me the simplest and most suitable, because all the other cells of the body are derived 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. from it, and because it is, in the strictest sense, the stem-rather 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, some- thing quite different in this stem-cell from what we find in the two parent-cells, from the coalescence of which it has issued. Y'hc life of 'the stem-cell or cytula is the product or resultant of the paternal life- movement that is conveyed in the spermato- zoon ani tin- maternal lifc-movcmoit 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 tells, known as "the segmentation sphere" or ''segmentation cells." The process is most clearly observed in the ova of the e< hinoderms (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 pre- liminary changes, which are very neces- sary—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 pronu- cleus uiasculiuus). The impregnation of the ovum com- mences 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 im- pregnation 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 con- verted 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 vet 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 separa- tion from the large mother-cell takes CONCEPTION place in the same way as in ordinan "indirect cell-division,1 Heme, the pol.n cells are probably to be conceived as " abortive ova,"or " rudimentarj ova," which proceed from a simple original ovum b) cleavage in the same way thai 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, cadi furnished with a fourth of the original nuclear matter (the hereditary chromatin) ; and each of these lour descendant cells becomes a spermato- . ready tor impregnation. Thus is prevented the doubling of the chromatin in the coalescence Of (he two nuclei at con- ception. As the two polar cells are i \- t aided and lost, and have no further part in the fertilisation of the OVum, we no^d not discuss (hem any further. But we must give more attention to the female pro-nu< leus which alone remains after the extrusion of the polar cells and the dis- sol\ mil; of the germinal vesicle ( Fig. 2 ^ <• *). 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 ot the act of impregnation, is the .stem-nucleus, or the first segmentation nucleus (archicaryon) that is to say, the nucleus of the new-horn embryonic Mem- cell or "first segmentation cell." This stem-cell is the starting-point of the sub- sequent embryonic processes. Hertwig has shown that the tiny trans- parent ova of the echinoderms are the most convenient for following the details of this important process of impregnation. We can, in this ease, easily and sue. fully accomplish artificial impregnation, and follow the formation of the stem-cell step by st,.p 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- mi'& •;v.:. '.'. ••;•.;«•.• • Fig. 25.— Impregnation of the ovum of a star-fish. (From tierhoig.) 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 protopla: m o( the ovum < C J. study of the rise and structure of the stem- cell. The first question that arises is as to the behaviour of 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 pheno- menon of heredity is inseparably connected with the reproductive process, we may further conclude that these two copu- lating nuclei "convey the characteristics which are transmitted from parents to offspring." In this sense 1 had in 1866 (in the ninth chapter of the General Morphology) ascribed to the reproductive 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 sperma- tozoon the work of stimulating and originating its development. The stimu- lus which it gave to the ovum was some- times thought to be purely chemical, at other times rather physical (on the prin- ciple of transferred movement), or again a mystic and transcendental process. This error was partly due to the imper- fect knowledge at that time of the facts of impregnation, and partly to the striking CONCEPTION 57 difference in the sues 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 demon itrated, the sperma- tozoon was believed to disappear in t ! u- ovum without leaving a ti ice. 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 th.it the tiny sperm-cell i- not Subordinated to, but co-ordinated with, the large ovum. The nuclei of the two cells, as the vehicles of tlu- 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 The striking differences of the res pe< live sexual cells in size and shape, which tsioned the erroneous views of earlier scientists, an- i i ilj explained ow the principle of division >>f labour. The inert, motionless ovum grows in >rding to tlu- quantity >>f 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 icek the ovum ami bore its way into its yelk. These differences are very eon-.pirn.ius in the higher animals, hut they are much less in the lower animals. In those protists (unicellular plants and. animals) which have tlu- first rudiments of sexual repro- duction tlu- two copulating cells are at lust quite equal. In these cases the act of impregnation is nothing more than a Impregnation of the ovum of the sea-urchin. (sk) moves towards the larger nucleus oi the ovum (ek). the radiating mantle of protoplasm. (From Hertmg.') In Fig. j6 the little sperm-nucleus In Fig. 27 they nearly touch, and .ire surrounded by 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 holh parents, and that on the average they are equally distributed. I say "on the average," hecause 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 char- acters (the sexual glaiids) are concerned. But it is also possible tint the determina- tion of the latter — the weighty deter- mination 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 which comes from both parents in the act of conception. sudden growth, in which the originally simple cell doubles its volume, and is thus prepared for reproduction dell- division. Afterwards slight differences are seen in the size oi 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 si/e. Quite in harmony with this new con- ception of the equivalence of the two ■j,on.ia\^, or the equal physiological im- portance of the male and female SCX-Cells and their equal share in the prOO heredity, is the important fact established by Hertwig (1875), thai m normal impreg- nation only one single .spermatozoon 0 CONCEPTION 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 oi the fortunate penetrator are excluded, and die without. But it" the ovum passes into a morbid state, it" it is made stiff by .1 lowering of its temperature or stupefied with narcotics (chloroform, morphia, nicotine, etc.), two or mor« spermatozoa may penetrate into its yelk-body. We then witness polyspermism. The more II rtwig chloroformed the ovum, the more spermatozoa were able to bore their w.i\ into its unconscious body. Fie 28.— Stein-cell of . a rabbit, magnified 200 times. In the centre of the granular protoplasm of the fertilised ovum (> 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 saj that the individual physiological character of each of these cells is due to its molecular plastklule - movement. Heine, the plastn/u/e-movement of the cytula is the resultant of the combined plastidule-tnovements of the female ovum a fid the male sperm-cell.1 Chapter VIII. THE GASTR^A THEORY There is a substantial agreement through- out the animal world in the fust 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 v'crv lowest and simplest forms of animal life ; these remain unicellular throughout life. To this group belong the anuvb.e, greg.u in.e, rhi/opods, infusoria, etc. As their whole organism consists of a single cell, thev can never form germinal layers, or definite strata of cells. Hut all the other animals— all the tissue-forming animals, or metaroa, as we call them, in contra- distinction 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, his long been known as "segmenta- tion." Sooner or later the segmenta- tion-cells join together to form a round (at first, globular) embryonic sphere (bias tula) ; they then form into two very different groups, and arrange themselves rhe plasson ..f tin- •tern-cell or cytula may, from the anatomical point o( view, be rea-arded .is homo- t, aW, a Vri';,l'ri'?" 'kt' ' ,a, °, t,,e nU,'Ur-1- Tir * n"1 ' — '<-'< With our In political asc ripl ,„ lO the plastidules (or molecules of tin- plasson) ol a complex molecular structure. Tlu- complexity of ll.is ,'s the (Tcatcr ,n Proportion to the complexity ol 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. 6o THE ('.. \ s TR. EA Tin: oh' 1 • in two separate strata the two primary u'nal layers. These enclose a diges- tive cavity, the primitive gut, with an opening, the primitive mouth. We give the name of \\w gastrula to the important embryonic form thai has these primitive organs, and the name of gastrulation to the formation of it. This ontogenetic process, lias a very threat significance, and is the real starling-point of the construction oi the multicellular animal The fundamental embryonic processes of the cleavage of the ovum and the formation oi~ 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 gastraea 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, cars, gill- arches, etc. — from the similar forms of .the other higher mammals, we may con- fidently assume that they agree in the earliest embryonic processes, segmenta- tion and the formation of germinal layers. This has not yet, it is true, been estab- lished by observation. We have never yet had occasion to dissect a woman immediately after impregnation and examine the stem-cell or the segmenta- tion-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. Hut 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, there- fore, understand it altogether in itself. In order to do this, we nave 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 seg- mentation 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.). Hut 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 (Li'ihihtus), the arrow-worm ( Sagitta), and many of the echinoderms and cnidaria, such as the common star- fish and sea-urchin, many of the medusa- and corals, and the simpler sponges ( Olynthus), We may take as an illus- tration the palingenetic segmentation and germinal layer-formation in an eight-fold insular coral, which I discovered in the Red Sea, and described as Monoxenia Daiwinii. 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 con- sequence of this, the protoplasm is con- stricted 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 sub<- divides 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.1 The final result of this 1 The number of segmentation-cells thus produced increases geometrically in the original gastrulation, or the purest paleogenetic 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. THE GASTR/EA THEORY 6l Fie. r>.— Castrulp.tion of a corp' f Afonoxtnia Darwiaffl A, I?. stem-cell (cytuln) or impregnated ovum. In Fi^f. A (immediate!) after Impregnation) the nudctw w invuuble. la IV:. B (a little later) il i* quite clear. C two « ■••mi. ntation-cclU. I) lour segmentation-cells. K mulberry-formation (morula): !•" bla»to«phcrc (bl.i-.tul.il. C I>I.i-.IiiIa (transverse section). II dcpula, or hollowed Mantilla (tronsvereq section). I gastnua (lonyitiiJiM.il ~ ^ti.Mii. K gantrula, or tup *phort, external appearance. THE GASTR/EA THEORY repeated cleavage is the formation of a globular cluster oi similar segmentation- cells, which we call the mulberry-forma- tion or morula. The cells are thickly pressed together like the parts oi a mul- berry or blackberry, and this gives a lumpy appearance to the surface oi the sphere ( rig. E).' 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 segmenta- tion-cells are loosened, and all rise to the surface. There they are Rattened by mutual pressure, and assume the shape ot truncated pyramids, and arrange them- selves side by side in one regular layer (Figs. F, ti). This layer of cells is called the germinal membrane (or hlastoderm) ; 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 blast via or blastospkere* In the case oi our coral, and oi many other lower forms of animal life, the young embryo begins at once to move independently and swim about in the water. A tine, long, thread-like process, a sort oi 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 blast ula to move about in a rotatory fashion. In many other animals, espe- cially those in which the embryo developes 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, 1 The segmentation-cells which make up the morula after the close ot the palingenetic cleavage seem usually to be quite similar, and to present no differences as to size, form, and composition. That, however, doc s not prevent them from differentiating into animal and vegetative cells, even during- the cleavage. -' 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 blastocyst is. This cenogenetic gastrocystis and the palingenetic blastula are sometimes very wrongly comprised under the common name o( blastula or vesicula blastoderm ica. the wall o\' which is composed of a single layer oi cells, such as the volvox, the rnagosphaera, syhura, etc. We shall speak further of the great phylogenetic significance oi this fact in the nineteenth Chapter. 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 oi' tWO layers oi' 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 oi' 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 ( blastoca'l ) which is disappearing, and the primitive gut- cavity ( progastcr) 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 prodiuvs 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, echind- derms and annelids, cnidaria and sponges — proceed from one and the same embry- onic form. In illustration I give a few THE GASTRMA THEORY 63 pure gastrula forms from various groups of animals (Figs. 30 35, explanation given beta* 1 a< hi. In view o( this extraordinary signifi- cance of the gastrula, we must make » wi \ 1 areful study of its oi iginal sti ucture. As ;i rule, the typical gastrula is ver) small, being invisible to the naked eye, or half round, or even almost round, and in others lengthened out, or almost C) limit ii al. 1 give the name of primitive gut ( pro- gasU 1 ) and primitive mouth (fnostoma) to the internal cavity o\ the gastrula-body and its opening; because this cavity is the ftrst rudiment o\ the digestive cavity of Fig. 31, Fig. si. Rio. 33. Fto. 34 Fig. 30. Fig. 35- Fig. 30 (A).— Gastrula of a very simple primitive-gut animal or pastraead (gastrophysema). ( lla.:k,-i.) Fig. 31 (R).— Gastrula Of a worm (Sag-il/a). (From h'oivaleishy.) Fig. 32 (C).— Gastrula of an echinoderm (star-fish, (/raster), not completely folded in (dcpula). (From Alexander Agnssit.) Fig. 33 (D).— Gastrula of an arthropod (primitive crab. \au/>lius)(as 32). I'k;. 31 ( E).— Gastrula of a mollusc (pond-snail. Limntems). (From Karl Rabl.) Fig. 35 (!•').— Gastrula of a vertebrate (lancclet, 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). at the most only visible as a fine point under very favourable conditions,- and measuring generally zhv to rtr of an inch (less frequently A 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 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 '- ii)> ;U1(J molluscs (Fig. 34), and, finally, in a slightly modified form, in the lowest vertebrate (the amphioxus, Kg. 35)- The gastrulation of the amphioxus is especially interesting because this lowest and oldest of all the vertebrates is ol the highest significance in connection with the evolution of the vertebrate stem, and therefore with thai of man (compare Chapters XVI. and XVII.). Jut as the comparative anatomist traces the most elaborate features in the structures of the various i lasses of vertebrates to divci development from this simple primitive vertebrate, so comparative embryology traces the various secondary forms of vei tc- brate gastrulation to the simple, primary formation of the germinal layers ;.> the Pro. 37.- Cells from the two primary permlnal layers ol the mammal (from both layer* of the blasto- derm), i larger and darker cells of the inner stratum, the vegetal layer or entoderm. ? smaller and clearer cells from the outer stratum, the animal la\.r or ectoderm. amphioxus. Although this formation, as distinguished from the cenogenetic modi- fications 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 ./, /> . Heine the blastoderm, which forms the single-layer wall of the globular blastula at the end of ilu i Icavagc-process, does not consist r£ (... THE CASTR.KA THEORY 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, k) is not quite globular, but forms a Battened spheroid with unequal poles of it^ vertical axis. While the blastula is being folded into a CUD at the vegetal pole of its axis, the difference in the size of the blastodermic cells increases (Fig. 38 A E) ; it is most conspicuous when the invagination is complete and the segmentation-cavity has Fig. 38.— Gastrulation of the amphioxus, from H'atschck (vertical section through the axis of the ovum). A, /?, C three stages in the formation of the blastula ; D, H curving of the blastula ; F complete gastrula. h segmentation- cavity, g primitive gut-cavity. 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. 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 itswholelengthin 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. 30 ?-), will be the anterior or belly-side, the opposite, flatter side will form the b.u'U ( a ' j . 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 fust 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 m 04J t h , or "the primitive cells of the mesoderm." In consequence of these considerable varia- tions arising in the course of the gastru- lation, the primitive uni-axial form of the archigastrula in the amphioxus has al- ready 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 conser- vative heredity, this palingenetic form of gastrulation which they have from their Till-: a 1 S 7/>\ EA THEOR \ ' 67 earliest common ancestors. But this is not the case with the great majority oi the animals. With these the original embryonic process has been gradually more or less altered in the course of millions oi years by adaptation to new conditions oi development. Both the segmentation oi the ovum and the sub- sequent gastrulation have in this w.n been considerably changed. In fact, Mmm variations have become so great in the course oi time thai the segmentation was not rightly understood in most animals, and the gastrula was unrecog- nised. It was not until I had made an extensive comparative Study, lasting a considerable time (in the wars [866 75), in animals oi the most diverse classes, that I succeeded in showing the same Common typical process in these appa- rently very different forms of gastrulation, and tracing them all to one original form. 1 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 f/nta- gastrula. The reader will find a scheme oi 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 oi gastrulation is the change in the nutrition of the ovum and the accumula- tion 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 forma- tive-yelk (with the germinal vesicle) then usually gathers at oiw pole and the food- yelk at the other. The first is the animal, and the second the vegetal, pole of the vertical a\is oi the ovum. In these " telolecithal " ova, or ova u iih the yelk at one end (for instance, in the cyclostoma and amphibia), the gastrula- tion then usually takes place in such a way that in the cleavage o( the impreg- nated QVUm the animal (usually the upper) half splits up more quickly than the vegetal (lower). The contractions of the active protoplasm, which effect this con- tinual 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 Fig. 30— Gastrula of the amphioxus, seen from left Side (diagrammatic median section). (From Hatschek.) £- primitive gut, u primitive month, p peri- stomal pole-cells, /entoderm, e ectoderm, (/dorsal side, V ventral side. more but smaller, and in the former fewer but larger, cells. The animal cells pro- duce 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 segmen- tation, and the ova in question holoblastic, or " whole-cleaving." It is otherwise with the second chief group of ova, which he distinguished from these jia meroblaslic, or "partially-cleaving": to this class belong the familiar large eggs pf birds and reptiles, and of most fishes. The inert mass of the passive food-yelk is so 68 THE GASTR.EA THEORY large in these cases thai the protoplasmic contractions o\ the active yolk cannot effect any further cleavage. In conse- quence, there IS only a partial .segmenta- tion. While the protopfasm.in the animal section o\ the ovum continues briskly to divide, multiplying the nuclei, the deuto- plasm 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 oi 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 appro- priate a portion of the food-yelk, and thus form a real "yelk-cell" ( mcrocyte). When this vegetal cell-formation con- tinues for a long time, after the two primary germinal layers have been formed, it takes the name of the "after-segmen- tation." 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 nourish- ment within the foetal membranes. According as the yelk-fopd accumulates at the centre or at the side of the ovum, we distinguish two groups of dividing ova, periblastic and discoblastic. In the periblast^ the food-yelk is in the centre, enclosed fnside 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 dis- coblastic 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 arc 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 investi- gation and philosophic consideration. These, however, have been mastered by the comparative cmbryological research which has been conducted by a number of distinguished observers during the last few decades especially the brothers Hertwig, Rabl, Kuplfcr,Seli'nka,Ruckert, 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 Gasttula and the Segmentation of the Animal OVUM [not translated], in 1875. As it is very important to under- stand 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 disco- blastic vertebrates, nevertheless in every case a blastula is developed from the morula, as in the holoblastic ova. 6. Also, in every case, the gastrula developes from the blastula by curving or invagination. 7. The cavity which is produced in the foetus 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 ; thccells (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. THE GASTRMA THEORY 6<> towards the dorsal side of the embryo; the vertical axis of the primitive gut is thus gradually converted into horizontal. u>. 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 tin.' " properistoma," or region of the anus. From this important point the formation «. ■> f 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 lor in vain, is found always, and is nothing else than the familiar "primitive groove.**— Of this we shall see mora as we proceed. Meantime we realise that gastrulation may he reduced to one and the same process in all the vertebrates. Moreover, the various forms it takes in the invertebrates can always he reduced to one of the four types of segmentation described above. In relation to the dis- tinction between total and partial seg- mentation, the grouping of the various forms is as follows : — I. Palingenetic (primitive) segmentation. II. Cenogenetic segmenta- tion (modified by adaptation). 'i. liqual segmen-' tation (bell-gastrula). j. Unequal teg- men tation (hooded f;as- trnla). J. Diseoid seg-\ mentation (discoid gas- trula). 4. Superficial seg- mentation (spherical gastrula). A. Total scg- mentation (without inde- pendent lood- yelk). B. Partial seg- mentation (with indepen- dent food- yelk). The lowest metazoa we know — namely, the lower zoophyta (sponges, simple polyps, etc.) — remain throughout life at a stage ol development which differs little from the gastrula ; their whole body consists ol two layers of cells. This is a fact of extreme importance. We sec 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 con- clusion : " Man and all the other animals which pass through the two-layer stage, 01 gastrula-form, in the course of their embryonu development, must descend from a primitive simple stem-form, the whole body of which consisted throughout life (as is the Case W Ith the lower /oophvta to-day) merel) ol two cell-strata «>r ger- minal law 1 • " W'i u ill call this primitive stem-form, « ith whi( h we shall deal more full) later on, ihcjpas/nra thai is to say, " primitive-gul animal." According to this g^st raja-theory there was originally in all the multicellular animals tmeorgan with the same structure and function. This was the primitive L,mt ; and the two primary germinal layers which form its wall must also he irded as identical in all. This im- portant 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 hlastula ; and, on the other hand, by the fact that in every case the same funda- mental 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 through- out life, the gastrseads, the simplest sponges (Ofyntnus J, 8X\d polyps (Hydra), these two groups of functions, animal and vegetative, are strictly divided between the two simple primary layers. Through- out 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 ai ts throughout life as a gut-lining, or nutri- tive layer of enteric cells, and often also yields the reproductive cells. The best known of these " gastraads," or "gastrula-like animals," is the common fresh-water polyp (ffydtn J. 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. Hut 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 primi- tive ancestor, the gns/nta (of. Chapter XIX.). 7o THE GASTRASA Tlll-.ORY In all other animals, particularly the vertebrates, the gastrula is merely a brief transitional >taoo. Here the two-layer stage oi the embryonic development i^ quickly succeeded by a three-layer, and then a four-layer, >-tat,rc. with the appearance of the four superimposed germinal layers we reach again a firm and steady Btanding-grourid, 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 arc indicated by the letters a-g : a Zoophyta. b Annelida, c MoIUlUCa. d 1-chinodcrma. <■ Articulata. J Tunicata. g Vcrtcbr ila. I. Total Segmentation. Holoblastic ova. Castrula without separate food-yelk. Hologastrula I. Primitive Segmentation. Archiblastic ova. Bell-gastrula (archigastrula.) a. Many lower zoophyta (sponges, hydrapolyps, medusae, .simpler corals). b. Many lower annelids (sagitta, phoronis, many nematoda, etc., terebratula, argiopc, pisidium). c. Some lower molluscs. d. Many echinoderms. e. A tew lower articulata (.-.ome branchiopods, copepods : Tar- digrades, pteromilina). /. Many tunicata. g. The aerania (amphioxus). II. Unequal. Segmentation. Amphiblastic ova. Hooded-gastrula (amphigastrula). a. Many zoophyta (sponges, medusae, corals, siphonophorae, ctenophora). b. Most worms. C. Most molluscs. d. Many eehinoderms (viviparous species and some others). e. Some of the lower articulata (both Crustacea and tracheata). /'. Many tunicata. g. Cyclostoma, the oldest fishes, amphibia, mammals (not includ- ing man). . II. Partial Segmentation. Mcroblastic Gastrula with separate food-yelk. Merogastrula. III. Discoid Segmentation. Discoblastic ovai. Discoid gastrula. IV. Superficial Segmentation. Periblastic ova. Spherical-gas- trula. c. Cephalopods or cuttle-fish. e. Many articulata, wood-lice, scorpions, etc. g. Primitive fishes, bony fishes, reptiles, birds, monotremes. e. The great majority of the arti- culata (crustaceans, myriapods arachnids, insects). THE GAS Th'l /.. I TION OF THE I rER TE BR. 1 TE 7' Chapter IX. THE GASTRULATION OV THE VERTEBRATE1 T 1 1 ;•: remarkable processes o\ gastrulation, ovum-segmentation, and formation oi' germinal layers present a most con- spicuous variety. There is to-day only the lowest oi the vertebrates, the amphi- oxus, that exhibits the original form o\ those processes, or the palingenetic gastru- lation which we have considered in the pceceding chapter, and which culminates m the formation of the archigastnjla (Fig. 38). In all other extant verlehrates these fundamental processes have been more or less modified .by adaptation to the conditions o( 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 con- siderably from each other. In order to grasp the unity that underlies the mani- fold 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 commen ancestor, "the primitive vertebrate." Hence the embryonic processes, by which eaoh 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 lorms 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 holo- bffUltC ova that is to say, ova with total, unequal segmentation ; while the younger Cyclostoma, most of the lishes, the cepha- lopods, reptiles, birds, and monot femes, have metoblastic ova, or ova with partial discoid segmentation. A 1 loser 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 compli- cated. In order to understand them properly, we must first consider the various modifications oi 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 segmen- tation. In order to understand the whole process rightly and follow the formation of the germinal layers and the gastruja, 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.2 In this way we get a definite axis of the ovum with two poles. To give a clear ■ C(. Balfour's Manual of Comparative Embryology, vol. ii.; Theodore Morgan's The Development of the Frog's Egg. 3 The_ colouring of the cjjgs of the amphibia is caused by the accumulation of dark<louring 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. 7' THE GASTRVLATION OF THE VERTEBRATE iiio.i of the segmentation of this ovum, it is iv^t to compare it with a adobe, on the surface of which are market! 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 heme the whole process lias been given the name of furcation. In reality, however, this " furcation, " which was formerly regarded as ■ very mysterious process, is 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 ( /)' ). An hour later a second furrow arises in the sains way, and this cuts the first at a right angle (Fig. 40 C). TIid ovum is thui divided into four equal Fig. 40.— The cleava'ge of the frog's ovum (magnified ten times). A stem-cell. R the first two segmen- tation-cells. C tour cells. I) eight cells (4 animal and 4 vegetative). R twelve cells (8 animal and 4 vegetative), /•sixteen cells (8 anima! and 8 vegetative). G twenty-four cells (16 animal and 8 vegetative). H thirty-two cells. / forty-eight cells. A' sixty-four cells. L ninety-six cells. .»/ iCkj cells (128 animal and ^2 vegetative). nothing but the familiar, repeated cell- segmentation. Hence also the segmenta- tion-cells which result from it are real cells. 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 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 (Kig. 40/)). 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 THE CASTRULATION OF THE VERTEBRATE 73 four new longitudinal divisions extend gradually to the lower cells, and the number rises from twelve to sixteen ( /■' j. Then a second circular furrow appears, parallel to the first, and nearer to the north polo, so thai we maj compare it to the north polai rink-. In this wa\ we pet twenty-four segmentation-cells six- teen upper, smaller, and darker ones, and light smaller and brighter ones below in succession forty, forty-eight, fifty-six, and at last si\t\-iour cells ( /, K ). In the meantime, the two hemispheres differ more and more from each other. Whereas the sluggish lower hemisphere long remains at thii tv-t wo < ells, the lively northern hemisphere briskly sub-divides tune, producing firsl sixtv-four and then i-'S (ells (I., .)/). Thus we reach a stage in which we count on the surface i-'ii. 41. Fie. 43. » :■:■. s .V Fio. 44. ••' 44 Four vertical sections of the fertilised ovum of the toad, in four successive stages 01 jtevelopment I he letters have the same meaning throughout: F segmentation-cavity, ©covering of same \n dorsal half ot the embryo, /'ventral half). P yelk-stopper (white round field at the lower pole) if yelk-cells of the entoderm (Rcmaks "glandular embryo"). .V primitive gut cavity (progastei or Ruscoman alimentary cavit) 1 1 he primitive mouth (prostoma) ,s dosed l>> the yelk-stopper, P. » partition between the primitive cut eaviLyr/V; and the segmentation cavit) 1 /■ j. *f section ot the large circulai lip-border ol the primitive moUth (the Ruscoman anus), rhe line ol dots between * and V indicates the earlier connection of the yelk-stopper (PI •run the central mass ol th>- yelk-celtar^J. In Pig. 44 the ovum has turned 90-. so that the back of the embryo is uppermost and the ventral *ide down. (From Stricter.) (<■'•)■ 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- tuo cells altogether (II). Then eighl new longitudinal lines are formed at the north pole, and these proceed to divide, firsi the darker tells above and afterwards the lighter southern cells, and finally reach the south pole. In this way we ge* of the ovum 12S small cells in the upper half and thirty-two large ones in the lower half, or itKi altogether. The dis- similarity of the two halves increases : while the northern breaks up into a greal number ol small tells, the southern con- sists of a much smaller number ol' larger « ells. Finally, the dark cells of the uppei half grow almost over the surface ot' the ovum, leaving- only a small circular spot n THE G \STRULATJON OF THE VERTEBRATE ut the south pole, where the large and cleai cells of the lower half are visible. This white region .it the south pole corre- sponds, as uv shall see afterwards, to the primitive mouth of the gastrula. The Fig. 45.— Blastula of the water-salamander ( Triton), fh segmentation-cavity, dz yelk-cells, r» border-zone, (From Hertwig.) 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. In the meantime, a large cavity, full of fluid, has been formed within the globular body the segmentation-cavity or embry- onic cavity [Mastoccel, Figs. 41-44 F). It extends considerably as the cleavage proceeds, and afterwards assumes an almost semi-circular form (Fig. 41 /'"). The frog-embryo now represents a modi- lied 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 cntoderm- cells (Figs. 41-44 A'). This is the primitive gut-cavity or the gastric cavity of the gastrula, prqgaster or archen- teron. It was first observed in the ovum of the amphibia by Rusconj, 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 (he border of the primitive mouth, the most important pari of the embryo (Fig. 44 *, V\ Soon the primitive gut" cavity stretches further and further at the expense o\ the segmentation-cavitj ( /■'), until at last the latter disappears alto- gether. The two cavities are only sepa- rated by a thin partition (Fig. 43$). with the formation o\ the primitive gut our ffOg-embryo has reached the gastrula Stage, though it is clear that this eenoge- netic amphibian gastrula is very different from the real palingenetic gastrula we have considered (Figs. 30 36). In the growth o( this hooded gastrula we cannot sharply mark off the various stages which we distinguish successively in thebell-gastrula as morula and gastrula. Nevertheless, it is not difficult to reduce the whole cenogenetic Or disturbed devel- opment o\~ tliis amphigastrula to the true palingenetic formation of the archigas- trula ol' the amphioxus. 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 parti- cularly 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 belter retained the original palingenetic character. It was first described by Scott and Osborn (1879), and Oscar Hertwig especially made Fig. 4C— Embryonic vesicle of triton (blastula), outer view, with the transverse fold of the primitive mouth (u). (From Hertwig.) a careful study of it (1881), and rightly pointed out its great importance in help- ing us to understand the vertebrate development. Its globular blastula (Fig. 45) consists of loosely-aggregated, yelk- THE GASTRULATION OF THE VERTEBRA TE 75 filled entodermic cells or yelk-cells (tUi J in the lower vegetal half; the upper, animal half encloses the hemispherical segmentation-cavity (fh)t the curved roof o( which is formed oi two or three Fie 47 Sagittal section of a hooded-embryo (dffiultt) of trlton (hlastula at the coinrnencetneat oi H-i-t nil.ition ). ak outer germinal layer, ik inner ger- minal layer, /it segmentation-cavity, »r-..il and ventral lip-' oi the mouth, r/c yelk-cells. (From I let-twig.) strata of small ectodermic cells. At the point where the latter pass into the former (at the equator of the globular vesiele) we have the border /.one ( ft ). The folding which leads to the formation of the gastrula takes place at a spot in this border zone, the primitive mouth (Fig. ■ Unequal segmentation takes place in some oi the cyclostoma and in the oldest fishes in just the same wav 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 thelowest real fishes (Selachii); hence I divided the group oi the cu lostoma in 1886 from the real tithes with which they were formerly associated, and formed of them a special class iA~ vertebrates. The ovum-segmentation in our common river- lamprey ( Petromyson /luviatilis ) was described by Max Schultze in 1856, and afterwards bv Scott (18S2) and Goette (1890). Unequal total segmentation follows the same lines in the oldest tithes, the selachii and ganoids, which are directly-descended from the cyclostoma. The primitive fishes (Sfhnhii ), which we must regard as the ancotral group of the true fishes, were generally considered, until a short time ago, 10 be discoblastic. It was not until the beginning o\' the twentieth century that Bashford Dean made the important discovery in Japan that oi\c oi the oldesl living fishes oi the shark type ( CestrUCiOn japohicus) has the same toial unequal segmentation as the amphiblastic plated tisiu's fganoides).1 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 oi gasii illation. The oldest and most conservative forms oi the modi in ganoids are 1 he scaley sturgeons ( Stu- ritmesj, plated fishes of great evolu- tionary importance, the eggs oi which are eaten as ca\iare; their cleavage is not essentially different from that of the lampre) - and the amphibia. On the other hand, the most modern oi the plated fishes, the beautifully scajed bony pike of the North American rivers ( F.cpidosteus ), approaches the osseous fishes, and is dis- coblastic like them. A third genus (' . I rnia ) 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 Fig. 48.— Sagittal section of the gastrula of the water-salamander ( Triton ). (From Hertivlg.) Letters as in Fig. 17 : except /> \ elk-stopper, tnk be- ginning oi" the middle germinal layer. 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 1 Bashford Dean, ffebUastie CUavag* in the Egg of a Shark, C . ■>/ ' •«< urn ju/xiim us MacU-ay. Aniiottitluites zouloglrue jajtonensrs, vol. iv., Tokio, 1901. 7" THE GASTRUl IT/ON OF THE VERTEBRATE ami lungs. Of the older dipnoi ( Paladip- iit-itstii ) we have now only one specimen, the remarkable Cemfodus of East Aus- tralia ; its amphiblastic gastrulation lias been recently explained by Richard Semon (cf. Chapter XXI.) That of the twp and batrachia, belong to the old, conser- vative groups Of Our Stem. Their unequal OVUm-Segmentation and gastrulation have many peculiarities iri detail, but can always be reduced with comparative ease to the original cleavage and gastrulation Fio. 49 — Ovum-segmentatlon in the lamprey (PetromyonJtuvuUilis), in four successive stages. The ill colls of the upper (animal) bemisphcre divide much more quickly than the cells of the lower (vegetal) Sill hemisphere Fig. arisen through the accumulation ofa store of food-stun al the vegetal pole, a ."nutritive yelk " being thus formed in con- trast to the " formative yelk." Neverthe- less, the gastrula is formed here, as in the previous cases, by the folding or invagina- tion o\ the blastula. We can, therefore, reduce this cenogenetic form of the discoid segmentation to the palingenetk form oi the primith e 1 leavage. This reduction is tolerably easy and confident in the case of the small ovum of our deep-sea bony ti-h, hut it becomes embryonic development and consumed by the embryo. The latter devefopei solely from the li\ ing formative yelk oi the stem- cell. This is equally true of the ova of our sm. ill bony fishes and o\ the colossal QVa o\ the primitive fishes, reptiles, and birds. The gastrulation of the primitive fishes or selachii (sharks and rays) lias been carefully studied of late years by Riickert, Rabl, and 11. E. Ziegler in particular, and is very important in the sense tint this group is the oldest among living fishes, and their gastrulation can be derived directly from that of the colo- stoma by the accumulation of a large quantity of food-yelk. The oldest sharks (Cestracion) still have the unequal seg- mentation inherited from the cyclostoma. But while in this case, as in the case o\' In. ., L Ovum-segmentation of a bony fish. A first cleavage of the stem-cell (cytula), B division of same into fou* si-gment.ition-cells (only two visible). C the germinal disk divides into the blastoderm ( b) and the periblast ( p). J nutritive yelk, f fat-globule, c ovolemma, z space between the ovolcmma and the ovum, filled with a clear thud. 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. Fre- quently 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 ! 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 com- pletely 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 Seg- mentation-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 So THE GASTRULATION OF THE, VERTEBRATE (Fig. 56 tuf) ; it extends gradually from this spot (which corresponds to the Kih- conian amis of the amphibia) forward and around, so thai the primitive mouth becomes first crescent-shaped and then Fie. 54— Discoid gastrula (discogastrula) of a bony nsh. t ectoderm, i entoderm, «■ border-swelling or primitive mouth, n albuminous globule of the nutri- tive velk, /Mat-globule o( same, c external membrane (ovolcmma), d partition between entoderm and ecto- derm (earlier the segmentation-cavity). circular, and, as it opens wider, surrounds the ball of the larger food-yelk. Essentially different from the wide- mouthed discoid gastrula of most of the selachii is the narrow-mouthed discoid gastrula (or epigastrula) of the amniotcs, the reptiles, birds, and monotremes ; between the two — as an intermediate stage — we have the amp/iigastiula 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 ( Gymnophionn, Coecilia, or Pcro- nwla ), 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 em- bryonic development has been ex- plained 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 Sey- chelles (1897). It is only by the historical and comparative study of these that we can understand the difficult and obscure gastru- lation of the amniotcs. The bird's \;^ 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 sludv, 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 si/e, 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 cither 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 ( prolovum ) 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 dk }'■ •" elk-nuclei, /V/ fine-grained yelk, gd coarse-grained > ilk. <^>. -Longitudinal section of the blastula of a shark (PrUtiurw) at the beginning of g.i TOO! RiicEcrt.) (Seen from the left.) /'tore end. // Kind end, />' segmentatioh-cavil y, ud first t gastrula- traec >>t shows a slight trace o( concentric layers in the hard-boiled egg (Fig. 15 r). We also find in the hen's Cgg, when we break the shell and take out the yelk, a round small white disk at it i 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. 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 irregularly ( D, E). Itl 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, tht se segmentation-cells form a round, lens- shaped disk, which corresponds to the morula, and is embedded in a small depression of the white velk. 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. „yS). The small segmentation-cavity (jh) is very Hat and much compressed. The upper or dorsal wall ( (lw ) is formed of a single layer of clear, distinctly separated cells ; this THE GASTRULATION OF THE VERTEBRATE corresponds to the upper or animal hemi- sphere oi the triton-blastula (Fie. 45). The lower or ventral wall of the flat dividing space ( vwj is made up of larger and darker segmentation-cells ; it corre- sponds to the lower or vegetal hemisphere of the blastula of the water-salamander (Fig. 45 db). The nuclei of the yelk-cells, which are in this case especially numerous at the edge o\ the lens-shaped blastula, travel into the white yelk, increase In /age, ami contribute even to the further growth o\ the germinal disk by furnishing it with food-stuff. The invagination or the folding inwards of the bird-blastula takes place in this 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 primi- tive mouth ; this is the primitive gul (Fig. (K) u<{). 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 (ivd), in which a number of yelk-nuclei fdk) are distributed. There is a brisk multiplication of these at the edge o\ the germinal disk-, especially in the neighbour- hood ol the- sic kle-shaped primitive mouth. We learn from sections through later Stages of this discoid bird-gastrula that Fig. 57.— Diagram of discoid segmentation in the bird's ovum (magnified about ten times). Only the formative yelk (the tread) is shown in these six figures (A FJ, 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. case also at the hinder pole of the subse- quent 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 con- tiguous parts. In the middle of its hind border there is a white, crescent-shaped groove — Roller'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, the primitive gut-cavity, extending forward from the primitive mouth as a flat pouch, undermines the whole region of the round flat Icns-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 invagina- tion, though greatly disguised, can thus be clearly seen in this case, as Cioette and Rauber, and more recently Duval (Fig. Oi), have shown. '1 he older embryologists (Pander, Rier, Remak), and, in recent times especially, THE GASTRUL AT/OX OF THE VERTEBRATE 83 His, Kolliker, and others, said (hat the two primary germinal layers of the hen's ovum the oldest and most frequent subject of observation I arose by hori- zontal cleavage of a simple germinal disk. In opposition to this accepted view, I affirmed in my Gastma Theory (1873) ils surface. 1 endeavoured to establish this view by the derivation of the verte- brates from one source, and especially by proving that the bird-^ descend from the reptiles, and these from the amphibia. If this i-, correct, the discoid gastrula of \ the amniotes must have been formed by Wtt div fh ivd Vertical SCCtlon Of the bJastula Of a hen fdiscoblastula). fh segmentation-cavity, dw dorsal wall of same, '.-v ventral wall, passing directly into the while yelk (rod). (From Duval.) Fh.;. 59.— The germinal disk of the hen's ovum at the beginning of gastrulatioh ; A before incubation, li in the first hour ot incubation. I Prom h'ollrr.) As germinal disk, X' its fore and H its hind border ; cs embryonic shield, s sickle-groove, sk sickle knob, d yelk. Fh;._«xi.- Longitudinal section of the germinal disk of a siskin (dhcogastrulm). (From Duval.) u l primitive gut. vl, hi fore and hind lips of the primitive mouth (or sickle-edge) ; ak outer germinal layer, ik inner germinal layer, dk \ elk-nuclei, wd white yelk. Fie. 61.— Longitudinal section of the discoid gastrula of the nightingale. (From Duval.) ud primitive gut, vl, hi tore and hind lips of the primitive mouth ; ak, ik outer and inner germinal layers ; vr fore- border of the discogastrula. that the discoid bird-gastrula, like that of all other vertebrates, i> formed by folding (or invagination), and that this typical process is merely altered in a peculiar w.i\ ;md disguised by the immense accu- mulation of lood-yelk and the Rat spread- ing of the discoid blastula at one part of 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 ] have mentioned (Cioetle.Rauber, and Duval) have decisively proved this E *4 TIII-: GASTRULATION. OF THE VERTEBRATE recently for the birds ; and the same lias been done for the reptiles by the fine studies of Kupfter, Beneke. Wenkebat h, and others. In the shield-shaped ger- minal disk of the lizard (Fig, 6a), 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 the primitive spit. The fore (dorsal) and hind (ventral) lips of the transverse furrow correspond exactly to the lips ol the primitive mouth (or sickle-groove) in the birds. The gastrulation of the mammals must I first advanced this fundamental prin- ciple in my essay On the Oastntlation oj Mammals (1*77), and sought to show in this way that 1 assumed a gradual degeneratian of the food-yelk and the yelk-sac on the way from the proreplilcs to the mammals. "The cenogenetic process of adaptation," I said, "which has occasioned the atrophy of the rudi- mentary yelk-sac of the mammal, is per- fect ly 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 super- fluous 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 usc- | ^ less yelk-sac was bound to atrophy by embryonic adapta- tion." My opinion met with little approval at the time; it was vehemently attacked by Kol- liker, Hensen, and His in par- ticular. However, it has been gradually accepted, and has recently been firmly estab- lished 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,' ^**-^ ~fj-~" Heape's and Licberkuhn's on -- .•>.•>»-»,. »: the mole, Kupffcr and KeibePs .Fie. 62.— Germinal disk of the lizard (Lacertaagilis). (From on tne rodents, Bonnet's on ^^indffSrmi^SSla^""' " embryonk *hic,d' A^and lIlc ruminants, etc. From the general comparative point ot Uo a~ ■ a c .1 • • ■ , . view. Carl Rabl in his theory of De derived from his special embryonic the mesoderm, Oscar Hertwig in the latest development of the reptiles and birds.' edition of his Manual (icpz), and Hubrecht This latest and most advanced class of the vertebrates has, as we shall see after- wards, evolved at a comparatively recent date from an older group of reptiles ; and all these amniotes must have come origi- nally from a coram a stem-form. Hence the distinctive ei ibryonic 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. 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 Wilhclm Haacke and Caldwell provided a proof of the long-suspei ted and very interesting fact, that the lowest mammals, the monotremes, lay eggs, like the birds and reptiles, and arc not viviparous like the other mammals. Although the gas- trulation of the monotremes was not really known until studied by Richard Till-: GASTRULATION OF THE VERTEBRATE 85 Semon in 1894, there could lx- little doubt, in view of the great size of their food-yelk, thai their ovum-segmentation was discoid, and led to the formation of a sickle-mouthed discogastrula, as in the /* — b - Fie.. 6?. Ovum of tho opossum (Didelpfys) divided into four. (From Selenka.) /> the four Wig, III! III lllillll 1 1 till r directive body, c UOnudeatcd dated matter. f> albumin-membrane. Case of the reptiles and birds. Heiue I had, in 1875 in my essay on The Gastrula and Ovum - segmentation of' Animals), counted the monotremes among the lis- coblastic 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 (n Australia (1894), the first description and correct explanation of the discoid gastru- lation of the monotremes. The fertilised ovaofthe two living monotremes^ Echidna and Oruit/ior/ivne/ius j are balls oi 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 .is large. The structure of the plentiful yelk, and especially the rela- tion of the yellow and the while 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. 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 gastrula* tion 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-laycred embryo forming from the cleavage. Hut in the monotremes the formation of the ceno- genetic entoderm docs not precede the invagination ; hence in this case the con- struction of the germinal layers is less modified than in the other amniota. The marsupials, a second SubrClaSS, come next to the oviparous monotremes, the oldest of the mammals. Hut as in their case the food-yelk is already atro- phied, and the little ovum devclopes 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 placental s. The fertilised ovum of the opossum ( Didelphys)ds\ ides, according to Selenka, first into two, tl. 11 four, then eight equal cells; hence the segmentation is at first Flo. 64.— Blastula of the opossum (Didelf>hys). (From Selenka.) a animal pule of the blastula, v vegetal pole, en mother-cell of the entoderm^ ex ecto- dermic nils. > spermia, ib unoudeated yclk-balls (remainder of the food-yelk). / albumin membrane. equal or homogeneous. Hut in the course of the cleavage a larger cell, distinguished by its less clear plasm and iis containing more yelk-granul< mother cell of the entoderm, Fig. 04 e»)t 86 THE GASTRULATION OF THE VERTEBRATE Separates from the others; the latter multiply more rapidly than the former. As, further, a quantity oi Quid gathers in the morula, we yet a round blastula, the wall of which is of varying thickness, like that o\ the amphioxus (Fig. 38 E) and the amphibia (rig. 45). Ine upper or animal hemisphere is formed o\ a large number o\ small cells ; the lower or vegetal hemisphere oi a small number o( large cells. One of the latter, distin- guished by its si/e (Fig. <>4 en), lies at the vegetal pole o\ the biastula-axis, at the point where the primitive mouth after- wards appears. This is the mother-cell oi the entoderm ; it now begins to multiply by cleavage, and the daughter- cells (Fig. (15 i) spread out from this spot gastrula (Fig. 6(>) gradually changes into globular, a larger quantity o\' fluid accumulating in the vesicle. At the same lime, the entoderm spreads further and further over the inner surface of the ectoderm ft). A globular vehicle is formed, the wall o( which consists o\ two thin simple strata oi cells ; the Cells o( the outer germinal layer are rounder, and those oi the inner layer Hatter. 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 Fig. 65. over the inner surface of the blastula, rthough 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. 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 gastrula- tion of the opossum the oval shape of the FlO. 66. Frc. 65. — Blastula of the opossum ( Didelphys) at the beginning of gastrulation. (Prom Selenka.) t ectoderm, i entoderm, a animal polo, u primitive mouth at the vegetal pole. /' segmentation-cavity, d unnucleated yelk-halls (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), curtailed cenogenetieally in the placentals than in the marsupials. It was first accurately known to us by the distin- guished 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 1 Fig. 68) are of different sizes and natures ; the difference is sometimes greater, sometimes less (Fig. 69). Om: of these tirst daughter-cells of the ovum is a little THE <:. 1 s'/'av '/.. I TION OF THE I ERTEBR. \ TE 87 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 oi the segmentation-cells in the mammal embryo lias reached ninety- six (in the rabbit, about seventy hours after impregnation) the foetus assUm form very like the archigastrula (Fig. 7.2). The spherical embryo consists of a central mass oi thirty-two soft, round cells with dark nuclei, which aie Rattened into polygonal shape by mutual pressure, and colour dark-brown with osmic acid (Fig. 72 /'). This dark central group of Cells i-> surrounded by a lighter spherical membrane, consisting oi sixty-four cuhe- shaped, small, and line-grained cells which lie close together in a single Stratum, and only colour slightly in osmic acid (Fig. ;j <•). The authors who regard this embryonic form as the primary gastrula oi the placental conceive the outer layer as the ectoderm and the inner as the entoderm. The entodermic membrane is only interrupted at one spot, onv} two, or three oi the ectodeiinic cells being loose there. These form the \ elk- stopper, and till up the mouth oi the gastrula ( a j . The central primitive gut- cavity ( d I is full of entodermic cells. The uni-axial type oi the mammal gastrula is accentuated in this way. However, opinions still differ considerably as to the real nature oi this " provisional gastrula" of the placental and its relation to4 the blastula into which it is converted. As the gastrulation proceeds a large spherical blastula is formed from this peculiar solid amphigastrula oi the placental, as we saw in the case of the marsupial. The accumulation oi fluid in the solid gastrula (Fig. 7; . I ) leads to the formation oi 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 f*PJ> 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 surfaceof the outer layer, the cell-stratum of which forms the wall oi the hollow sphere ; the extension proceeds from the [ vegetal towards the animal pole. lb. cenogenetic gastrulation oi the placental has been great l\ modified by mdary adaptation in the various groups oi this most advanced and youngest sub-class oi the mammals. Thus, iov instance, we find in manj of the rodents (guinea-pigs, mice, etc.) apparently a temporary inversion oi the two germinal layers. This is due to a folding of the blastodermic wall by what is called the "girder," a plug-shaped growth oi Raubcr's "roof-layer." It is a thin layer Of fiat epithelial cells, that, is freed from the surface of the blastoJerm in some oi the rodents; it has nomoio significance in connection with the general course of placental gastrulation than the conspicuous departure from the usual Fig. 67.— Longitudinal section through the oval gastrula of the opossum (Via- >«>)■ (From SiU-nka.) £ primitive mouth, « ectoderm, i entoderm, d \ elk remains in the primitive j,ruln."avity ( it J. globular shape in the blastula oi some of the ungulates. In some pigs aiitl ruminants it grows into a thread-like, long and thin tube. Thus the gastrulation of the placentals, which diverges most from that oi 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 oi the blastoderm does not form a completely closed (only open at the primitive mouth) blind sa< . .is is usual ; but this blind sac has a wide opening at the ventral curse (opposite to the dorsal mouth) ; and through this opening the primitive gut communicates from the lirst with the embryonic cavity of the blastula. The folded crest-shaped 88 THE GASTRULATION OF THE VERTEBRATE entoderm grows with a free circular border on the inner surface o( the ento- derm 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 Fig. 68.— Stem-cell of the mammal ovum (from the rabbit). £■ stem-nucleus, n inn liar corpuscle, ,* pro- toplasm of the stem-cell, = modified zona pellucida, h outer albuminous membrane, s dead sperm-cells. Fig. (x).— Incipient cleavage of the mammal Ovum (from the rabbit). The stem-cell has divided into two unequal cells, one lighter (e) and one darker ft), z zona pellucida, h outer albuminous membrane, 5 dead sperm-cells. 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 prows the essential unity of gastru- lation in all the vertebrates, in spite of the striking differences in«the various classes. In order to complete our consideration of the important processes of segmental- 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, 0 zona pellucida, h outer albuminous membrane. Fig. 71.— Mammal ovum with eight segmenta- tion-cells (from the rabbit), e four larger and lighter cells, i four smaller and darker cells, z zona pellucida, h outer albuminous membrane. tion and gastrulation, we will, in conclu- sion, 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, THE GASTRULATION OF THE VERTEBRATE So spiders, myriapods, and nabs. The dis- tinctive form of gastrula tint comes of it is the "vesicular gastrula" ( l\ri- gastrula). In the ova which undergo tliis super- ficial cleavage the formative yelk is sharply divided from the nutritive yelk, a^ in the preceding cases of the o\a of birds, reptiles, fishes, etc.; the formative yelk alone undergoes cleavage. Hut while in the 0V8 with discoid gastrulation the formative yelk is not in the centre, but at one pole of the uni-a\ial 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 accu- mulated in the middle of the ova. As the Segmentation only affects the former and not the latter, it is hound 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 ol 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, completelv closed vesicle, the internal cavitv 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 sub- stances. As this quantity of food-velk 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 agiee 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 re- mainder of the segmentation - cavity) remains full of food - yelk, which is gradually used up. This is the only material difference between our vesicular gastrula ( pcrigastiula ) and the original form of the bell-gastrula (arthigastmla). Clearly the one has been developed from the other in the course of time, owing to the accumulation of {bod-yelk in the centre of the ovum.' 1 On t>n_- reduction of all forms of R.astrulation to 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 .is cenogenetic and derivative. The un- equal, 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 forma- tion OI the food-yelk, and the increasing antithesis between animal and Vegetal halves of the ovum, 01 between ectoderm (skin-layer) and entoderm (gut-layer). Fie. 72.— Gastrula of the placental mammal (epigastrula from the rabbit), lonKiiinlin.il Mction through the axis, c ectodermic cells (sixty-four, lighter and smaller), 1* entodermic cells (thirty-two, darker and larger), (/central entodermic cell, fining the primitive gut-cavity, o peripheral entodermic cell, stopping up the Opening o( the primitive mouth (yelk-stopper in the Kusconian anus). The numbers of careful studies of animal gastrulation that have been made in the Jast 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 metazoawas not the gastrula, but the " planula " a double- walled vesicle wit h 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. the original palingenetic form see especially the lucid treatment of the subject in Arnold Lang's Manual of Comparative Auatumx (iSSS), Part I. oo 77/ A' ((/■:/.().]/ THEORY It was also shown tint wli.it is called delamination the rise of the two primary germinal layers by the folding of the surface ofthe blastoderm (for instance, in the Geryomda and other medusae) was a secondary formation, due to cenogenetic they attach themselves to the inner wall ofthe blastula, and forma second internal epithelial layer that is to say, the ento- derm. In these and many other contro- versies oi~ modern embryology the first requisite for dear and natural explanation Fig. 73.— Gastrula of the rabbit. A as a solid,, spherical cluster of cells, B changing: into the embryonic Vesicle, bp primitive mouth, i'p ectoderm, liy entoderm. 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 ; is a careful and discriminative distinction between palingenetic (hereditary) and cenogenetic (adaptive) processes. If this is properly attended to, we find evidence everywhere ofthe biogenetic law. Chapter X. THE CCELOM THEORY The two "primary germinal layers" which the gastnea theory has shown to be the first foundation in the construction of the body arc found in this simplest form throughout life only in animals of the lowest grade — in the gastneads, olynthus (the stem-form of the sponges), hydra, and similar very simple animals. In all the other animals new strata of cells arc formed subsequently between these two primary body-layers, and these are gene- rally comprehended under the title of the middle layer, or mesoderm. As a rule, the various products of this middle layer after- wards constitute the great bulk of the animal frame, while the original entoderm, or internal germinal layer, is restricted to the clothing of the alimentary canal and its glandular appendages ; and, on the other hand, the ectoderm, or external germinal layer, furnisher, the outer cloth- ing of the body, the skin and nervous system. In siime large groups of the lower animals, such as the sponges, corals, and fiat-worms, the middle germinal layer THE C(ELOM THEORY 9r rem. mis a tingle connected mass, and most of the body is developed from it ; I these have been called the three-l.i\ ei vd mctazoa, in opposition to the two-layered animals described. Like the two-lavered , animals, they have no body-cavity — ' that is to say, no cavity distinct' from the alimentary system. On the oilier hand, all the higher animals have this real body- cavity (cmloma)% and so .tie called cr/o- maria. In all these we can 'distinguish lour secondary germinal layers, which develop from the two primary layers. To the same class belong all true ver- malia (excepting the platodes), and also the higher typical animal stems that have been evolved from them molluscs, echinoderms, articulates, tunicates, and vei tebrates. of nutrition in the multicellular animal- body, is the oldest and most important organ of all the metazoa, and, together with the primitive mouth, is formed in every Case in the gastrula as the primitive gUt ; it is onl) at a much later Stage lh.it the body-cavjty, which is entirely wanting in the ccelenterata, is developed in some ot the metazoa between the ventral and the bods wall. The two cavities are entirely different in content and purport The alimentary cavity ( cntcmn J s. | the purpose of digestion ; it contains water and food taken from without, .is well as the pulp (chymus) formed from this by digestion. On the other hand, the body-cavity, quite distinct from the gut and closed externally, has nothing to do with digestion ; it encloses the gut Fra. :t The body-cavity (coeloma) is therefore a new acquisition of the animal body, much younger than the alimentary system, and of great importance. I first pointed OUt this fundamental significance of the coelom in my Monograph on the Sponges (1872), in the section which draws a dis- tinction between the body-cavity and the gut-cavity, and which follows immediately on the germ-layer theory and the ancestral tree ot the animal kingdom (the first sketch of the gastraea theory). Up tO that time these two principal cavities oi the animal body had been confused, or very imper- fectly distinguished ; chiefly because Leuckart, the founder of the ccelenterata group(i84X), has attributed a body-cavity, but not a gut-cavity, to these' lowest metazoa. In reality, the truth is just the other way about. The ventral cavity, the original organ Fig. 75. Fics. 74 and 7S. _ Diagram of the four secondary germinal layers, transverse »ecU\ n through the metazene embryo: Fig. 74 oi an annelid, Fig. 7^ of a vermauan. «/ primitive gut, M ventral glandular layer, THE CCELOM THEORY surface of the ventral wall; hence the) are palled confining or limiting layers. Between them are the two middle-layers, or mesoblasts, which enclose the body-: : y. Fia 76. — Coelomula of sagitta (gastrula with a couple of ccelom-pouches. (Prom Kotoalevxky.) bl.p primitive mouth, al primitive gfut, fv coelom-folds, in permanent mouth. The four secondary germinal layers are so distributed in the structure of the body in all the ccelomaria (or all metazoa that have a body-cavity) that the outer two, joined, fast together, constitute the body-wall, and the inner two the ventral wall ; the two walls are separated by the cavity of the ccelom. Each of the walls is made up of a limiting layer and a middle layer. The two limiting layers chiefly give rise to epilhelia, or covering- tissues, and glands and nerves, while the middle layers form the great bulk of the fibrous tissue, muscles, and connective matter. Hence the latter have also been called fibrous or muscular layers. The outer middle layer, which lies on the inner side of the skin-sense-layer, is the skin fibre-layer ; the inner middle layer, which attaches from without to the ventral glandular layer, is the ventral fibre layer. The former is usually called briefly the parietal, and the latter the visceral layer or mesoderm. Of the many different names that have been given to the four secondary germinal layers, the following are those most in us'e to-day :— h Neural, The lwo sccondarv layer germinal layers (neuroblast). \ *f „u. bod ^ . II. Parietal i. Epithelial layer J n. fibrous. (myoblast). IM- Visceral^ Thctwosic.onJ;iry r ii i , germinal layers IV. Lnteral m. Fibrous. layer J (ctitvroblast). i. Skin-sense- layer (outer limiting layer). 2 Skin -fibre- layer (outer middle layer). 3. Gut - fibre - layer (inner middle layer). 4 Gut -gland- layer (inner limiting layer). IV. Epithelial. The firsl scientist to recognise and clearly distinguish the four secondary germinal layers was Baer. It is true that he was not c|tiile clear as to their origin and further significance, and made several mistakes iii detail in explaining them. But, on the whole, their great importance did not escape him. How- ever, in later years his view had to be given up in consequence of more accurate observations. Remak then propounded a three-layer theory, which was, generally accepted. These theories oi cleavage, however, began to give way thirty years ago, when Kowalcvsky ( 1 87 1 ) showed that in the case of Sagitta (a very clear and typical subject of gastrulation) the two middle germinal layers and the two limiting layers arise not by cleavage, but by Folding by a secondary invagination of the primary inner germ-layer. This invagination or folding proceeds from the primitive mouth, at the two sides of which (right and left) a couple of pouches are formed. As these coelom-pouches or. ccelom-sacs detach themselves from the primitive gut, a double body-cavity is formed (Figs. 74-6). The same kind of culom-formation as in sagitta was afterwards found by Kowalevsky in brachiopods and other invertebrates, and in the lowest vertebrate — the amphioxus. Further instances were discovered by two English embryologists, to whom we owe very considerable advance D Fie. 77. — Coelomula of sagitta. in section. (From //(';•/;»r visceral mesoblast, //; body-cavity, »if> parietal mesoblast, ak outer germinal layer. in ontogeny H. Ray-IJankestcr and F. Balfour. On the strength of these and other studies, as well as most extensive research of their own, the brothers Oscar and Richard Hertwig constructed in 1881 THE <■(//<>:/ THEORY 93 the Coelom Theory. In order to appre- ciate fully the great merit of this illumi- nating and helpful theory, one must remember what a chaos of contradictor] views was then represented by the " problem of the mesoderm, "or the much- disputed "question of the origin of the middle germinal layer." The coelom theory brought souk- light and order into this infinite confusion by establishing the following points: i. The body-cavity originates in the great majority of animals (especially in all the vertebrates) in the same way as in sagitta : a couple of pouches or sacs are formed by folding inwards at the primitive mouth, between the two primary germinal layers ; as these pouches detach from the primitive i^ut, a pair of co?lom-sacs (right and left) are formed ; the coalescence of these produces a simple body-cavity 2. When these coelom-embryos develop, not as a pair of hollow pouches, but as solid layers of cells (in the shape of a pair of mesodermal iks) a- happens in the higher verte- brates we have a secondary (cenogenetic) modification of the primary (palingeneticj structure ; the two walls of the pouches, inner and outer, have been pressed toge- ther by the expansion of the large food- yelk. 3. Hence the mesoderm consists from the first of two genetically distinct layers, which do not originate by the cleavage of a primary simple middle layer (as Remak supposed). 4. These two middle layers have, in all vertebrates, and the tfreat majority of the invertebrates, the same radical significance for the con- struction of the animal body ; the inner middle layer, Or the visceral mesoderm, (gut-fibre layer), attaches itself to the original entoderm, and forms the fibrous, muscular, and connective part of the visceral wall ; the outer middle layer, or the parietal mesoderm (skin-fibre-layer), attaches itself to the original ectoderm, and forms the fibrous, muscular, and connective part of the body-wall. 5. It is only at the point of origination, the primitive mouth and its vicinity, that the four secondary germinal layers are directly connected ; from this point the two middle layers advance forward separately between the two primary germinal layers, to which they severally attach themselves. 6. The further separation or differentia- tion of the four secondary germinal layers and their division into the various tissues and organs take place especially in the later fore-part or head of the embryo, and extend backwards from there towards the primitiv e mouth. All animals in which the bodv'-c.ivily demonstrably arises in this way from the primitive gut (vertebrates, tunicatOS, echinoderms, articulates, and a part of the vermalia) were comprised by the Hertwigs under the- title of enterocarla, and were contrasted with the other groups of the pseudocetfa (with false body-cavity) and the ccelenterata (with no body-cavity). However, this radical distinction and the views as to classification which it occa- sioned have been shown to be untenable. Further, the absolute differences in tissue- formation which the HcrtwigS set up between the eiitcrocula and pseudocoela cannot be sustainod in this connection. Voi' these and other reasons their ccelom- theoryhas been much criticised and partly Frn. 78.— Section of a young sagitta. (From Herhvig.) dh visceral cavity, ik and aM inner and outer limiting layers. »ii' and »i/> inner and outer middle layers, Ik body-cavity, tint and vm dorsal and visceral mesentery. abandoned. Nevertb. less, it has rendered a great and lasting set A c In the solution of the difficult problem ol the mesoderm, and a material part of it will certainly be retained. I consider it an especial merit of the theory that it has established the identity of the development of the two middle layers in all the vertebrates, and has traced them as cenogenetic modifica- tions back to the original palingenctic form of development thai wc still find in the amphioxus. Carl Rabl comes to the same conclusion in his able Theory of Ihc Mesoderm, and so do Rav-Lankcster, Rauber, Kupffer, Ruckort, Selenka, Hatschek, and others. There is a general agreement in these and main other recent writers that all the different forms of ccelom-construction, like those of gastru- lation, follow one and the same Strict hereditary law in the vast vci tcbraic stem ; in spite of their apparent differences, they v), and are still con- nected with the primitive gut by wide apertures ; they also communicate for a short time with the dorsal side (Fig. 77 *0' Soon, however, the ccelom-pouches com* pletely separate From each oilier and from the primitive gUl ; at the same time they enlarge so much that they close round the primitive gut (Fig. 78). Hut in the middle line of the dorsal and ventral sides the pouches remain separated, their approaching walls joining here to form a thin vertical partition, the mesentery (dm and vm). Thus Sagitta has throughout life a double body-cavity (Fig. 78 Ik), and the gut is fastened lo the body-wall both above and below by a mesentery — below Fig. PlGS. 79 and 80. — Transverse section Of amphioxus-larvae. (From Hatschek.) Fi£. 79 at the commence- ment of caelom formation (still without segments). Fig. 8o at the stage with four primitive segments, ak, ile, mk outer, inner, and middle germinal layer, h/> horn plate, nip medullary plate, ch chorda, * and * disposition of the ccelom-pouches, Ih body-cavity. also a most remarkable branch of the I extensive vermalia stem. It was therefore j very gratifying that Oscar Hertwig (1880) fully explained the anatomy, classification, and evolution of the chaetognatha in his careful monograph. The spherical blastula that arises from the impregnated ovum of the sagitta is converted by a folding at one pole into a typical archigastrula, entirely similar to that of the Monoxeriia which I described (Chapter VIII., Fig. 29). This oval, uni- axial cup-larva (circular in section) becomes bilateral (or tri-axial) by the growth of a couple of ccelom-pouches from the primi- tive gut (Figs. 76, 77). To the right mid left a sac-shaped fold appears towards the top pole (where the permanent mouth, m, by the ventral mesentery (vm), and above by the dorsal mesentery (dm). The inner layer of the two ccelom-pouches ( mv) attaches itself to the entoderm (ik), and forms with it the visceral wall. The outer layer (tn/>) attaches itself to the ectoderm (ak), and forms with it the outer body-wall. Thus we have in Sagitta a perfectly clear and simple illustration of theoriginal coslomation of theenteroccela. This palingenetic fact is the more impor- tant, as the greater part of the two body- cavities in Sagitta changes afterwards into sexual glands — the fore or female part into a pair of ovaries, and the hind or male part into a pair of testicles. Coelomation takes place with equal clearness and transparency in the case of THE CCELOM THEORY 95 the amphioxus, the lowest vertebrate, and it* nearest relatives, the invertebrate tuhi- cates, the sea-squirts. However, in these two stems, which we class together .i> Ckordonia, this important process is more complex, as two other processes are a dated with it -the development of the chorda from the entoderm and the separa- tion of the medullary plate or nervous centre from the ectoderm. Here again the skulless amphioxus has preserved to our own time by tenacious heredity the chief phenomena in their original Form, while it lia-> been more or less modified hv embryonic adaptation in all the other vertebrates (with skulls). Hence we must once more thoroughly understand the palingenetic embryonic features of the lancelet before we i^o on to consider the cenogenetic forms of the craniota. borders of the concave medullary plate fold towards each oilier and grow under- neath the horny-plat.-, a cylindrical tube is formed, the medullary tube (Fig. 8a >/); this quickly detaches itself altogether from the horny-plate. At each side of the medullary tube, bel ween it and the alimen- tary tube Ba dh), the two parallel longitudinal folds grow out of the dorsal wail of the alimentary tube, and tl form the two coelom-pouches Figs. 80 and 81 //;). This part of the entoderm, which thus represents the first structure of the middle germinal layer, is shown darker than the rest of the inner germinal layer in Figs. ;) and the homy-plate \tUtJt the beginning of the outer skin or epidermis. As the parallel the chorda or axial rod, is being formed between the two ccelom-pouches. This first foundation of the skeleton, a solid cylindrical cartilaginous rod, is formed in the middle line of the dorsal primitive gut-wall, from the entodermal cell-streak that remains here between the two ccidom- pouches (Figs. 79-82 ch). The chorda appears at first in the shape ■ f a flat longitudinal fold or a shallow groove (Figs. 80, 81) ; it does not become a solid cylindrical cord until after separation from the primitive gut (Fig. 82). Hence we might say that the dorsal wall of the primitive gut forms three parallel longi- tudinal folds at this important period — one single fold and a pair of folds. The single middle fold becomes the chorda, and lies immediately below the groove of the ectoderm, which becomes the medullary 96 THE CCELOM THEORY uilv ; the pair of folds to the right and left lie at the sides between the former and the latter, and form the coelom- pouches. The pan o\ the primitive gui that remains after the cutting off of tlu-s.- three dorsal primitive organs is the per- (Figs. 83, X4, in the third period of development acoording to Elatschek). (Strabo and Plinius give the name of cordula or cordyla to young lisli larvae.) I asoribe the inmost phylogenefic signifi- cance to it, as it is found in all the chorda- - h Fig. 84. Figs. 8j and S4— Chordula of the amphioxus. Fig. 83 median longitudinal section (ecu from the left). Fig. 84 transverse section. (From Hatschtk.) In Fig. 8j the ccelom-pouches are omitted, in order to show the chordula more clearly Fig. S4 is rather diagrammatic, h horny-plate, m medullary tube, n wall of same (11 dorsal. >:' ventral), ch chorda. >if> neuroporus, /<<• e malis neurentericus. d gut-cavity, >■ gut dorsal wall, b gut ventral wall. • y ;lk-ceUs in the latter. 11 primitive mouth, o mouth-pit, p promesohlasts (primitive or polar cells of the mesoderm), -,v parietal layer, v visceral layer of the mesoderm, c ccelorn,/ rest oi the segmentation-cavity. Fig. 86. Figs. 85 and 86. —Chordula of the amphibia (the ringed adder). (From Goettd.\ Fig 85 median longitudinal section (seen from the left), Fig. 86 transverse section (slightly diagrammatic). Lettering as in Figs. 8j and 84. mancnt gut ; its entoderm is the gut- animals ftunicates as well as vertebrates) gland-layer or enteric layer. in essentially the same form. Although I give the name of chordula or chorda- the accumulation of food-yelk greatly larva to the embryonic stage of the modifies the form of the chordula in the vertebrate organism which is represented higher vertebrates, it remains the same by the amphioxus larva at this period in its main features throughout. In all THE CCELOM THEORY 07 cases the nerve-cube ( »i ) lies on the dorsal side o\' the bilateral, worm-like body, the gut-tube (d) ow the ventraJ side, the chorda (ch) between the two, on (he long um--, and the cuelom pouches (ij at eacb side. In every case these descend from an ancient common ances* u .ti form, which we may call ( 'kotvlaa. We should regard tins long-extinct ChonUto, it a were still in existence, .^ a special class of unarticulated worm (chordaria). Ii is especially noteworthy that neithei ud .. Fun. 87 anJ 83. -Diagrammatic vertical section of coelomula-ambryos of vertebrates. (From Hjrtmif.) Kiij. 87. verticil ■CCtJOII tii>vu;h tlu- primitive month. Fijf. 88, Vertical section befjor* the primitive m Mlth. " primitive m >uth, tt / primitive rut d yelk. medullary plat-.', el chorda plate, a> and <£ outer and inner germinal layers, pb parietal and vb visceral mcsoblast. Pto. 89 Figs. 89 and g->.— Transvjrs3 S3Ction of ccelomuia embryos of trit^r. (From Hertw'p.) Fig. 89, section through the primitive mouth. Fig. 90, section in front of the primitive mouth, u primitive mouth, ifh gut-cavity, dz yelk-cells. df> yelk-stopper, ak outer and ik inner germinal layer, pb parietal and vb visceral middle layer, m medullary plate, ch chorda. primitive organs develop in the same way from the germinal layers, and the same organs always arise from them in the mature chorda-animal. Hence we may conclude, according to the laws of the theory of descent, that all these chordoma or chordata (tunicates and vertebrates) the dorsal nerve-tube nor the ventral gut- tube, nor even the chorda that lies between them, shows any trace of articu- lation or segmentation ; even the two CCeiom-sacs are not segmented at first (though in the amphioxus they quickly divide into a series of parts by transverse 98 THE CCELOM THEORY folding). These ontogenetic facta are of the greatest importance for the purpose of learning those ancestral forms of the vertebrates which we have to seek in the group o\ the unarticulated vermalia. The CCelom-pOUCheS were originally sexual glands in these ancienl chordoma. From the evolutionary point of view the CCelom-pOUCheS are, in any case, older than the chorda ; since they also develop Fie. 91 A. R, C. -Vertical section of the dorsal part of three triton-embryOS. (From Hertwig.) In Fig. A the medullary swellihgs (the parallel borders of the medullary plate) begin to rise : in Fig. B they grow towards each other: in F'g. C they ioin and form the medullary tube, mp medullary plate, mf medullary folds. « nerve-tube. rJi chorda. Iti body-cavity, mki and mlt? parietal and visceral mesoblasts. ;/: primitive- segment cavities, ak ectoderm, ik entoderm. ouches, the layers of which inner and outer) close with each other. In common parlance we still call a pouch or pocket by that name, whether it is full or empty. It is different in ontogeny ; in some of our embryological litera- ture ordinary logic does not count for very much. In many of the manuals and large treatises on science it is proved that vesicles, pouches, or sacs deserve that name only when they are inflated and filled with a clear fluid. \yhen they are not so filled (for instance, when the primitive gut of the gastrula is filled witli yelk, or when the walls of the empty ccelom-pouches are pressed together), these vesicles must not be cavities any longer, but "solid structures." The accumulation of food-yelk in the ventral wall of the primitive gut (Figs. 85, 86) is the simple cause that converts the sac-shaped ccelom-pouches of the acrania into the leaf-shaped ccelom-streaks ot the craniotes. To convince ourselves of this we need only compare, with Hertwig, the palingenetic ccclomula of the amphioxus (Figs. 80, 81) with the corresponding cenogenetic form of the amphibia (Figs. 89-90), and construct the simple diagram that connects the t wo (Figs. 87, 88). I f we imagine the ventral half of the primitive gut-wall in the amphioxUS embryo (Fig*. 7*1 84) distended with food-yelk, the vesicular ccelom-pouches (Ik J must be pressed together by this, and forced to extend in the shape of a thin double plate between the gut-wall and body-wall (FigS. 86, 87), This expansion follows a downward and forward direction. They are not directly connected with those tuo walls. The real unbroken connection between the two middle layers and the primary germ-layers is found right at the back, in the region of the primitive mouth (Fig. 87 «). At this important spot we have the source of embryonic development ( blnslinrcne), or "zone of growth," from which the eulomation (and also the gastrulation) originally proceeds. Hertwig even succeeded in showing, in the Civlomula-cmbryo of the water sala- mander ( Triton), between the first struc- tures of the two middle layers, the relic of Fig. 02.— Transverse section of the chordula-embryo Of a bird (from a hen's egg at the clOM of the first day of incubatiqn). (From Kolliker.) h horn-plate (ectoderm), tn 'medullary plate, Rf dorsal folds of same, P'v medullary furrow, ch chorda, uivh median (inner) part of the middle layer (median wall of the ccelom-pouches), sp lateral (outer) part of same, or lateral plates, uinh structure of the body- _cavity, dd gut-fjland-layer. this the body-cavity, wliich is represented in the diagrammatic transitional form (Figs. 87, 83). In sections both through the primitive mouth itself (Fig. 89) and in front of it (Fig. 90) the two middle layers ( pb and vb) diverge from each other, and disclose the two body-cavities as narrow clefts. At the primitive mouth itself (Fig. 90 u) we can penetrate into them from without. It is only here at the border of the primitive mouth that we can show the direct transition of the two middle layers into the two limiting layers or primary germinal layers. The structure of the chorda also, shows the same features in these ccelomula- embryos of the amphibia (Fig. 91) as in the amphioxus (Figs. '79-82). it arises from the entodermic cell-streak, which forms the middle dorsal line of the primi- tive gut, and occupies the space between the flat ccelom-pouches (Fig. 91 A). THE CCELOM THEORY While the nervous centre is formed here in the middle line of the back and separated from the ectoderm as "medul- lary tube," there takes place at the same time, directly underneath, the severance of the chorda from the entoderm- (Fig. 91 .1, />, t '). Under the" chorda is formed (out of the ventral entodermic half of the gastrula) the permanent gut or viscera! cavity (t-ntcmn) (Fig. 91 />', i ('). All these important modifications take place at first in the fore or head-part of the embryo, and spread backwards from there ; here at the hinder end, the region of the primitive mouth, the important . border of the mouth (or properistoma) possible as a matter of fact ; even the older illustrations showed an essential identity of features. Thus forty years ago Kdlliker save, in the first edition of his Human Embryology (l86l), some sections of the chicken-embryo, the features of which could at once be reduced to those already described and explained in the sense of Hertwig's coelom-theory. A section through the embryo in the hatched hen's egg towards the close of the first day of incubation shows in the middle of the dorsal sulfate a broad cctodcrinic medul- lary groove (Fig. ()-' R /'), and under- neath the middle of the chorda ( ' c/i J and at each side of it a couple of broad meso- derms layers (sp). These enclose a narrow space or cleft ( uwh J, which is nothing else than the structure of the body-cavity. The two layers that enclose it— the upper parietal layer ( hpl ) and the lower visceral layer (of)— are pressed together from without, but clearly distin- _j,Pt cK tin ao jp cfj jf Fig. 93.— Transverse section of the vertebrate-embryo of a bird (from a hen's c^a on the second day of incubation). (From Kdlliker.) h horn-plate, nir medullary tube, ch chorda, uu< primitive segments, uwh primitive-segment cavity (median, relic of the ccelom). s/> lateral coelom-cleft, hf>l skin-tibre-laver, df gut- fibrc-layer, ung primitive-kidney passage, no primitive aorta, dd gut-gland-layer. remains for a long lime the source of development or the zone of fresh construc- tion, in the further building-up of the organism. One has only to compare care- fully the illustrations given (Figs. 85-91) to see that, as a fact, the cenogenetic ccelomation of the amphibia can be deduced directly from the palingenctic form of the acrania (Figs. 79-84). The same principle holds go6d for the amniotes, the reptiles, birds, and mammals, although in this case the pro- cesses of ccelomation are more modified and more difficult to identify on account of the colossal accumulation of food-yelk and the corresponding notable flattening of the germinal disk. However, as the whole group of the amniotes has been developed at a comparatively late date from the class of the amphibia, their ccelomation must also be directly trace- able to that of the latter, This is really guishable. This is even clearer a little later, when the medullary furrow is closed into the nerve-tube (Fig. 93 nir). Special importance attaches to the fact that here again the four secondary ger- minal layers are already sharply distinct, and easily separated from each other. There is only one very restricted area in which they are connected, and actually pass into each other ; this is the region of the primitive mouth, which is contracted in the amniotes into a dorsal longitudinal cleft, the primitive groove. Its two lateral lip-borders form ifot primitive streak, which has long been recognised as the most important embryonic source and starting- point of further processes. Sections through this primitive streak (Figs. 94 and 95) show that the two primary ger- minal layers grow at an early stage (in the discoid gastrula of the chick, a few hours after incubation) into the primitive THE CiKf.OM THEORY mr streak (x), and that the two middle layers extend outward from this thickened axial plate ( v ) to the right and left between the former. The plates o\' the ccelom-layers, the parietal skin-fibre-layer f»i) and the viscera] gut-fibre-layer (/)* are seen to be still pressed close together, and only -diverge later to form the body- cavity. Between the inner borders of the tWO Hat coclom-pouches lies the chorda (Fig. 95 x), which here again developes from the middle line oi the dorsal wall of the primitive gut. Coelomation takes place in the verte- brates in just the same way as in the birds and reptiles. This was to be ex- four secondary germinal layers consists of a single stratum of cells. Finally, we must point out, as a fad oi the utmost importance for our anthropo- geny and o\ great general interest, that the rour-laj ered coelomula o\' man has just the same construction as that of the rabbit (Pig. 96). A vertical section that Count Spee made through the primitive mouth Or streak of a wry young human ger- minal disk (Fig. 97) clearly shows thai here again the four secondary germ- layers are inseparably connected only .it the primitive Streak, -ind (hat here also the two Battened ccelom-pouches ( ' mk ) extend outwards to right and left from annn * y (i f v /« Tic. 94. Fig. .,.5 Fk-.s. 94 Md ^-Transverse section of the primitive streak (primitive mouth) of the chick Fig-. />/-) along the 'primitive streak (at the folding-point of the blastula), and from this spot (the border of the primitive mouth) the middle germinal layers (ink) grow out to right and left between the preceding. In the fine illus- tration of the ccelomula of the rabbit which Van Beneden has given us (Fig. 96) one can clearly see lhat each of the the primitive moutn between the outer and inner germinal layers. In this ease, too, the middle germinal layer consists from the first of two separate strata ol cells, the parietal (mpj and visceral ( >nv ) mesoblasts. These concordant results of the best recent investigations (which have been confirmed by the observations of a numbei of scientists I have not enumerated) prove the unity of the vertebrate-stem in point o( coelomation, no less than of gastrulation. In both respects the in- valuable amphioxus -the sole survivor of the acrania— is found to be the original model that has preserved for us in palin- genetic form by a tenacious heredity these THE CCELOM THEORY most important embryonic processes. From this primar) model of construction we can cenogencticallj deduce all the embryonic forms oi the other vertebrates, the craniota, by secondarj modifications. M\ thesis of the universal formation o\ the gastrula by folding of the blastula has now lu-i.il clearly proved for all the verte- brates ; so also lias been Hertwig's thesis o\ the origin of the middle germinal layers bj the folding of a couple of ccelom- pouches which appear at the border ol IIIV ////> typical, unarticulated, worm-like form, which has .in axial Chorda between the dorsal nerve-tube and the ventral gut- tube. This instructive chordula (Figs, 8l 86) provides a valuable support of our phytogeny ; it indicates the important moment in our .stem-history at which the stem of the chordoma (tumcates and ver- tebrates) parted tor ever from the diver- gent stems of the other meta/oa (articu- lates, ecHinoderms, and molluscs), I may express here my opinion, in the Wk^mmi Fu; 96 -Transverse section of the primitive groove (or primitive mouth) of a rabbit. (From Van lioirdrn.) pr primitive mouth, ul lips oi same (primitive tips), nk and ik Outer and inner germinal layers, mk middle germinal layer, mf> parietal layer, mv visceral layer oi the mesoderm. Fig. 97 —Transverse section of the primitive mouth (or groove) of a human embryo (at the ccelomula stage). (From Count Sf>cc.) f>r primitive mouth, ul lips of same (primitive folds), ak an4 ik outer and inner germinal layers, mk middle layer, m/> parietal layer, niv visceral layer of the mesohlasts. the primitive mouth. Just as the gastra?a- theory explains the origin and identity of the two primary layers, so the cotlom- tlieory explains those of the four secondary layers. The point of origin is always the properistoma, the border of the original primitive mouth of the gastrula, at which the two primary layers pass directly into each other. Moreover, the ccelomula is important as the immediate source of the chordula, the embryonic reproduction of the ancient, form of a chordaia-thcory', that the charac- teristic chordula-larva of the chordoma has in reality this great significance — it is the typical reproduction (preserved by heredity) of the ancient common, stem- form of all the vertebrates' and tunicates, the long-extinct Chordcea. We will return in the twentieth chapter to these worm-like ancestors, which stand out as lumirious points in the obscure stem-history of the invertebrate ancestors of our race. .,., THE VERTEBRATE CHARACTER OF MAN ">J Chapi er XI. THE VERTEBRATE CHARACTER OF MAN W'k hive how secured a number of firm stand ing-placcs in the labyrinthine course of our individual development by our stud) of the important embryonic forms which \vc have railed the cytula, morula, blastula, gastrula, coelomula, and chord- Ula. But we have still in front of us the difficult task of deriving the complicated frame of the human body, with all its different parts, organs, members, etc., from the simple form of the chordula. We have previously considered the origin of this four-layered embryonic form from the two-layered gastrula. The two primary germinal layers, which form the entire b 'dv of the gastrula, and the two middle layers of the ccelomula that develop between them, are the four .simple cell- Strata, or epithelia, which alone 140 to the formation of the complex body of man and the higher animals. It is so difficult to understand this construction that wc will first seek a companion who may help u> out of many difficulties. This helpful associate is the science of comparative anatomy. Its task is, by comparing the fully - developed bodily forms in the various groups of animals, t "> learn the general laws of organisation according to which the body is con- structed ; at the same time, it has to determine the affinities of the various groups by critical appreciation of the degrees of difference between them. Formerly, this work was conceived in a teleologic al >eiise, and it was sought to find traces of the plan of the Creator in the actual purposive organisation of ani- mals. But comparative anatomy has £on'. \CTER OF )/. I V '"5 neither skull nor vertebrae, and no extremities or limbs. Even the human embryo passes through .1 stage In which it has no skull oi vertebrae ; the trunk is quite simple, and there is yel no trace of arms and legs. At this stage of develop- ment man, like even1 other higher verte- bi ate, is essentially similar to the simplest vertebrate form, which we now find in only one living specimen. This one lowest vertebrate that merits the closesl study undoubted!) the most interesting of all the vertebrates after man is the famous lancelet or amphioxus, to which w e have already often referred. As we are going to studs it more closely later on (Chapters XVI. and XVII.), I will only make one or two passing observations o]\ it here. The amphioxus lives buried* in the sand of the sea, is about one or two inches in length, and has, when fully developed, the shape of a very simple, longish, lancet- like leaf ; hence its name of the lancelet. The narrow bodj is compressed on both sides, almost equally pointed at the fore and hind ends, without any trace oi I nal appendages or articulation oi the body into head, neck, breast, abdomen, etc. Its whole shape is so simple that its first discoverer thought it was a naked snail. It was not until much later half a century ago - that the tiny creature was Studied more carefully, and was found to be a true vertebrate. More recent inves- tigations have shown that it is of the greatest importance in connection with the comparative anatomy and ontogeny of the vertebrates, and therefore with human phylogeny. The amphioxus reveals the great secret of the origin of the vertebrates from the invertebrate vermalia, and in its development and structure connects directly with certain lower tunicates, the ascidia. When we make a number of sections of the body of the amphioxus, firstly vertical longitudinal sections through the whole body from end to end, and secondly transverse sections from right to left, we get anatomic pictures of the utmost in- structiveness (cf. Figs. 98-102). In the main they correspond to the ideal which We form, with the aid of comparative anatomy and ontogeny, oi the primitive type or build of the vertebrate the long extinct form to which the whole stem owes its origin. As we take the phylo- genetic unity of the vertebrate stem to be beyond dispute, and assume a common origin from a primitive stem-form for all the vertebrates, frOffl amphioxus to man, \\i aie justified in forming a definite morphological idea of this primitive verte- brate ( J'ri>spuiit(\lus or VeitebraaJ. We need only imagine a few slight and unessential changes in the real sections of the amphioxus in order lo have this ideal anatomic figure or diagram oi the primitive vertebrate form, as we see in rigs. ii* id.'. The amphioxus departs SO link from this primitive form thai we may, in a certain sense, describe it as a modified "primitive vertebrate."' The outer form of our hypothetical primitive vertebrate was at all events very simple, and probably more or less similar to that oi the lancelet. The bilateral or bilateral-sj mmetricaJ body is stretched out lengthways and compressed at the sides (Figs. 98 too), oval in section (Figs. 101, 102). There are no external articulation and no external appendages, in the shape oi limbs, legs, or tins. On the other hand, the division oi the body into two sections, head and trunk, was probably clearer in ProspOfufylus than it is in its little-changed ancestor, the amphioxus. In both animals the fore >i head-half oi the bodv contains different organs from the trunk, and different oi\ the dorsal from on the ventral side. As this impor- tant division is found even in the sea-squirt , the remarkable invertebrate stem-relative of the vertebrates, we may assume that it was also found in the prochordonia, the common ancestors i^i both stems. It is also very pronounced in the young larvae of the cyclostoma ; this fact is particularly interesting, as this palingenetic larva-form is in other respects also an important con- necting-link between the higher verte- brates and the acrania. The head of the acrania, or the anterior half of the body (both oi the real am- phioxus and the ideal prospondylus), contains the branchial (gill) gut and heart in the ventral section and the brain and sense-organs in the dorsal section. The trunk, or posterior half of the body, con- tains the hepatic (liver) gut and sexual- 1 The ideal figure of the vertebrate as given in Figs. 98-102 is a hypothetical scheme or diagram, that has been chiefly constructed on the lines ot' the amphioxus, hut with a certain attention to the comparative anatomy •and ontogeny of the ascidia and appendicular!.! on 1 he- one hand, and of the cyclostoma and selachii on the other. This diagram has no pretension whatever to he an "exact picture," hut merely an attempt to recon- struct hypothetic-ally the unknown and long extinct vertebrate stem-form, "an ideal "archetype." /'///■: VERTEBRATE CHARACTER OF max nn r a:v g m| a r ,,ts t 0 T-r.;s. 98-102.— The ideal primitive vertebrate (prosppndylus). Diagram I^cJrideEES view. 1 ,k, 101 transverse section through the head ..caual tfand. (gonad.)./ corium. « kid"ey!5en g? ££ Ltflhei. LSft '"i / *-'"et' r srin;i1 "»™S - *«*,, hvpopl^Munnary append^, Ju.ic^±.^r t H^v^^^;;^ -"' ^''^ -in). THE l -ERTEHR. \ TE ( //. I /.'. \ < TER OF M. I .v 107 glands in the ventral pan, and (he spinal marrow and most of tin- muscles in the dorsal part. In the longitudinal section of the ideal vertebrate (rig. <)X) we have in the middle of the body a thin and flexible, but stilt', cylindrical rod, pointed at both ends (ch). It goes the whole length through the middle of the body, and forms, as the central skeletal axis, the original structure of the later vertebral column. This is the axial rod, or chorda dorsaJis, also called chorda vertebral**, vertebral cord, axial cord, dorsal cord, notoekorda, or, briefly, chorda. This solid, but flexible and elastic, axial rod consists of a cartilaginous mass of cells, and forms the inner axial skeleton or central frame of the body; it is only found in vertebrates and tunicates, not in any other animals. As the first structure of the spinal column it has the same radical significance in all vertebrates, from the amphioxus to man. But it is only in the amphioxus and the Cyclostoma that the axial rod retains its simplest form throughout life. In man and all the higher vertebrates it is found only in the earlier embryonic period, and is afterwards replaced by the articulated vertebral column. The axial rod or chorda is the real solid chief axis of the vertebrate body, and at the same time corresponds to the ideal k>ng-axis, and serves to direct us with some confidence in the orientation of the principal organs. We therefore take the vertebrate-body in its original, natural disposition, in which the long-axis lies horizontally, the dorsal side upward and the ventral side downward (Fig. 98). When we make a vertical section through the whole length of this long axis, tin body divides into two equal and sym- metrical halves, rit^ht and left. In each half we have originally the same organs in the same disposition and connection ; Only their disposal in relation to the vertical plane of section, or median plane, is exactly reversed : the left half is the reflection of the right. We call the two halves antimera (opposed-parts). In the vertical plane of section that divides 1 In two halves the sagittal I " arrow ") axis, or " dorsoventral axis," ^oes from the back to the belly, corresponding to the sagittal seam of the skull. But when we make a horizontal longitudinal section through the chorda, the whole body divides into a dorsal and a ventral half. The line of section that passes through the body from right to left is the trans- verse, frontal, or lateral axis. The (wo halves of tin. vertebrate body that are separated by this horizontal trans', else axis and by the chorda have quite different characters. The dorsal half is mainly the animal part of the body, and contains the greater part of what are called the animal organs, the nervous system, muscular system, osseous system, etc. the instruments of move- ment and sensation. The ventral half is essentially the vegetative half of the body, and contains the greater part of the vertebrate's vegetal organs, the visceral and vascular systems, sexual system, the instruments of nutrition and repro- duction. Hence in the construction of the dorsal half it is chiefly the outer, and in the construction of the ventral half chiefly the inner, germinal layer thai is engaged. Each of the two halves developes in the shape of a lube, and encloses a cavity in which another tube is found. The dorsal half contains the narrow spinal-column cavity or vertebral canal above the chorda, in which lies the tube-shaped central nervous system, the medullary tube. The ventral half contains the much more spacious visceral cavity or body-cavity underneath the chorda, in which we find the alimentary canal and all its appendages. The medullary tube, as the central nervous system or psychic 0114,111 of the vertebrate is called in its first stage, consists, in man and all the higher vertebrates, of two different parts : the large brain, contained in the skull, and the long spinal cord which stretches from there over the whole dorsal part of the trunk. Even in the primitive vertebrate this composition is plainly indicated. The fore half of the body, which corre- sponds to the head, encloses a knob- shaped vesicle, the brain ( ' gh ) ; this is prolonged backwards into the thin Cylindrical tube of the spinal marrow (r). Hence we find here this very important psychic organ, which accomplishes sensa- tion, will, and thought, in the vertebrates, in its simplest form. The thick wall of the nerve-tube, which runs through the long axis of the body immediately over the axial rod, encloses a narrow central canal tilled with fluid (Figs. 98-102 r). We still find the medullary tube in this very simple form for a time in the embryo of all the vertebrates, and it retains this form in the amphioxus throughout life ; THE 1 7 7." TEBR. I TE ( 11. I R< \ < TER OF M. I N only in the latter case the cylindrical medullary tube barely indicates the sepa- ration or brain and spinal cord. The lancelet's medullar) tube runs near!) the whole length of the body, above the chorda, 'm the shape of ,i long thin tube of almost equal diameter throughout, and there is onlv a slight swelling of it right at the Front to represent the rudiment of a Cerebral lobe. It is probable that this peculiarity of the amphioxus is connected with the partial atrophy of its head, as the ascidian larvae on the one hand and the young cyclostomaon the other clearly show a division of the vesicular brain, 01 head marrow, from the thinner, tubular .spinal marrow. Probably we must trace to the same phylogenetic cause the defective nature of the sense organs of the amphioxus, which we will describe later (Chapter XVI.). ProspOndylus, on the other hand, probably had three pairs of sense-organs, though of a simple character, a pair of, or a single olfactory depression, right in front' (Figs. q8, ()(), nit), a pair of eyes (au) in the lateral walls of the brain, and a pair of simple auscultory vesicles (g) behind. There was also, perhaps, a single parietal or " pineal " eye at the top of the skull (epiphysis, e). In the vertical median plane (or middle plane, dividing the bilateral body into right and left halves) we have in the acrania, underneath the chorda, the mesentery and visceral tube, and above it the medullary tube ; and above the latter a membranous partition of the two babes of the body. With this partition is con- nected the mass of connective tissue which acts as a sheath both for the medullary tube and the underlying chorda, and is, therefore, called the chord-sheath (perichorda) ; it originates from the dorsal and median part of the cnelom-pouches. which we shall call the skeleton plate or " sclcrotom " in the craniote embryo. In the latter the chief part o( the skeleton— the vertebral column and skull ^ de\ elopes from this chord- sheath ; in the acrania it retains its simple form as a soft connective matter, from which are formed the membranous parti- tions between the various muscular plates or myotomes (Figs. 98, s differentiated in length, and has two apertures — a mouth for taking in food (Figs. 98, 100 md) and an anus for the ejection of unusable matter or excre- ments ( at ' ). With the alimentary canal a number of glands are connected which are of great importance for the vertebrate body, and which all grow out of the canal. Glands of this kind are the salivary glands, the lungs, the liver, and many smaller glands. Nearly all these. glands aie wanting in the acrania ; probably there were merely a couple of simple hepatic tubes (FigS. • w .j/. / .v task the derivation of the simple bod\ of the primitive vertebrate fimm the chordula is concerned, the articulate parts or metamera are o\' secondary interest, and we need not Lro into them just now. The characteristic composition of the vertebrate body devdopes from the em- bryonic structure in the same way in man thai this answer is just as certain and precise in the case ol the origin of man from the mammals. This advanced vertebrate class is also monophyletic, or h is evolved from one common stem- group of lower vertebrates (reptiles, and, earlier still, amphibia). This follows from the fact that the mammals are £ Fig. 103 A, R, C. D.— Instances of redundant mammary glands and nipples (hype>-»iasii<;t>t). A a ir of small redundant breasts (with two nipples on the left) above the large normal ones ; from a 45-year-old erlin woman, who had had children 17 times (twins twice). (From Ilansemami.) B the highest number : ten nipples (all giving milk), three pairs above, one pair below, the large normal breasts ; from a 22-year-old servant at \S arschau. (From jVrugebaur.) C three pairs of nipples: two pairs on the normal glands and one pair above ; from a 19-year-old Japanese girl. D four pairs of nipples : one pair above the normal and two pairs of small accessory nipples underneath ; from a 22-year-old Bavarian soldier. (From M'iederslieim.) as in all the other vertebrates. As all competent experts now admit the mono- phyletic origin of the vertebrates on the strength of this significant agreement, and this "common descent of all the vertebrates from one original stem-form " is admitted as an historical fact, we have found the answer to "the question of questions." We may, moreover, point out clearly distinguished from the other classes of the stem, not merely in one striking particular, but in a whole group of distinctive characters. It is only in the mammals that we find the skin covered with hair, the breast- cavity separated from the abdominal cavity by a complete diaphragm, and the larynx provided with an epiglottis. The THE VERTEBRATE CHARACTER OF MAN "3 mammals alone have three small aus< ul- tory bones in the tympanic cavitj .t feature thai is connected with the charac- teristic modification o( their maxillary joint. Their red blood-cells have no nucleus, whereas this is retained in nil other vertebrates. Finally, it is only in the mammals that we find the remarkable function of the breasl structure which has given its name to the whole class the feeding of the young by the mother's milk. The mammary glands which serve this purpose arc interesting in so many ways that we may devote a few lines to them here. As is well known, the lower mammals, especially those which beget a numbei ol young al a time, have several mammar) glands at the breast. Hedgehogs and sows have five pairs, mice four or five pairs, dbgS and squirrels lour pails, cats and hears three pairs, most of the ruminants and many of the rodents two pairs, each provided with a teat or nipple (iihistos). In the various genera o( the half-apes (lemurs) the number varies a good deal. On the other hand, the bats and apes, which only beget cue young at a time as a rule, have onlv one pair <.<( mammary glands, and these are found at the breast, as in man. These variations in the number or structure o\' the mammary apparatus (mammarium) have becomed oubly inter- esting in the light of recent research in comparative anatomy. It has been shown that in man and the apes we often find redundant mammary glands ( hvper- mastism ) and corresponding teats (hyper- thelism) m both sexes. Fig. 103 shows four cases o( this kind — A, B, and C o\ three women, and D of a man. They prove that all the above-mentioned numbers may be found occasionally in man. Fig. 103 A shows the breast of a Berlin woman who had had children seventeen times, and who has a pair of small accessory breasts (with two nipples on the left one) above the two normal breasts; this is a common occurrence, and the small soft pad above the breast is not infrequently represented in ancient statues of Venus. In Fig. 103 C we have the same phenomenon in a Japanese girl of nineteen, who has two nipples on each breast besides (three pairs altogether). Fig. 103 /) is a man of twenty-two with four pairs of nipples (as in the dog), a small pair above and two small pairs beneath the large normal teats. The ' maximum number o\ live pairs (as fi> the sow and hedgehog) was found in a Polish servant o\ twenty-two who had had several children ; milk was given hv > .a li nipple; there wire three pairs of redundant nipples above and one paii underneath the normal and very large breasts ( Fig. 103 />'). A numbei of recent investigations (especially among recruits) have shown that these things are not uncommon in the male as well as the female sex. They can only be explained by evolution, which attributes them to atavism and latent heredity, The earlier ancestors of all the primates (including man) were lower placentals, which had, like the hedgehog (one oi' the oldest forms of the living placentals), several mammary glands (live or more pairs) in the abdominal skin. In the apes and man only a couple of them are normally developed, but from time to time we get a development o\ the atrophied structures. Special notice should be taken of tiie arrangement of these accessory mammae; they form, as is clearly seen in Fig. 103 B and /), two long rows, which diverge forward (towards the arm - pit), and converge behind in the middle line (towards the loins). The milk-glands of the pplymastic lower placentals are arranged in similar lines. The phylogenetic explanation of poly- mastism, as given in comparative anatomy, has lately found considerable support in ontogeny. Hans Strahl, E. Schmitt, and others, have found that there are always in the human embryo at the sixth week (when it is three-fifths of an inch long) the microscopic traces of five pairs ol mammary glands, and that they are arranged at regular distances in two lateral and divergent lines, which correspond to the mammary lines. Only one pair of them — the central pair — are normally developed, the others atro- phying. Hence there is for a time in the human embryo a normal hyperthelism, and this can only be explained by the descent of man from low«.r primates (lemurs) with several pairs. But the milk-gland o( the mammal has a great morphological interest from another point of view. This organ for feeding the young in man and the higher mammals is, as is known, found in both However, it is usually active only in the female sex, and yields the valuable " mother's milk " ; in the male sex it is "4 THE VERTEBRATE CHARACTER OF MAX small and inactive, ■ real rudimentary organ of no physiological interest. Never- theless, in coi tain caseswe find the breasl as fully developed in man as in woman, and it ma) give milk for feeding the young. We have a striking instance of this gynecomastism (largo milk-giving breasts in a male) in Fig. 104. 1 owe the photo- graph (taken from life) to the kindness oi' Dr. Ornstein, of Athens, a German physi- cian, who has rendered service by a number of anthropological observations (for instance, in several cases oi tailed mj st.i\ in Ceylon (at Beltigemma) in [881. A young Cinghalese in his twenty- fifth year was brought to ine as a curious hermaphrodite, half-man and hall-woman. His targe breasts gave plenty of milk ; he was employed as " male nurse" to suckle a new-born infant whose mother had died at birth. The outline of his body was softer and more feminine than in the Greek shown iruFig. 104. As the Cingha- lese are small of stature and of graceful build, and as the men often resemble the women in clothing (upper part of the body naked, female dress on the lower Fig. 104.— A Greek gynecomasty men). The gynecomast in question is a Greek recruit in his twentieth year, who has both normally developed male organs and very pronounced female breasts. It is noteworthy that the other features of his structure arc in accord with the softer forms of the female sex. It reminds us of the marble statues of hermaphrodites which the ancient Greek and Roman j sculptors often produced. But the man would only be a real hermaphrodite if he had ovaries internally besides the (exter- nally visible) testicles. I observed a very similar case during part) and the dressing of the hair (with a comb), 1 first took the beardless youth to be a woman. The illusion was greater, as in this remarkable case gynecomastism was associated with iryptorchism — that is to say, the testicles had kept to their original place in the visceral cavity, and had not travelled in the normal way down into the scrotum. (Cf. Chapter XXIX.) Hence the latter was very small, soft, and empty. Moreover, one could feel nothing of the testicles in the inguinal canal. On the other hand, the male organ was very small, but normally developed. It was EMBR I '( Wit " sun: I DA ND GERMIN. \ l'l i '/. . I AY/. I "5 clear that this apparent hermaphrodite also was a real male. Another caseof practical gynecomastism has been described by Alexander von Humboldt. In a South American forest he found a solitary settler whose wife had died in child-birth. The man had laid the new-horn child on his own hreast in despair ; and the continuous stimulus of tlie child's sucking movements had revived the activity of the mammary gland . It i-< possible that nervous suggestion had some share in it. Similar cases have been often ohserved in recent years, even among other male mammals (such as sheep anil goats). The great scientific interest of these facts is in their heating on the question of heredity. The stem-history of the mammarium rests partly on its embry- ology (Chapter XXIV.) and partly on the facts of comparative anatomy and physio- logy. As in the lower and higher mam- mals the monotremes, and most of the marsupials) the whole lactiferous r.ppa- ratus is only found in the female ; an 1 as there are trai es of it in the male only in .1 few younger marsupials, there can Ixj no doubt that these important organs were originally found only in the female mammal, and that they were acquired hy these through a special adaptation to hahits of life. Later, these female organs were com- municated to both sc\es by heredity; and they have been maintained in all persons of either sex, although they are not physio- logicallyactive in the males. This normal permanence of the female lactiferous organs in both sexes of the higher mam- mals and man is independent of any selection, and is a fine instance of the much-disputed "inheritance oi acquired characters." Chapter XII. EMBRYONIC SHIELD AND GERMINATIVE AREA The three higher classes of vertehrates which \\i call the amniotes — the mammals, birds, and reptiles are notably distinguished by a number of peculiarities of their development from the five lower > of the stem -the animals without an amnion (the anamnia). All the amniotes have a distinctive embryonic membrane known as the amnion (or " water- membrane "), and a special emhryonic appendage — the allantois. They have, further, a large yelk-sac, which is filled witlj food-yelk in the reptiles and hirds, and with a corresponding clear fluid in the mammals. In consequence of these later-acquired structures, the original features of the development of the amniotes are so much altered that it is very difficult to reduce them to the palin- genetk embryonic processes of the lower amnion-less vertebrates. The gastraja theory shows us how to do this, hy repre- senting the embryology of the lowest vertebrate, the skull-less amphioxus, as the original form, and deducing from it, through a series of gradual modifications, the gastrulation and ccvlomation of the craniota. It was somewhat fatal to the true con- ception of the chief emhryonic processes^ of the vertebrate that all the older embryo- logists, from Malpighi (1687) and Wolff (1750) to Baer(i828) and Remak (1850), always started from the investigation of the hen's egg, and transferred to man and the other vertebrates the impressions they gathered from this. This classical, object of embryologies! research is, as we have seen, a source of dangerous errors. The large round food-yelk of the bird's egg causes, ^n the first place, a fiat discoid expansion of the small gastrula, and then so distinctive a development of this thin round embryonic disk that the controversy as to its significance occupies a large part of embryological literature. One of the most unfortunate errors that this led to was the idea of an original W n6 EMBRYONIC SHIELD AND GERM 1 NATIVE AREA antithesis of germ and yolk. The latter was regarded as a foreign body, extrinsic to the real germ, whereas it is properly a part of it, an embryonic organ oi nutrition. Many authors said there was no tra^i.' of the embryo until a later stage, and outside the yelk ; sometimes the two- layered embryonic disk itself, at other times only the central portion of it (as distinguished from the germinative area, which we will describe presently), was taken to he the first outline of the embryo. primitive gut. This, is clearly shown by the ova of the amphibia and cyclostoma, which explain the transition from the yelk-less ova of the amphioxus to the large yelk-tilled ova of the reptiles and birds. It is precisely in the study of these difficult features that we see the incal- CUlable value of phylogenetic considera- tions in explaining complex ontogenetic facts, and the need of separating ceno- genetic phenomena from palingenetic. ^ Fig. 105.— Severance of the discoid mammal embryo from the yelk-sac, in transverse section (diagrammatic). A The germinal disk (h, hf) lies Hat on one side of the branchial-gut vesicle (kb). B In the middle of the germinal disk we find the medullary groove ( >nr ), and underneath it the chorda I ch). C The gut- fibre-layer (df) has been enclosed by the gut-gland-layer (dd). D The skin-fibre-layer (hf) and gut-fibre-layer (df) divide at the periphery; the gut (d) begins to separate from the yelk-sat or umbilical vesicle ( nb). E The medullary tube ( mr ) is closed ; the body-cavity (c ) begins to form. F The prevertebral w,) begin to grow round the medullary tube (mr) and the chorda (ch ) : the gut fV; is cut off from the umbilical vesicle (nb). H The ▼ertebra; (iv) have grown round the medullary tube (mr) and chorda ; the body-cavity is closed, and the umbilical vesicle has disappeared. The amnion and serous membrane are omitted. The letters have the same meaning throughout: h horn-plate, mi- medullary tube, hf skin-fibre-layci. iv provertebra;, eh chorda, r body-cavity or cceloma, df gut-fibre-layer, dd gut-gland-layer, d rut-cavity, ■nb umbilical vesicle. In the light of the gastraea theory it is hardly necessary to dwell on the defects of this earlier view and the erroneous conclusions drawn from it. In reality, the first segmentation-cell, and even the stem-cell itself and all that issues there- from, belong to the embryo. As the large original yelk-mas* in the undivided egg of the bird only represents an inclosure in the greatly enlarged ovum, so the later contents of its embryonic yelk - sac (whether yet segmented or not) are only a part of the entoderm which forms the This is particularly clear as regards the comparative embryology of the verte- brates, because here- the phylogenetic unity of the stem has been already estab- lished by the well-known facts of paleon- tology and comparative anatomy. If this unity of the stem, on the basis of the amphioxus, were always borne in mind, we should not have these errors con- stantly recurring. In many cases the cenogenetic relation of the embryo to the food-yelk has until now given rise to a quite wrong idea of EMBRYONIC SHIELD AND GERM I NATIVE AREA i»7 the first and most important embryonic processes in the higher vertebrates, and baa occasioned ;i number of false theories in connection with them. Until thirty yens ago the embryology of the higher vertebrates always started from the position that the first structure of the embryo i- ;i fiat, leaf-shaped di-k ; it was for this, reason that ilu' cell-layers that compose this germinal disk (also called germinative ana) are called "germinal layers." This flat germinal disk, which i- round at first and then oval, .o)o\ which is often described as the tread 01 cicatricula in the laid hen's egg, i- found at a certain part of the surface of the large globular food-yelk. 1 am convinced that it i- nothing else than the discoid, flattened gastrula of the birds. At the beginning of germination the Hat embryonic disk cur\e> outwards, and separates on the inner. side from the underlying large velk- ball. In this way the fiat layers are con- verted into lubes, their edges folding and joining together (Fig. 105). As the embryo grows at the expense of the food- yelk, the latter becomes smaller and smaller; it i* completely surrounded by the germinal layers. Later still, the remainder of the food-yelk only forms a small round sac, the yelk-sac or umbilical vesicle (Fig. 105 ///>). This is enclosed by the visceral layer, is connected by a thin stalk, the yelk-duct, with the central part of the gut-tube, and i- finally, in most of the vertebrates, entirely absorbed by this (H). The point at which this takes place, and where the gut finally closes, is the visceral navel. In the mammals, in which the remainder of the yelk-sac remains without and atropine-, the yelk- duct at length penetrates the outer ventral wall. At birth the umbilical cord proceeds from here, and the point of closure remains throughout life in the skin as the navel. As the older embryology of the higher vertebrate- was mainly based on the chick, and regarded the antithesis of embryo (or fori native-yelk) and food-yelk (or yelk-sac) as original, it had also to look upon the fiat leaf-shaped structure of the germinal disk as the primitive embryonic form, and emphasise the fact that hollow grooves were formed ol these flat layers bv folding, and closed tubes by the joining together of their edges. This idea, which dominated the whole- treatment of the embryology of the higher vertebrates until thirty years ago, was totally false. The ga-.tr.ca theoiy, which has it- chief application here, teaches us that it i- the very reverse of the truth. Tlu cup-shaped gastrula, in the body- wall of which the two primary germinal layers appear from the fust as closed tube-, i- the original embryonic form of all the vertebrates, and all the multi- cellular invertebrates ; and the flat ger- minal disk with it- superficially expanded minal layers i- a later, secondary form, due to the cenogenetic formation Of the large food-yelk and the gradual -piead of llie germ-layers over its surface. Heme the ac tual folding of the germinal layers and their conversion into tubes is not an original and primary, but a much later and tertiary, evolutionary process. In the phytogeny of the vertebrate em- bryonic process we may distinguish the following three stages : — A. First Stage : Primary (palingenetic) embryonic priH - 13. Second Stage Secondary (cenogenetic) embryonic process. C. Third Stage ; Tertiary (cenogenetic) embryonic pro. The germinal layers form irom tlu tirvt closed tubes, the one- layered blastula being converted into the tWO- layered gastrula bj invagination. i-ye!k. ( Amfihioxus.) The germinal laj ersspread out li af-wise, to.xl- yelk gathering in the ventral entoderm, and a I elk - sac being formed from the middle of the gut-tube. (Amphibia.) The germinal 11 in a flat germinal di.sk, the borders of which join to- gether and form closed tubes, separating Irom the central yelk- sac. ( Amniotes.) .\- this theory, a logical conclusion from the gastraa theory, has been fully substantiated by the comparative study ol gastrulation in the last few decades, we must exactly reverse the hitherto pre- valent mode of treatment. The yelk-sac is not to be treated, as was done formerly, as if it were originally antithetic to the embryo, but as an essential part of it, a part of its visceral tube. The primitive gut of the gastrula has, on this view, been divided into two parts in the higher animals as a result of the cenogenetic formation of the food-yelk — the perma- nent gut ( mctagastcr), or permanent alimentary canal, and the yelk-sac (leci- thonm j, or umbilical vesicle. This is very clearly shown by the comparative onto- genj ol the fishes and amphibia. In these cases the whole yelk undergoes cleavage at first, and forms a yelk-gland, composed of yelk-cells, in the ventral wall uS EMBRYONIC SHIELD AND GERMINATIVE AREA of the primitive gut. Hut it afterwards k omes so large thai a part of the yelk does nol divide, and is used up in the yelk-sac thai is cut off outside. When we make a comparative study of the embryology of the amphioxus, the i .;, the chick, and the rabbit, there cannot, in my opinion, be any further doubt as to the truth oi this position, which 1 have held for thirty yens. Hence in the light of the gastraea theory we must regard the features of the amphi- oxus as the only and real primitive structure among all the vertebrates, de- parting very little from the palingenetic embryonic form. In the cyclostoma and the frog those features are, on the whole, not much altered cenogcnetically, but Fig. if/;.— Tt'e visceral embryonic vesicle (blastocyst, a or gastrocystis) of a rabbit (the " blastuta " or vesicula blastodermica of other writers), a outer envelope (o/olemma) b skin-layer or ectoderm, forming the entire wall of theyelk- i gTOUPE of dark cells, representing the visceral layer or entoderm. Fic. 107.-- The same in section. Letters as above, d cavity of the vesicle. (From Hisckoff.) they are very much so in the chick, and most oi all in the rabbit. In the bell- gastrula of the, amphioxus and in the hooded gastrula of the lamprey and the frog the germinal layers are found to be closed tubes or vesicles from the first. On the other hand, the chick-embryo (in the new laid, but not yet hatched, egg) is a Mat circular disk, and it was not easy to recognise tin's as a real gastrula. Rauber and Goette have, however, achieved this. As the discoid gastrula grows round the large globular yelk, and the permanent gut then separate-, from the outlying yelk- sac, we find all the processes which we have shown (diagrammatical!)) in Fig. 108 — processes that were hitherto re- garded as principal acts, whereas they are merely secondary. The oldest, oviparous mammals, the monotremes, behave in the same wa) as the reptiles and birds. Hut the corres- ponding embryonic processes in the vivi- parous mammals, the marsupials and placentals, are very elaborate and dis- tinctive. They were formerly quite mis- interpreted ; it was not until the publica- tion of the studies of Kdward van Beneden (1875) and ''u' 'aU'r research of Selenka, Kuppfer, Rabl, and others, that light was thrown on them, and we were in a position to bring them into line with the principles of the gastraea theory and trace them to the embryonic forms o( the lower vertebrates. Although there is no independent food-\ elk, apart from the formative yelk, in the mammal ovum, and although its segmentation is total on that account, nevertheless a large yelk-sac is formed in their embryos, and the "embryo proper" spreads leaf-wise over its surface, as in the reptiles and birds, which have a large food-yelk and par- tial segmentation. In the mammals, as well as in the latter, the Hat, leaf-shaped germinal disk sepa- rates from the yolk- sac, and its edges join together and form tubes. How can wc ex- plain this curious anomaly ? Only as a result of very characteristic and peculiar cenogenetic modifications oft ho embryonic process, the real causes of which must be sought in the change in the rearing of the young on the part of the viviparous mammals. These are clearly connected with the fact that the ancestors of the viviparous mammals were oviparous amniotes like the present monotremes, and only gradually became viviparous. This can no longer be questioned now that it has been shown (1884s! that the monotremes, the lowest and oldest of the mammals, still lay eggs, and that these develop like the ova of the reptiles and birds. Their nearest descendants, the marsupials, formed the habit of retaining the eggs, and developing them in the EMBRYONIC SHIELD AND GERM/NATIVE AREA jio oviduct; the latter was thus converted into a womb (uterus). A nutritive fluid thai was secreted from its wall, and passed through the wall of the blastula, now served to feed the embryo, and took the place of the food-yelk. In this way the original food-yelk of the monotremes gradually atrophied, and at last disap- peared so completely that the partial ovum-segmentation of their des< endants, the rest of the mammals, once more became total. From the discogustrula of the former was evolved the distinctive epigastnda of the latter. It is only b\ this phylogenetic explana- tion that we Can understand the formation and development of the peculiar, and hitherto totally misunderstood, blastula of the mammal. The vesicular condition of the mammal embryo was discovered 200 years ago 1 1077) by Regner de Graaf, He found in the uterus or a rabbit four days after impregnation small, round, loose, transparent vesicles, with a double envelope. However, Graaf's discovery p.is-^j without recognition. It was not until 1N27 that these vesicles were re- discovered by Baer, and then more closely studied in 1S42 bv Bischoff in the rabbit 1 •", 107). They are found in the womb of the rabbit, the dog, and other small mammals, a lew days after copula- tion. The mature ova of the mammal, when they have left the ovary, are ferti- lised either here 01 in the oviduct imme- diately afterwards by the invading sperm- cells.1 (As to the womb and oviduct see Chapter XXIX.) The cleavage and for- mation of the gastrula take place in the oviduct. Either here in the oviduct or after the mammal gastrula has passed into the uterus it is converted into the globular vesicle which is shown externally in Fig. 106, and in section in Fig. 107. The thick, outer, structureless envelope that encloses it is the original ovolemma or zona pcllucida, modified, and clothed with a layer of albumin that has been deposited on the outside. From this stage the envelope is called the external membrane, the primary chorion or pro- chorion f a J. The real wall of the vesicle 1 In man and the other mammals the fertilisation of the ova probablv takes place, as a rule, in the oviduct ; here the ova, which issue from the female ovary in the shape of the Graafian follicle, and enter the inner aper- ture of the oviduct, encounter the mobile sperm-cells of the male seed, which pass into the uterus at copulation, and from this into the external aperture of the oviduct. Impregnation rarely takes place in the ovary or in the womb. enclosed In it consists of a simple layer Ofectodermic cells ( b ), which are flattened by mutual pressure, and generally hexa- gonal ; a light nucleus shines through their fine-grained protoplasm (Fig. 108). At oi\t: part ft J inside this hollow ball we find a circular disc, formed of d.uker, softer, and rounder cells, the dark-grained entodermic cells (Fig. 109). The characteristic embryonic form that the developing mammal now exhibits has up to the present usually been tailed the "blastula" (Bischoff), "sac-shaped ein- bryo " (Baer), "vesicular embryo " ( vesi- cula blastodermic*!, or, briefly, blasto- SpheeraJ. The wall of the hollow vesicle, which consists of a single laver of cells, was called the " blastoderm," and was Supposed to be equivalent to the ccll-laver of the same name that forms the wall of the real blastula of the amphioxus and Fig. 109. Fie.. 108. — Four entodermic cells from the embryonic vesicle of the rabbit. Fig. 109. — Two entodermic cells from the embryonic vesicle of the rabbit. many of the invertebrates (such as Mono- xenia, Fig. 29 F, (J). Formerly this real blastula was generally believed to be equivalent to the embryonic vesicle of the mammal. However, this is by no means the case. What is called the " blastula " of the mammal and the real blastula of the amphioxus and many of the inverte- brates are totally different embryonic structures. The latter (blastula) is palin- genetic, and precedes the formation of the gastrula. The former (blastodermic vesicle) is cenogenetic, and follows gas- trulation. The globular wall of the blastula is a real blastoderm, and consists of homogeneous (blastodermic) cells ; it is not yet differentiated into the two primary germinal layers. But the globu- lar wall of the mammal vesicle is the differentiated ectoderm, and at one point in it we find a circular disk of quite different cells— the entoderm. The round EMBRYONIC SHIELD AND GERM1NATIVE AREA cavity, filled with fluid, inside the real blastula is the segmentation-cavity. But the similar cavity within the mammal vesicle is the yelk-sac cavity, which is which we have considered previously (Chapter IX.). For these reasons it is very necessary to recognise the secondary embryonic vesicle in the mammal (gastri* Fig. Fio. 112 Fig. Fig. mo.- Ovum of a rabbit from the uterus, one sixth of an inch in diameter- The embryonic vcsicle/AJ has withdrawn a little from the smooth ovolcmma fa). In the middle of the ovolemina we seethe round germinal disk (blastodiscus, c), at the edge o( which (at d) the inner layer of the embryonic vesicle is already beginning to expand. (Pigs, i io-i 14 from Bisckoff.) Fit;, mi. The same OVUm, seen in profile. Letters as in Fig. 1 10. Fig. 112. OVUm Of a rabbit from the uterus, one- fourth of an inch in diameter. The blastoderm, is already for the most part two-layered (b). The ovo- lemina. or outer envelope, is tutted (a). Fig. 113. The same ovum, seen in profile. Letters as in Fig. 1 1 2. FlG. 114. — Ovum Of a rabbit from the uterus, one- third of an incfa in diameter. The embryonic vesicle is now nearly every where two-layered (kj, only remaining cine-layered txTow (at d). Fig. 1:4 connected with the incipient gut-cavity. This primitive gut-cavity passes directly into the segmentation-cavity in the mam- mals, in consequence of the peculiar ceno- genelic changes in their gastrulation, cystis or blastocysts) as a characteristic structure peculiar to this class, and dis- tinguish it carefully from the primary blastula of the amphioxus and the inver- tebrates. EMBRYONIC SHIELD AND CFRM1NAT1VE AREA The small, circular, whitish, and opaque spot which the gastric disk (Fig. too) forms at a certain part of the surface oi the clear and transparent embryonic vesicle has long been known to science, iio. 115. -Round germinative area of the rabbit, divided into the centra] light areara/wa /v. lucida) and the peripheral dark ana (area opaea) The litflit ar.M teenu darker >>n account ol tfu> Jar!; ground appearing through it. and compared to the germinal disk of the birds and reptiles. Sometimes it has been called the germinal disk, sometimes the germinal spot, and usually the germina- tive area. From the area the further development oi the embryo proceeds. However, the larger pan oi the embry- onic vesicle oi the mammal is not directly used iov building up the later body, but for the construction oi the temporary umbilical vesicle. The embryo separates from tbis in proportion as it grows at its expense ; the two are only connected by the yelk-duct (the stalk o{ the yelk-sai i. and this maintains the direct communica- tion between the cavity oi the umbilical vesicle and the forming visceral cavity (Fig. ">5 • The germinative area or gastric disk of the mammal consists at first (like the germinal disk of birds and reptiles merely of the tWO primary germinal layers, the ectoderm and entoderm. Hut soon there appears in the middle of the circular disk between the two a third stratum of cells, the rudiment oi the middle layer ov fibrous layer ( mesoderm ). This middle germinal layer consists from the first, as we have seen in the tenth Chapter, of two separate epithelial plates, the two layers of the lom-pouches (parietal and visceral). However, in all the amniotes (on account of the large formation oi yelk) these thin middle plates are so firmly pressed together that the) seem to represent a single layer. It is thus peculiar to the amniotes that the middle ol the germina- tive area is composed of tour germinal layers, the two limiting (or primary) layers and the middle layers between them (Figs. 96, 97). These four second- ary germinal layers can be clearly dis- tinguished as soon as what is called the sickle-groove (or " embryonic sickle") seen at the hind bordei ofthegerminati area. At the borders, however, the ger- minative area o\ the mamma1 only con- sists oi tWO layers. Tin- resi of the wall oi the embryonic vesicle consists at first (but onl) foi a short time in most oi the mammals) ,a a single layer, the outer germinal layer. From this stage, however, the whole wall of the embryonic vesicle becomes two-laj 1 red. The middle of the germina- tive area i- much thickened by the growth oi the cells of the middle layers, and the inner layer expands at the same time, and increases at the border of the disk ill round. Lying close on the outer layer throughout, it grows oxer its inner surface at all points, covers first the upper and then the lower hemisphere, and at last closes in the middle oi the inner layer (Figs. 110 114). The wall oi the embry- onic vesicle now consists throughout oi two layers oi cells, the ectoderm without and the entoderm within. It is only in the centre oi the circular area, which I'n.. 116. Oval area, with the opaque whitish border oft hi- dark area without. becomes thicker and thicker through the growth of the middle layers, that-it is made up of all four layers. At the same time, small structureless tufts or warts are deposited on the surface oi the outer 1 2a EMBRYONIC SHIELD AND GERMINATIVE AREA ovolemma or prochorion, which has been raised above the embryonic vesicle (Figs, 112 114 a). We may now disregard both the outer oxolemma and 'the greater pari of the Fie. .17 Oval germinal disk of the rabbit, magnified about ten times. As the delicate, half transparent disk lies on a black ground, the pellucid area looks like a dark ring, and (he opaque area (lying outside it) like a white ring. The, oval shield in die centre also looks whitish, and in its axis we see' the dark medullary groove. (From Biscltoff.) vesicle, and concentrate our attention on the germinative area and the four-layered embryonic disk. It is here alone that we find the important changes which lead to the differentiation of the first organs. It is immaterial whether we examine the germinative area of the mammal (the rabbit, for instance) or the germinal disk o\ a bird or a reptile (such as a lizard or tortoise). The embryonic processes we are now going to consider are essentially the same in all members of the three higher classes of vertebrates which we call the amniotes. Man is found to agree in this respect with the rabbit, dog, ox, etc.; and in all these mammals the ger- minalive area undergoes essentially the same changes as in the birds and rep'tiles. They are most frequently and accurately studied in the chick, because we can have incubated hens' eggs in any quantity at any stage of development. Moreover, the round germinal disk of the chick passes immediately after the beginning of iocu- bation (within a few hours) from the two- layered to the four-layered stage, the two- layered mesoderm developing from the median primitive groove between the ectoderm and entoderm (Figs. X2-95). The first Change in the round germinal disk of the chick is that the cells at its edges multiply more briskly,, and form darker nuclei in their protoplasm. This gives rise to a dark ring, mote or less sharply set off from the lighter centre of the germinal disk (Fig. 115). From this point the latter takes the name of the "light area" (area pellueidaj, and the darker* ring is called the "dark area" (area opaca). (In a strong light, as in FigS. 115 117, the light area seems dark, because the dark ground is seen through it; and the dark area seems whiter). The circular shape of the area now changes into elliptic, and then immediate!} into oval (Figs. Il6, 117). One end seems to be broader and blunter, the other narrower and more pointed; the former corresponds to the anterior and the latter to the pos- terior section of (he subsequent body. At the same time, we can already trace the characteristic bilateral form of' the body, the antithesis of right and left, before and F'<\ ''8r-,pear-shaped germinal shield of the rabbit (eight days old), magnified twenty times, rf medullary groove. // primitixc groove (primitive mouth ). (J'rom ho/likcr.) behind. This will be made clearer by the " primitive streak," which appears at the posterior end. At an early Stage an opaque spot is seen in the middle of the clear germinative EMBRYONIC SHIELD AND GERMIN \TIVE AREA i*3 area, and this also passes from a circular to .m oval shape.' At first this shield- shaped marking is very delicate and bai ely perceptible j but it soon becomes clearer, and now stands oul as an oval shield, surrounded by two rings or areas (Fig. 117). The inner and brighter ring is the remainder o\ the pellucid area, and the dark outer ring the remainder o( the opaque area ; the opaque shield-like spol itself is the first rudiment of the dorsal pari oi the embryo. We give it briefly ment " and " germinative area " arc used in many different senses and this has led to a t'ata! confusion in embi vonic literature -wo must explain very clearly the real significant e of these importanl embryonic paits o\ the amniote. It will be useful lo do SO in a series ol" formal prin- ciples : - 1. The so-called " first trace o\ the embryo " in the amniotes, or the embry- onic shield, in the Centre of the pellucid area, consists merely of an early differen- ^- --. v s. Fig. no. — Median longitudinal section of the gastrula of four vertebrates. (From Pah!.) A iscogastrufa ol 1 iharkfPrittinrus). B amphigastrula of a sturgeon ( Accipenser), C amnhitjastrula of an u:i.: / '/'.-.•/.... 1 /» ...*: — .^..l-, .*.* «m -».,%..: ■»... 1. 1:. ,....- ...,\ ' .......i /. .1 -.1 i:_ .,1* a,~ »«.k.m ..».-.. .*w ui^Ci.t^;.i''iriii.i 01 .1 snarK i / rtst turns /. /> anipm^.txirui.i 01 a siurj^ron f .1 1 < tpi'Hsrr J. e ampni^astruia 01 amphirmim f Triton J. D epigastrula of an amniote (diagram), a ventral, b dorsal lip of the primitive mouth the name of .embryonic shield ov dorsal shield. In most works this embryonic shield is described as "the Inst rudiment o\- trace ol" the embryo, " ov " primitive embryo." Bui this is wrong, though it rests on the authority of Baerand Bischoff. As a matter o( fact, we already have the embryo in the stem-cell, the gastrula, and all the subsequent Stages. The embryonic shield is simply the tirst rudiment of- the dorsal part, which is the earliest todevelop. As the older nanus o\ " embryonic rudi- tiation and formation of the middle dorsal parts. 2. Hence the best name for it is " the dorsal shield," as I proposed long ago. 3. The germinative area, in which the firsl embryonal blood-vessels appear at an early stage, is not opposed as an external are. 1 to the "embryo proper," but is a part of it. 4. In the same way, the yelk-sac or the umbilical vesicle is not a foreign external '-4 / MBR ) -().\'/( ' SHIELD . 1 ND GERMIN* 1 11 1 /. . ! RE. I appendage of the embryo, but an outlying part of its primitive gut. 5. Tne dorsal shield gradually separates from the germinative area and the yelk- sac, its edges growing downwards and folding together to form ventral plates. (). The yelk-sac and vessels of the ger- minative area, which soon spread over its whole Surface, are, therefore', real embryonic organs, or temporary parts of the embryo, nnd have a transitorj impor- tance in connection with the nutrition of the growing later body; the latter maybe called the " permanent body " in COUtrast to them. The nlation of these cenogenetic features of the amniotes to the palin- genetic structures of the older non- amniotic vertebrates may be expressed in the following theses : The original gastrula, which completely passes into tlie embryonic body in theacrania, cyclos- torna, and amphibia, is early divided into two parts in the amniotes the embrvonic shield, w hich represents the dorsal outline of the permanent body ; and the temporary embrvonic organs of the germinative area and its bloou-vessels, which soon grow over the whole of the \ elk-sac. The differences which we find in the various classes of the vertebrate stem iii these important particulars can only be fully understood when we bear in mind their phylogenetic relations on the one hand, and, on the other, the cenogenetic modi- fications of structure that have been brought about by changes in the rearing of the young and the variation in the mass of the food-yelk. We have already described in the ninth chapter the changes which this increase and decrease of the nutritive yelk causes in the form of the gastrula, and cspeciallv in the situation and shape of the primitive mouth. The primitive mouth or prostoma is originally a simple round aperture at the lower pole of the long axis ; its dorsal lip is above and ventral lip below. In the amphioxus this primitive mouth is a little eccentric, or shifted to the dorsal side (Fig. 39). The aperture increases with the growth of the food-yelk in the cyclo- stoma and ganoids; in the sturgeon it lies almost on the equator of the round ovum, the ventral lip ( a ) in front and the dorsal lip (b) behind (Fig. 1196). In the wide- mouthed, circular discoid gastrula of the selachii or primitive fishes, which spreads quite Hat on the large food-V elk, the anterior semi-circle of the border of the disk is the ventral, and the posterior semi- circle the dorsal lip (Fig, IIQ .1). The amphiblastic amphibia are directly con- nected with their earlier fish-ancestors, the dipneusts and ganoids, and further the oldest selachii ( ( 'estracfon); they have retained their total unequal segmentation, and their small primitive mouth (Fig. 119 (',— Embryonic vesicle of a seven-days-old rabbit with oval embryonic shield 1 Groat above, a from the side. (From Ktilliker.) ttg dorsal shield or embryonic spot. In/? the upper half of the vesicle is made up of the two primary germinal layer-,, the lower (up to ge) onl) Iron the outei '■■ rabbit (Fig. 96) ; moreover, the peculiar course of the gastnilation is just the same. The germinative area forms in the human embryo in the same way as in the Other mammals, and in the middle part of this we have the embryonic shield, the purport of which we considered in the previous chapter. The next changes in the embryonic disk, or the "embryonic spot," take place in corresponding fashion-. These are- the changes we are now going to consider more closely The chief part of the oval embryonic shield is at first the narrow hinder end ; limit this median furrow are the side lips of the primitive mouth, right and left. In this way the bilateral-symme- trical type of the vertebrate becomes pronounced. The subsequent head of the amniote is developed from the broader and rounder fore-half of the dorsal shield. In this fore-hall" of the dorsal shield .1 median furrow quickly makes its appear- ance (Fitf. 123 >-/). This is the broader dorsal furrow or medullary groove, the first beginning of the central nervous system. The two parallel dorsal or medullary swellings that enclose it grow DORSAL BODY AX/) YEXTRAL /it)/))' together over i! afterwards, .md form the medullary tube. As is seen in transverse sections, it is formed only of the outer germinal layer (Figs. 05, 136). Hie lips of the primitive mouth, however, lie, as we know, at the important point where the outer layer bends over the inner, and from which the two ccelom pouches grow between the primary germinal layers. Thus the median primitive lurrow ( pr) Fig. 121.— Oval embryonic shield of the rabbit (A of six days eighteen hours, B of eight days). (From Kolliker.) ps primitive streak, pr primitive groove, arg area germinalis, sw sickle-shaped ter- minal growth. */ Fig. Fig. 122.— Dorsal shield fagl and germinaiive area of a rabbit-embryo ol cicrhi days. (From A'dlliker.) pr primitive groove, r/idoraal lurrow Fig. 123. — Embryonic shield of a rabbit of eight days. (From Vnn Beneden.) pr primitive groove, en canalis neurentericus. nk nodus ncurcntericus (or " Henscn's ganglion"), kf head-process (chorda). DORSAL BODY AND VENTRAL /u>nv i*7 in the hind-halt and the median medullary furrow (rf) in the fore-half of the oval shield are totally different structures, although the latter seems to a superficial observei to be merely the forward con- tinuation o\ the former. Hence the\ Fig. 124— Longitudinal section of the ccelomula ot amphioxus (from tin- left). 1 entoderm, d primitive gut, en medullary duct, n nerve tube, »' mesoderm, 1 lirsi primitive segment, r ffk>m-p<>whffii, (From Hatsckek.) were Formerly always confused. This error was the more pardonable as imme- diately afterwards the two grooves do actually pass into each other in a very remarkable way. The point of transition is the remarkable neurenteric canal (Fig. 1240V). But the direci connection which is thus established (.Iocs not last long ; the two are soon definitely separated by a partition. The enigmatic neurenteric canal is a very old embryonic organ, and of great phylogenetic interest, because it arises in the same way in all the chordoma (both tunicates and vertebrates). In everj case it touches or embraces like an arch the posterior end o\ the chorda, which has been developed lure in tvont out of the middle line oi the primitive gut (between the two ccelom-folds of the sickle groove) ("head-process," Fig. 123 if). These very ancient and strictly hereditary struc- tures, which have no physiological signifi- cance to-day, deserve [as " rudimentar) organs") our closes! attention. The tenacity with which the useless neuren- teric canal has been transmitted down to man through the whole series of verte- brates is of equal interest for the theory of descent in general, and the phytogeny of the chordonia in particular. The connection which the neuren- teric canal (Fig. 123 en) establishes between the dorsal nerve-tube ( n) and the ventral gut-tube (d) is seen very plainly in the amplnoxus in a longi- tudinal section of the nc'oinula, as soon as the primitive mouth is completely closed at its hinder end. The medullary tube has >tiii at this stage an opening at the forward end, the neuropi \i>^ (Fig. 83 n/>). This opening also is after- ,;, wards closed. There are then two completely closed canals ovei other the medullar) tube above and the gastric tube below, the two being separated by the chorda. The same features as in the .11 rania are exhibited by the related tunicates, the as, idi.e. Again, we find the neurenteric Canal in just the same form and Situation in the amphibia. A longitudinal section of a young tadpole (Fig. 1 -'5) shows how we may penetrate from the still open primitive mouth (xj either into the wide primitive gut-cavity (al) or the* narrow overlying nerve- tube. A little later, when the primitive mouth is closed, the narrow neurenteric canal (Fig. 1 10 tte) represents the an heel connection between the dorsal medullar) canal (ntc) and the ventral gastric canal. In the amniotes this original curved form ot the neurenteric canal cannot be found at first, because here the primitive mouth travels completely over to the Fig. 125.— Longitudinal section of the ehordula of a frog. (From Jial/on::) nc iur\c-tube, x canalis niurcntericus, al alimentary canal, yk yclk- cclls, m mesoderm. dorsal surface of the gastrula, and is con* verted into the longitudinal furrow we call the primitive groove. Hence the primitive groove (Fig. 1 28 pr)^ examined from above, appears to be the straight 128 /h)/c\ u /H)/)V AND VENTRAL BODY continuation ot the fore-lyintj younger medullary furrow (me) md The divergent hind legs of the latter embrace the anterior end of the former. After- wards we haw the complete closing of the primitive mouth, the dorsal swellings /»' Fig. 126.— Longitudinal section of a frog-embryo. (From Goeltr.) m mouth, / liver, an amis, »,■ c&iialis neur- cntcricus, n/c medullary tube, />/; pineal body (epiphysis), ch chorda. While these important processes are taking place in liu- axial purl of the dorsal shield, it ^ external form also is changing. The oval form (Fig, 117) becomes liU> tin- sole of a shoo or sandal, lyre-shaped or finger- biscuit shaped (Fig. 130). The middle third does noi grow in width as quickly as tin.' posterioi , and still loss than the anterior third; thus the shape of the permanent body becomes somowh.it narrow at the waist. \t the same time, the oval form of the germinative area returns to a circular shape, and the inner pellucid area separates more clearly from tho opaque outer area (rig. 131,0V The completion of the circle m the area marks the limit of the formation of IMoodrvessels in the mesoderm. The characteristic sandal-shape ol the dorsal shield, which is Fk.. 127. Figs. 127 and 128— Dorsal shield of the chick. (From Balfoitr.} The medullary furrow ( me ). which is not yet \isiblc ui Fig. 130, encloses with its binder end the fore end of the primitive groove ( p>-j in Fig. 131, Pig. 1 joining to form the medullary tube and growing ovei it. Tho neurenteric canal then leads directly, in the shape of a narrow arch-shaped tube (Fig. 129 tie), from the medullary tube (sp) to the determined by the narrowness of the middle part, and which is compared to a violin, lyre, or shoe-sole, persists foi a long time in all the am motes. All mammals, birds, and reptiles have sub- gastric tube (peg)- Directly in front of slantially the same construction at this it is the latter end of the chorda (ch). ' stage, and even for a longer or shorter DORSAL fii)DV AND I E.\ I'RAL HOOY 129 period after the division of the primitive segments into the coeJom-fokls has begun (Fig, 1 \D The human embryonic shield assumes the siindai-tbrni in the second week 01 development; towards Pta 1 ^.—Longitudinal section of the hinder end of a chick. (From Ha //on r.) sf> medullary tube, connected with the terminal gut I i<> the ueupentfric canal (mj.rh chorda, pr neureoteric (or llenaen si ganglion, al allantoic ef ectoderm, ky entoderm, so , layer, xf> visccr.il layer, an anus-pit, am amnion the end of the week our sole-shaped embryo has .1 length of about one-twelfth of an inch (Fig. 133), The complete bilateral symmetry of the vertebrate body is verj early indicated in the oval form of the embryonic shield (Fig. 117) by the median primitive streak ; in the sandal-form it is even more pro- nounced (Figs. 131 135). In the lateral parts of the embryonic shield a darker central and a lighter peripheral /one become more obvious ; the former i> called the stem-zone (Fig. 134 st~), and the latter the parietal /.one ( pz ) ; from the first we get the dorsal and from the ■ad the ventral half of the body-wall. The stem-zone of the ammote embryo would be called more appropriately the dorsal zone or dorsal shield , from it developes the whole of the dorsal hall of the later body (or permanent body) — that is to say, the dorsal body ( episoma ). Again, it would be better to call the "parietal /one" the ventral /.one or ventral shield, from it develop the ventral "lateral plates,' which after- wards separate from the embryonic vesicle and form the ventral body fhypo- tomaj — that is to say, the ventral half of the permanent body, together with the body-cavity and the gastric canal that it encloses. The sole-shaped germinal shields of all the amniotes are still, at the stage of con- struction which Fig. 1 34 illustrates in the rabbit and Fig. 135 in the opossum, so like each Other lliat we cm eitlll distinguish them it all or onlj b) mean-, ol quite subordinate peculiarities in die size of the various parts. Moreover, die human sandal-shaped embryd cannot ai this stage be dis- tinguished from thoseol other mammals, and it particularly r< sembles thai of the rabbit. On the other hand, the outer form of these flat s.mdal- shaped embryos is very dif- ferent from the corresponding form of the lower animals, especiallythe a< rania (amphi- oxus). Nevertheless,' the body is just the same in the essen- tial features of its structure as thai we find in the chordula Of the latter (Figs. 83 86), and in the embryonic forms which immediately develop from it. The striking ex- ternal difference is here again due to the fact that in the palingenetic embryos of the amphioxus (Figs. 83, 84) and the amphibia [Figs. 85, 86) the gut- Fie. no. -Germinal area or germinal disk ot the rabbit, w ith sole-shaped embryonic shield. magnified about ten times The dear circular field (dj is tlu opaque area. The pellucid area (c) is 1> re- shaped, like the embryonic shield itself ( b ). In its a\is is seen the dorsal lurrow or medullary furrow (a J. (From />/.■>< fi wall and body-wall form closed tubes from the first, whereas m the cenogenetic embryos of the amniotes they are forced to expand leaf-wise on the surface owing to the great extension of the food-yelk. 13° DORSAL BODY AXD VENTRAL BODY It i> all the more notable thai the eai K iration o\ dorsal and ventral halves takes place in the same rigidly hereditary fashion in all the vertebrates. In botn the acrania and the craniota the dorsal body is about this period separated from the ventral body. In the middle pari of the body this division has already taken place by the construction of the chorda between the dorsal nerve-tube and the ventral canal. But in the outer or lateral proceed step by step with interesting changes in the ectoderm, while the ento- derm changes little at first. We can study these processes besl in transverse sections, made verticallj to the surface through the sole-shaped embryonic shield. Such a transverse section of a chick- embryo, at the end of the first day of incubation, shows the gut-gland layer as a very simple epithelium, which is spread like a leaf over the outer surface of the Fig Fig. 132. Fig. 131.— Embryo Of the OpOSSUm, sixty hours old, one-sixth of an inch in diameter. (From Selenka.) i the globular embryonic vesicle, a the round germinative area, b limit of the ventral plates, r dorsal shield, t> its fore part, u the first primitive segment, ch chorda, chShs fore-end, pr primitive groove (or mouth). Fig. 132.— Sandal-shaped embryonic shield of a rabbit of eight days, with the fore part of the germinative area (no opaque. ap pellucid area). (From h'olliker.) rf dorsal furrow, in the middle of the medullary plate, It, pr primitive groove (mouth), stz dorsal (stem) zone, p: ventral (parietal) zone. In the narrow middle part the first three primitive segments may be seen part of the body it is only brought about by the division of the coelom-pouches into two sections — a dorsal tpisomile (dorsal segment or provertebra) and a ventral hyposomite (or ventral segment) — by a frontal constriction. In the amphioxus each of the former makes a muscular pouch, and each of the latter a sex-pouch or eronad. These important processes of differen- tiation in the mesoderm, which we will consider more closely in the next chapter. food-yelk (Fig. 92). The chorda (ch) has separated from the dorsal middle line of the entoderm ; to the right and left of it are the two halves of the mesoderm, or the two ccelom-folds. A narrow cleft in the latter indicates the body-cavity ( uwh ) ; this separates the two plates of the coelom-pouches, the lower (visceral) and upper (parietal). The broad dorsal furrow ( Rf) formed by the medullary plate (rti) is still wide open, but is divided from the lateral horn-plate DO R. SAL /M)/)Y AND VENTRAL BODY i3t (h ) by the parallel medullary swellings, which eventually close. During these processes important change-- are taking place in the outer germinal layer (the " skin-Sense la\er"). The continued rise and growth of the dorsal swellings causes their higher parts to bend together at their free borders, approach nearer and nearer (Fig. 1 and finally unite. Thus in the end we Yelk-sac descent it is a thoroughly natural process. The phylogenetic explanation of it is that the central nervous system is the organ by means of which all intercourse with the outer world, all psychic action and sense-pei ception, area* complished ; hence it was hound to develop originally from the outer and upper surface of tin- body, Or from the outer skin. The medullary tube afterwards separates completely from Amnion Chorion Fio. '11- Fig. tit.— Human embryo at the sandal-stage, one-twelfth of an inch long, from the end of the second week, magnified twenty-five times. (From Count Spec.) Fig. 134— Sandal-shaped embryonic shield of a rabbit of nine days. (From Kslliker.) (Back view from above.) stz Stem-zone or dorsal shield (with eight pairs of primitive segments), pz parietal or ventral zone, ap pellucid area, af amnion-fold, h heart, ph pericardial cavity, vo omphalo-mcsenteric vein. ab eye-vesicles, vh fore brain, mh middle brain, hh hind brain, ma primitive segments (or vertebrae). get from the open dorsal furrow, the upper cleft of which becomes narrower and narrower, a closed cylindrical tube (Fig. 137 mr). This tube is of the utmost importance ; it is the beginning of the central nervcius system, the brain and spinal marrow, the medullary tube. This embryonic fact was formerly looked upon as very mysterious. We shall see pre- sently that in the light of the theory of the outer germinal layer, and is sur- rounded by the middle parts of the pro- vertebra- and forced inwards (Fig. 146). The remaining portion of the skin-sense layer (Fig. 93 h) is now called the horn- plate or horn-layer, because from it is developed the whole of the outer skin or epidermis, with all its horny appendages (nails, hair, etc.). A totally different organ, the prorenaJ . l32 DORSAL BODY AM) VENTRAL BODY (primitive kidney) duct (u/tg-J, is found to lx- developed at an early stage from the ectoderm. This is originally a quite simple, tube-shaped, lengthy duct, or straight canal, which runs from front to rear at each side ol the provertebrae (on the outer side, Kig. 93 ang). It ont;i- the first trace of it does not come from the skin-sense layen but the skin-fibre layer The inner germinal layer, or the gut- fibre layer (rig. 93 t/ci), remains un- changed during these processes. -\ little later, however, it shows a quite fiat, groove-like depression in the middle line afi k » , \: Fig. 135 — Sandal-shaped embryonic shield of an opossum (DuUUhys). three days old. (From SrUnha.) (Back new from above.) stz stem-zone or dorsal shield (with eigln pair, of primitive segments), pz parietal or ventral zone, un pellucid area, ao opadue area, hh halves of the heart, v lore-end. h hind-end In ttw median line \vt see the chorda ( ch ) througli the transparent medullar} tube ( m ) n primitive segment, pr primitive stre.k (or primitive mouth) nates, it seems, out of the horn-plate at the side of the medullary tube, in the gap that we find between the prevertebral and the lateral plates. The prorenal duct is visible in this gap even at the time of the severance of the medullary tube from the horn-plate. Other observers think that of the embryonic shield directly under the chorda. This depression is called the gastric groove or furrow This at once indicates the future lot of this germinal layer As this ventral groove gradually deepens, and its lower edges bend towards each other, it is formed into a closed tube, /v>/,\v !/- RODY AND VENTRAL BODY 'ii the alimentary canal, in the same way as the medullary groove grows into the medullary tube. The gut-fibre layer (Fig. '37/1' which lies on the gut-gland layer r'c/y, naturally follows it in its folding. Moreover, the incipient gut- wall consists from the first oi two layers, internally the gut-gland layer and exter- nally the gut-fibre layer. The formation oi the alimentary canal resembles that of the medullary tube to this extent in both cases a straighj groove ov fin low arises first (>t" all in the middje line of a fiat layer. The .edges of this furrow then bend towards each other, and join to forma tube (Fig. 137). Bui the two processes are really very different. The medullar) tube closes in its whole length, and forms a cylindrical tube, whereas the alimentary canal remains open in the middle, and its cavity con- tinues for a long time in connection with the cavity of the embryonic vesicle. The open connection between the two cavities is only closed at a very late stage, by the construction of the navel. The closing oi the medullary tube is effected from both sides, the edges oi the groove joining together from right and left. Hut the closing oi the alimentary canal is not only effected from right and left, hut also from front and rear, the edges oi the ventral growing together from every side towards the navel. Throughout the three higher classes oi vertebrates the whole oi this process of the construction of the gut is closely connected with the formation oi the navel, or wilh the separation of the embryo from the yelk-sac or umbilical vesicle. In order to get a clear idea of this, we must understand carefully the relation of the embryonic shield to the germinative area and the embryi mc vesi !e. This is done best by a comparison of the five stages which are shown in longitudinal section in Figs. 138 142 The embryonic shield (c ), which at first projects very slightly over the surface of the germ illa- tive area, soon begins to rise higher ftbove it, and to separate from the embryonic vesicle. At this point the embryonic shield, looked at from the dorsal . ^urface, shows still the original simple .sandal-shape. (Figs. 133-135). We do not yet sec any trace of articulation into head, neck, trunk, etc., or limbs. Hut the embryonic shield has increased greatly in thickness, especially in the anterior part. It now has the appearance oi a thick, oval swelling, strongly curved over the surface of the germinat ive area. li begins 1 ompletely from the embryonic vesicfe, with which it is con- nected at the ventral surface. As this severance proceeds, Hie back bends more and more ; in proportion as tin- embryo grows the embryonic vesicle decreases, and at lasl i( merely hangS as a small vesicle from the belly of the embryo (Fig. 142 ds). In consequence of the growth- movements which cause this severance, a groove-shaped depression is formed at the surface of the vesicle, the limiting furrow, which surrounds the vesicle in the shape oi a pit, and a circular mound or dam (Pig. 139 ks\ is formed at the outside of this pit by the elevation of the contiguous parts oi the germinal. vesicle. In order to understand clearly this important process, we may compare the embryo to ,1 fortress wilh its surrounding In. 1 ;\ Transverse section of the embryonic disk of a chick at the end of the first da> of inctiba> tio.1. magnified about twent) times The edges of the medullary plate I >n j. the medullar] swelling fwj, which separate the medullary Iron, the horn-prate (hj. arc bending towards each other, At each side of the chorda ( ch ) t lit primitive segment plates (u) have separated from the lateral plates fspj. A ^rut-jjland layer. ( From Remak. » rampart and trench. The ditch consists oi the outer part of the germinat ive area, and comes to an end at the point where the area passes into the vesicle. The important fold of the middle germinal layer that brings about the formation of the body-cavity spreads beyond the borders of the embryo over the whole germinative area. At first this middle layer reaches as far as the gcr.ninative area , the whole of the rest of the embryonic vesii k i onsists in the beginning onlv of the two original limiting layers, the outer.and inner ger- minal layers. Hence, as far as the ger- minative area extends the germinal layer splits into the tWQ plates we have already recognised in it, the outer skin-fibre layer and the inner gut-fibre layer. These two plates diverge considerably, a clear fluid gathering between them (Fig. 140 am). The inner plate, the gut-fibre layer, remains on the inner layer of the embry- onic vesicle (on the gut-gland layer). The »34 DORSAL nonv AND VENTRAL BODY outer plate, the skirt-fibre layer, lies close on the outer layer of the germinative area, or the skin-sense I. nor, and separates together with this From the embryonic vesicle. From these two united outer plates is formed a continuous membrane. This is the circular mound that rises higher and higher round the whole, embryo, and at last joins above it (Figs. 139-142 itm). To return to our illustra- tion Of the fortress, we must imagine the circular rampart to he extraordinarily high and towering \.ir ahove the fortress. Its edges bend over like the combs of an overhanging wall of rock th^t would enclose the fortress ; they form a deep hollow, and at last join together above. the original embryonic vesicle, starts from the open bell) of the embryo (Fig. 138 kk). In more advanced embryos, in which the gastric wall and the ventral wall are nearly closed, it hangs out of the navel-opening in the shape o( a small vesicle with a stalk (Figs. 141, 142 ,/\). The more the embryo grows, the smaller becomes the vitelline (yelk) sac. At first the embryo looks like a small appendage of the large embryonic vesicle. After- wards it is the yelk-sac, or the remainder of the embryonic vesicle, that seems a small pouch-like appendage of the embryo (Fig. 142 ds). It ceases to have any significance in the end. The very wide opening, through which the gastric cavity Fig 117.— Three diagrammatic transverse sections of the embryonic disk of the higher verte- brate, to show the origin of the tubular organs from the bending germinal layers. In Fig. A the medullary tube ( >i ) and the alimentary canal (a) are still open grooves. In Fig. B the medullary tube (n)and the dorsal wall are closed, but the alimentary canal (a) and the ventral wall are open ; the prorenal ducts ( u ) are cut off from the horn-plate (h) and internally connected with segmental prorenal canals. In Fig. C both the medullary- tube and the dorsal wall above and the alimentary canal and ventral wall below are closed. All the open grooves have become closed tubes ; the primitive kidneys are directed inwards. The letters have the same meaning in all three figures : h skin-sense layer, n medullary tube, u prorenal ducts, x axial rod, s primitive-vertebra, « dorsal wall, b ventral wall, c body-cavity or ceeloma, / gut-fibre layer, / primitive artery (aorta), v primitive vein (subintestinal vein), d gut-fibre layer, a alimentary canal. In the end the fortress lies entirely within the hollow that has been formed by the growth of the edges of this large rampart. As the two outer layers of the germina- tive area thus rise in a fold about the embryo, and join above it, they come at last to form a spacious sac-like membrane about it. This envelope takes the name of the germinative membrane, or water- membrane, or amnion (Fig. 142 ani). The embryo floats in a watery fluid, which fills the space between the embryo and the amnion, and is called the amniotic fluid (Figs. 141, 142 ah). We will deal with this remarkable formation and with the allantois later on (Chapter XV.). In front of the allantois the yelk-sac or umbilical vesicle (ds), the remainder of at first communicates with the umbilical vesicle, becomes narrower and narrower, and at last disappears altogether. The navei, the small pit-like depression that we find in the developed man in the middle of the abdominal wall, is the spot at which the remainder of the embryonic vesicle(the umbilical vesicle) originally entered into the ventral cavity, and joined on to the growing gut. The origin of the navel coincides with the complete closing of the external ventral wall. In the amniotes the ventral wall originates in the same way as the dorsal wall. Both are formed substantially from the skin-fibre layer, and externally covered with the horn-plate, the border section of the skin-sense layer. Both come into DORSAL BODY .\.\D VENTRAL Hop) '35 K* < Flu ij8. Fig. 140, Pi.. 1 0. Fig. 141. Fig. 142. Figs. 13&-142— Five diagrammatic longitudinal sections of the maturing: mammal embryo and Its envelopes. In Figs. 118-141 the longitudinal section passes through the sagittal or middle plane of the b.xiy. dividing the right and left halves; in Fig. 142 the embryo is seen from the left side. In Fig. 138 the tufted prochorion (3d) encloses the germinal vesicl :, the wall of which consists of the two primary layers. Between the outer (a) and inner (i) layer the middle layer (m) has been developed in the rdgion of the germinative area. In Fig. 139 the embryo (e) begins to separate from the embryonic vesicle (ds), while the wall of the amnion-fold 1 bout it (in front as head-sheath, ks, behind as tail-sheath, ss). In Fig. 140 the edges of the amniotic fold (am) rise together over the back of the embryo, and form the amniotic cavity (ah); as the embryo separates more completely from, the embryonic vesicle (ds) the alimentary canal (dd) is formed, from the hinder end of which the allantois grows (al). In Fig. 141 the allantois is larger; the yelk-sac (ds) smaller. In Fig. 142 the embryo shows the gilt-clefts and the outline of the two legs ; the chorion has formed branching villi (tufts.) In all four nguYes e — embryo, a outer germinal layer, m middle germinal layer, «" inner germinal layer, am amnion. (ks head-sheath, ss tail-sheath), ah amniotic cavity, as amniotic sheath of the umbilical cord, kh embryonic vesicle, ds yelk-sac (umbilical vesicle), dg vitelline duct, df gut-fibre layer, dd gut-gland layer, al allantois. vl^hh place of heart, d vitelline membrane (ovolcmma or prochorion). d' tufts or villi of same, sh serous membrane (serolemma). sz tufts of same, c h chorion, ch: tufts or villi, st terminal vein, r periccelom or serocoelom (the space, filled with 8uid, between the amnion and chorion). (From Kolliker.) «.* DORSAL BODY AXD I'KXTRAL BODY existence by the conversion of the four flat germinal layers of the embryonic shield into a double tube by folding from Opposite directions ; above, at the back, we have the vertebral canal which encloses Fig. .143. the medullary tube, and below, at the belly, the wall of the body-cavity which contains the alimentary canal (Fig. 137). We will consider the formation o( the dorsal wall first,- and that of the ventral wall after- wards (Figs. 143- 147). In the middle of the dorsal surface of the embryo there is originally, as we already know, the medullary ( in r J tube directly underneath the horn - plate (h J, from the middle part of which it has been devel- oped. Later, how- ever, the prever- tebral platesf ww^ grow over from the right and left between these originally con- nected parts(Figs. 145, 146). The upper and inner edges of the two prevertebral plates push between the horn-plate and medullary tube, force them away from each other, and finally join between them in a seam that corresponds to the middle line of the back. The coales- cence of these two dorsal plates and the closing in the middle of the dorsal wall take place in the same way as the medullary tube, which is henceforth en- closed by the vertebral tube. Thus" is formed the dorsal wall, and the medullary tube takes up a position inside the body. In the same way the provertebral mass grows afterwards round the chorda, and forms the vertebral column. Below this the inner and outer edge of the prover- tebral plate splits on each side into two horizontal plates, o( which the upper pushes between the chorda and medullary tube, and the lower between the chorda and gastric tube. As the plates meet from both sides above and below the chorda, they completely enclose ' it, and so form the tubular, outer chord-sheath, the shaath from Fig. 144. Fibs. 143-146.— Transverse sections of embryos (of chicks). Fig. 143 of the second, Fi^. 144 of the third. Fig-. 145 of the fourth, and Fig. 146 of the fifth day of incubation. Figs. 143-145 from Kbtlikrr, magnified about 100 times ; Fig. 146 from Remak, magnified about twenty tirncs^ h horn-plate, mr medullary tube, ung prerenal duct, un prorcnal vesicles, hf> skin-fibre layer, m=mu=m/> muscle-plate, uiv proverte- bral plate (ich cutaneous rudiment of the body of the vertebra, itb of the arch of the vertebra, wq the rib or transverse continuation), uwk prevertebral cavity, ch axial rod or chorda, sh chorda-sheath, bh ventral wall, g hind and v fore root of the spinal nerves, a—af—am amniotic fold, / body-cavity or cceloma, df gut-fibre layer, ao primitive aortas, aa secondary aorta, i>c cardinal veins, d—dd gut-gland layer, dr gastric groove, [n Fig. 143 the larger part of the right half, in Fig. 144 the larger part of the left half, of the section is omitted. Of the yelk-sac or remainder of the embryonic vesicle only a small piece of the wall is indicated below. which the vertebral column is formed (perichorda, Fig. 137 C, s ; Figs. 145 tnuh, 146). We find in the construction of the ventral wall precisely the same processes DORSAL BODY AND VENTRAL BODY 137 as in the formation of the dors.il wall (Fig. 137 /?, Fig. ,44 hp, Rg. 146 bh). It is formed on 1 lu- tl.n embryonic shield of the BmniotOI from the upper plates oi the yelk-sac (Fig. 105). 'The external navel in the skin is the definitive point of thei losing of the ventral wall ; this is visible in the developed body as a small depression. •"■ '4.v parietal zone. The right and left parietal plates bend downwards towards each other, and grow round the gut in the same way as the gut itself closes. The outer part of the lateral plates forms the \entral wall or the lower wall of the body, the two lateral plates bending eonsiderably on the inner side of the amniotic fold, and growing towards each other from right and left. While the ali- mentary canal is closing, the body- wall also closes on all sides. Hence the ventral wall, which encloses the whole ventral cavity below, consists o\' two parts, two lateral plates that bend towards each other. These approach each other all along, and at last meet at the navel. We ought, therefore, really to distinguish two .,a\els, an inner and an outer one. 1 he internal or intestinal navel is the dennitive point of the closing of the gut wall, which puts an end to the open communication between the ventral cavity and the cavity of the Fig. 146. 138 DORS A/. BODY A.XD lEXTRAL BODY With the formation o( (ho intern. il n tvel and the closing of the alimentary canal is connected the formation oi two cavities, which we call the capital and the pelvic sections o\ the visceral cavity. As the embryonic shield lies flat on the wall of the embryonic vesicle al lirst, and only gradually separates from it, its fore and hind ends are independent in the begin- ning ; on the Other hand, the middle part of the ventral surface is connected with the yelk-sac In- means oi the vitelline or umbilical duct (Fig. 147 m). This leads to a notable curving of the dorsal surface ; the head-end bends downwards towards the breast and the tail-end towards the A-. a result of these processes the embryo attains a shape that may be com- pared to a wooden shoe, or, better still, to .in overturned canoe. Imagine a canoe or boat with both ends rounded and a small covering before and behind ; if this canoe is turned upside down, so that the curved keel is uppermost, we have a fair picture of the canoe-shaped einbryo (Fig. 147). The upturned convex keel corresponds to the middle line of the back ; the small chamber underneath the fore-deck represents the capital cavity, and the small chamber under the rear- deck the pelvic chamber of the gut (cf. Fig. 140). Fig. 147.— Median longitudinal section of the embryo of a chick (fifth day of incubation), seen from the right side (head to the right, tail to the left). Dorsal body dark, with convex- outline. >f tlii alimentary canal. If Are were to go forward from there into the head-par) of the embryo, we should reach the capital cavity of the gut, the fore-end of which is closed up. The reader will ask: "'Where are the mouth and the anus?" These arc not at first present in the embryo. The whole o( the primitive gut-cavity is complete!) closed, and is merely connected in the middle by the vitelline duct with the equally closed cavitv of the embryonic vesicle (Fig. 140). The two later apertures of the alimen- tary canal the anus and the mouth — are secondary constructions, formed from the outer skin. In the horn-plate, at the spot where the mouth is found subsequently, a pit-like depression is formed, and this grows deeper and deeper, pushing towards the blind fore- end of the capital cavity; this is the mouth-pit. In the same way, at the spot in the outer skin where the anus is afterwards situated a pit -shaped depression appears, grows deeper and deeper, and approaches the blind hind-end of the pelvic cavity ; this is the anus-pit. In the end these pits touch with their deepest and innermost points the two blind ends of the primi- tive alimentary canal, so that they are now only separated from them by thin membranous partitions. This membrane firfafly disappears, and henceforth the alimentary canal opens in front at the mouth and in the rear by the anus (Figs. 141, 147). Hence at first, if we penetrate into these pits from without, we find a partition cutting them off from the cavity o\ the alimentary canal, which gradually disappears. The formation'of mouth and anus is secondary in all the vertebrates. During the important processes which lead to the formation of the navel, and of the intestinal wall and ventral wall, we find a number of other interesting' changes taking place in the embryonic shield of the amniotes These relate- chiefly to the prorenal ducts and the first blood-vessels. The prorenal (primi- tive kidney) ducts, which at first lie quite flat under the horn-plate or epiderm (Fig. Q3 "ng), soon back towards each other in consequence of special growth movements (Figs. 143 145 ung). They depart more and more from their point of origin, and approach the gut-gland layer. In the end tin v lie deep in the interior, on either side of the mesentery, underneath the ihoida (Fig. 145 "".<■'• At the same time, the two primitive aortas change their position (cf. Figs. 138 145 tio) ; they travel inwards underneath the chorda, and there coalesce at last u> form a single Secondary aorta, which is found under Fig. '48.— Longitudinal section of the fore half of a Chick-embryo at the end of the first day of incu- bation (seen from the left side), k head-plates, c h chorda. Above it is the blind fore-end of the ventral tube (m) ; below it the capital 'cavity of the put. d gut-gland layer, df gut-fibre la>cr. h horn plate, hh cavity of the heart, hk heart-capsule, ks head-sheath, kk head- capsule. (From Remak.) the rudimenlHry vertebral column (Fig. 145 ao). The cardinal veins, the first venous blood-vessels, also back towards each other, and eventually unite imme- diately above the rudimentary kidneys (Figs. 145 vc, 152 cav). In the same spot, at the inner side of the fore-kidneys, we soon see the first trace of the sexual organs. The most important part of this apparatus (apart from all its appendages) is the ovary in the female and the testicle 140 DORSAL BODY AND VEXTRAL BODY Mesoderm Bend of skull — Yelk-sac Head-gut (with gill-clelts) Primitive lungs Umbilical cord Terminal gut "" Rudimentary kidnevs - - Mesentery , _ Primitive kidneys ■ Allantoic duct Rectum Fig. 149 — Longitudinal section of a human embryo ol the fourth week, one-fifth of an inch long. magnified fifteen times. (From KollmannA Fig. 150. Pig. 150. -Transverse section of a human embryo of fourteen days, mr medullary tube, ch chorda, vu umbilical vein, mi myotome, mp middle plate, ug prorenal duct, Ih .body-cavity, * ectoderm, fih ventral skin, hf skin-fibre layer, df gut-6bre layer. (From Kollmann.) Fro. I5r. — Transverse Section Of a Shark -embryo (or young selachius). mr medullary tube, 1I1 ciiorda. a aorta, d gut, vp principal (or subintestinal) vein, ml myotome, mm muscular mass of the provertebra, mf> middle plate, ug prorenal duct, Ih body-cavity, e ectoderm of the rudimentary extremities, nut mesenchyme cells, 1 point where the myotome and nephrotome separate. (From H. E. Ziegtcr.} Til E ARTICULATION OF THE BODY '-P in the male. Both develop from a small part of the cell-lining of the body-cavity, at the spot where the skin-fibre layer and gut-fibre layer touch. The connection of this embryonic gland with the prorenal ducts, which lie close to it and awUHM most important relatione to it, is only sec ondary. Fie. 152 Transverse section of a duck-embryo with twenty-four primitive segments. (From fiai/our.) From a dorsal lateral joint of the medullar) tube fspcj the spinal ganglia fspgj gram out between it and the horn-plate, ch chorda, ao double aorta, hy put-gland layer, sfi rut-fibre layer, with blood-vessels in I vtion. mm muscle plate, in the dorsal wall of the inyocoel (episomite). Below the cardinal vein ( cav) is the prorenal duct fwd) and a segmental prorenal canal < st ). The skin-fibre layer of the body-wall (so) is continued n the amniotic fold (am ). Between the four secondary germinal layers and the structures formed from them there is formed embryonic connective matter w ith stellate cells and vascular structures (Hert wig's " mesenchym "). Chapter XIV. THE ARTICULATION OF THE BODY' The vertebrate stem, to which our- race belongs as one of the latest and. most advanced outcomes of the natural develop- ment of life, is rightly placed at the head of the animal kingdom. This privilege must be accorded to it, not only because man does in point of fact soar far above all other animals, and has been lifted to < The term "articulation" ls used in this chapter to denote both " segmentation " and " articulation " in the ordinary sense.— Trans. I4-' THE ARTICULATION OF THE BODY the position of '"lord of ctv.U ion ", but li o because the vertebrate organism fai surpluses all the other animal-stems in sue, in complexity o\ structure, and in the advanced character of its functions. From the point of view o( both anatomy and physiology, the vertebrate stem outstrips all the other, or invertebrate, animals. There is only one among the twelve stems o( the animal kingdom thai can in many respects be compared with the vertebrates, and reaches an equal, if not i greater, importance in many points. This is the stem of the articulates, com- posed of three classes : 1, the annelids (earth-worms, leeches, and connate forms); 2, the Crustacea (crabs, etc.) ; 3, the tracheata (spiders, insects, etc.). The stem of the articulates is superior not only to the vertebrates, but to all other animal-stems in variety of forms, number of species, elaborateness of individuals, and general importance in the economy of nature. When we have thus declared the verte- brates and the articulates to be the most important and most advanced of the twelve stems of the animal kingdom, the question arises whether this special posi- tion is accorded to them on the ground of a peculiarity of organisation that is common to the two. The answer is that this is really the case ; it is their segmental or transverse articulation, which we may briefly call metamerism. In all the vertebrates and articulates the developed individual consists of a series of successive members (segments or meta- mera = " parts ") ; in the embryo these are called primitive segments or somites. In each of these segments we have a certain group of organs reproduced in the same arrangement, so that we may regard each segment as an individual unity, or a special " individual " sub- ordinated to the entire personality. The similarity of their segmentation, and the consequent physiological advance in the two stems of the vertebrates and articulates, has led to the assumption of a direct affinity between them, and an attempt to derive the former directly from the latter. The annelids were supposed to be the direct ancestors, not only of the Crustacea and tracheata, but also of the vertebrates. We shall see later (Chapter XX.) that this annelid theory of the vertebrates is entirely wrong, and ignores the most important differences in the prganisation of the two stems. The internal articulation of the vertebrates is just as profoundly different from the external metamerism of the articulates as are their skeletal structure, nervous. system, vascular system, and so on. The articulation has been developed in a totally different way in the two stems. The un- articulated chordula (Figs. 83-86), which we have recognised as one of the chief palingenetic embryonic forms of the ver- tebrate group, and from which we have inferred the existence of a corresponding ancestral form for all the vertebrates and tunicates, is quite unthinkable as the stem-form of the articulates. All articulated animals came originally from unarticulated ones. This phylo- .genetic principle is as firmly established as the ontogenetic fact that every articu- lated animal-form dcvelopes from an unarticulated embryo. But the organisa- tion of the ernbryo is totally different in the two stems. The chordula-embryo of all the vertebrates is characterised by the dorsal medullary tube, the neurenteric canal, which passes at the primitive mouth into the alimentary canal, and the axial chorda between the two. None of the articulates, either annelids or arthro- pods (crustacea and tracheata), show any trace of this type of organisation. More- over, the development of the chief systems of organs proceeds in the opposite way in the two stems. Hence the segmenta- tion must have arisen independently in each. This is not at all surprising ; we find analogous cases in the stalk- articulation of the higher plants and in several groups of other animal stems. The characteristic internal articulation of the vertebrates and its importance in- the organisation of the stem are best seen in the study of the skeleton. Its chief and central part, the cartilaginous or bony vertebral column, affords an obvious instance of vertebrate meta- merism ; it consists of a series of cartila- ginous or bony pieces, which have long been known as vettebrce (or spondyli). Each vertebra is directly connected with a special section of the muscular system, the nervous system, the vascular system, etc. Thus most of the "animal organs" take part in this vertebration. But we saw, when we were considering our own vertebrate character (in Chapter XI.), that the same internal articulation is also found in the lowest primitive vertebrates, the acrania, although here the whole skeleton consists merely of the simple chorda, and is not at all articulated. THE ARTICULATION OF THE HODV •43 Hence the articulation does not proceed primarily from the skeleton, but from the muscular system, and is clearly deter- mined by the more advanced swimming- movements of the primitive chordonia- ancestors. " somites " or primitive segments to these so-called " primitive vertebrae." If the latter name is retained at all, it should only be used of the sclcrotom—i.e., the small part o{ the Somites from which the later vertebra does actually develop. Fig. 154. Fig. 155. Figs. 153-155 —Sole-shaped embryonic disk of the chick, in three successive stages of development, looked at from the dorsal turface, magnified about twenty times, somewhat diagrammatic. Fig. 153 with sic pairs of somites. Brain a simple vesicle < hb / Medullary furrow still wide open from x ; greatly widened at $. mp medullary plates, sp lateral plates, y limit of gullet-cavity ( sh) and fore-gut (vdj. Fig. 154 with ten pairs df- somites. Brain divided into three vesicles : v fore-brain, m middle-brain, h hind-brain, c heart, av vitelline-veins. Medullary furrow still wide open behind (z), tnp medullary plates. Fig. 155 with sixteen pairs of so"mites. Brain divided into five vesicles : v fore-brain, z intermediate-brain, m middle-brain, h hind-brain, n after-brain, a optic vesicles, g auditory vesicles, c heart, dv vitelline veins, nip medullary plate, MW primitive •vertebra. It is, therefore, wrong to describe the first rudimentary segments in the verte- brate embryo as primitive vertebrae or proloveitcbra; ; the fact that they have been so railed for some time has led to much error and misunderstanding. Hence we shall give the name of Articulation begins in all vertebrates at a very early embryonic stage, and this indicates tne considerable phylogenetic age of the process. When the chordula (Figs. 83-86) has completed its charac- teristic composition, often even a little earlier, we find in the amniotes, in the '44 THE ARTICULATION OF THE BODY middle of the solo-shaped emhryonic shield, several pairs of dark square spots, symmetrically distributed on both sides ol the chorda (Figs. 131-135). Transverse sections . (Fig. 9j m*) .show that they Fig. 1 .56. -Embryo of the amphioxus, sixteen Hours. Old, scon trom the back. (From /fatschek.) d primitive gut, u primitive- mouth, f> polar cells of the mesoderm, c ccelom-pouches, ,„ their first segment. « medullary tube, • entoderm, e ectoderm, i first segment-/old. belong to the stem-zone (episoma) of the mesoderm, and are separated from the parietal zone (hyposoma) by the lateral folds ; in section they are still quadrangular, almost square, so that they look something like dice. These pairs of "cubes" of the mesoderm are the first traces of the primitive segments or somites, the so-called " protovertebrse " (Figs '53-155 «*')• .. Among the mammals the em- bryos of the marsupials have three pairs of somites (Fig. 131) .after sixty hours, and eight pairs after seventy-two hours (Fig. 135). They develop more slowly in the embryo of the rabbit; this has three somites on the eighth day (Fig. 132), and eight somites a day later (Fig. 134). In the incubated hen's egg the first somites make their appearance thirty hours after in- cubation begins (Fig. 153). At the end of the second day the number has risen to sixteen or eighteen (Fig. 155). The articulation of the stem-zone, to which the somites owe their origin, thus proceeds briskly from front to rear, new transverse constrictions of the " proto- vertebral plates" forming continuously and successively. The first segment, which is almost half-way down in the embryonic shield of the amnlote, is the foremost of all ; from (his first somite is formed the fust cervical vertebra with its muscles and skeletal parts. It follows from this, firstly, that the multiplication of the primitive segments proceeds back- wards from the front, with a constant lengthening of the hinder end of the body ; and, secondly, that at the begin- ning of segmentation nearly the whole of the anterior half of the sole-shaped embryonic shield of the amniotc belongs to the later head, while the whole of the rest of the body is formed from its hinder Ijalf. We are reminded that in the amphioxus (and in our hypothetic primi- tive vertebrate, Figs. 98-102) nearly the whole of the fore half corresponds to the head, and the hind half to the trunk. The number of the metamera, and of the embryonic somites or primitive segments from which they develop, varies considerably in the vertebrates, according as the hind part of the body is short oris lengthened by a tail. In 'the developed man the trunk (including the rudimentary tad) consists of thirty-three metamera, the solid centre of which is formed by that number of vertebrae in the vertebral column .(seven cervical, twelve dorsal, five lumbar, Fig. is7— Embryo of the amphioxus, twenty hours old With five somites (Right view f for le'ft view see Fig ?Jft (From Hatschek.) Vlore end. //hind end. ak. mM; ,/out7r. middle and .nr *r germinal layers; dh alimentary ca.al. « neural tube, en canal.s neurentericus. usk ccelom-pouches (or pr.mitrve-scgment cavities). tlSi first (and foremost) primitive segment, r five sacral, and four caudal). To these we must add at least nine head-vertebrae, which originally (in all the craniota) con- stitute the skull. Thus the total number of the primitive segments of the human Till: ARTIL ( V..I 7'IOA OF THE BODY '45 body is raised to at least forty-two , it would reach forty-five to forty-eight if (according to recent investigations) the number of the original segments of the skull is put at twelve to fifteen. In tin- tailless or anthropoid apes the number o\ metamera is much the same as in man, only differing byoneortwo ; but it is much r in the long-tailed apes and mosl of the other mammals. In long serpents and fishes it reaches several hundred (sometimes n*j). modified embryonic ; I of the craniota. The articulation of the amphi- oxus begins at an early stage earlier tii. in in the cramotes. The t .» ■ coslom- pouches have hardly grown out of the primitive gut (Fig. 156*) when the blind fore pan of it (farthest awaj from the primitive mouth, u) begins to se.oarate by a transverse fold (sj : this is the first primitive segment. Immediatel) after- wards the hind part of the ccelom-pouches begins to divide into a series ol piec< Fig. 158. Figs. 158-160.— Embryo of the amphioxus, twenty-four hours old, with eight somites. (From Halschek.) Fijjs. 158 and 159 lateral view (from left). Fig. 160 seen from back. In Fig. 158 only the outlines ol the eight primitive segments are indicated, in Fig. [59 their cavities and muscular nails. Vfart end. //hind end. d gut, du under and dd upper wall of the gut, >;>■ canalis neurenterieus, nv ventral, >id dorsal wall of the neural tube, np neuroporus, dv fore pouch of the gut, eh chorda, »if mesodermic fold, pm polar cells of the mesoderm (ms), e ectoderm.' In order to understand properly the real nature and origin of articulation in the human body and that of the higher vertebrates, it is necessary to compare it with that of the lower vertebrates, and bear in mind always the genetic connec- tion of all the members of the stem. In this the simple development of the invalu- able amphioxus once more furnishes the key to the complex and cenogenetically new transverse tolds (F'i£. 157). The foremost of these primitive segments (its 1) is the first and oldest ; in Figs. 124 and 157 there are already five formed. They separate so rapidly, one behind the other, that eight pairs are formed within twenty-four hours of the beginning of development, and seventeen pairs twenty- four hours later. The number increases as the embryo grows and extends u6 THE ARTICULATION OF THE liODY backwards, and new cells are formed con- stantly (at the primitive mouth) from the two primitive mssodermic celIs(Figs. 159- l6o) This typical articulation of the two coslom-sacs begins verj earl) in the lance- let, before the) are yet severed from the primitive gut, so that at first each segment-cavity (us) still communicates by a narrow opening with the gut, like m\ intestinal gland. But this opening soon closes by complete severance, pro- ceeding regular!) backwards. The closed uppermost section, to the proncphridia or primitive-kidney canals, and from the lower to the segmental rudiments o\' the iezual glands or gonads. The partitions ol the muscular dorsal pieces (myotomes) remain, and determine the permanent articulation o( the vertebrate organism. But the partitions ol the large ventral pieces (gOttOtomes) become thinner, and afterwards disappear in pan, so that their cavities run together to form the metaccel, Ol the simple permanent bodv-cavity. The articulation proceeds in sub- mk Sk - 1), Fie. i(. Fios ,61 and .62 -Transverse section of shark -embryos (through the region of the kidneys) (From W,jh,- and HeHw.g.) In Fig xb> the dorsal segment-cavities*; arc alr< ad separatee from the hodv-iavity (7*7 but £eyare connected a little earlier (Fig ,.„>. nr neural tube, ch chorda! « 1, subchorial string ^aorta. Ltnr'ir , iPt '/ "T musc,e-p'-1,c- fP eut.s^plate. » connection ol latter (growth-zone), vn primitive kidneys i£f ■ 1 am "*.PPOre"al. «nals. •« po.nt where the> arc cut off. rr prerenal funnel, mi middle germ-layer (m*, parietal. w*2 vibceral), t* inner germ-layer (gut-gland layer). segments then extend more, so that their upper half grows upwards like a fold between the ectoderm (ok) and neural tube ( >/ ), and the lower half between the ectoderm and alimentary canal {ch; Fig. S2 d, left half of the figure). Afterwards the two halves com- pletely separate, a lateral longitudinal fold cutting between them [mk, ri^ht half of Fi^. 82). The dorsal segments ( sd j provide the must les o\ the trunk the whole length of tin- bodyOsa): this cavity after- wards disappear, On the other hand, the ventral parts give rise, from their stantially the same way in the other vertebrates, the craniota, starting from the CCelom-pOUcheS. But whereas in the former case there is first a transverse division of the ccelom-sacs (by vertical folds) and then the dorso-ventral division, the procedure is reversed in the craniota; in their c.isc each ol the long ccelom- pom lies first divides into a dorsal (primi- tive segment plates) and a ventral (lateral plates) section In a lateral longitudinal fold. Only the former an- then broken up into primitive segment!: In the subsequent vertical folds, while the latter (segmented THE ARTICULATION OF THE BODY M7 for a time in the amphioxus) remain undivided, and, by the divergence of their parietal and visceral plates, forma bodv- cavity thai is unified from the first. In this ease, again, it is clear that we must regard the features o\ the younger craniota as cenogenetically modified processes that can be traced palingeneticauyto the older acrania. We have an interesting intermediate Stage between the acrania and the fishes in these and many other respects in the cyclostoma (the hag and the lamprey, cf. Chapter XXI. ). Among the fishes the Belachii, or primi- tive fishes, yield the most important infor- mation on these and many other phylo- genetic questions (Figs* ''>'• &*)• *he 'careful studies of Ruckeit, Van Wijhe, H. E. Ziegler, and others, have given us most valuable results. The products of Fig. iCy Frontal (or horizontal-longitudinal) section of a triton-embryo with ttunee pairs of primitive segments, eft chorda, us primitive segments, ush their Cavity, ok horn plate. the middle germinal layer are partly clear in these- cases at the period when the dorsal primitive segment cavities (or mvoctvls, //) are still connected with the ventral hodv-tavity (///.• Fig. 161). In Fig. i<>-\a somewhat older embryo, these cavities are separated. The outer or lateral wall of the dorsal segment yields the cutis-plate (), the foundation of the connective corium. From its inner or median wall are developed the muscle- plate {mp, the rudiment of the trunk- muscles) and the skeletal plate, the forma- tive matter o\ the \ crtebral.column (sk). In the amphibia, also, especially the water - salamander (Triton), w> observe very clearly the articulation of the ccelom-pouches and the rise of the primitive segments from their dorsal hall (tl. Fig- 91, A, B, C). A horizontal longitudinal section of the salamander- embryo (Fig. 163) shows very clearly the series of pair-, oi these vesicular dorsal segments, which have been cut off on each side from the ventral side-plates, and lie to the righl and left of the chorda. The metamerism of" the amniotes a.;iecs in all essential points with that o! the I'll.. !'•) Fig. 165. Fig. 164.— The third cervical vertebra (human). lie. 165 —The sixth dorsal vertebra (human). three lower classes of vertebrates we have considered ; but it varies considerably in detail, in consequence of cenogenetic disturbances that are due in thefirst place- dike the degeneration of the ccelom- pouches) to the large development of the food-yelk. As the pressure of this seems to force the two middle layers together from the start, and as the solid structure oi the* mesoderm apparently belies the original hollow character of the sacs, the two sections of the mesoderm, which are at that time divided by the lateral fold — the dorsal segment-plates and ventral side- plates— have the appearance at fust oi solid layers of cells (Figs. 94-97)- And when the articulation of the somites begins in the sole-shaped embryonic shield, and a couple of protovertebne are developed in succession, constantly in- creasing in number towards the rear, Fig. 166 —The second lumbar vertebra (human). these cube-shaped somites (formerly called protovertebrae, or primitive vertebrae) have the appearance of solid dice, made up of mesodermic cells (Fig. 93). Neverthe- less, there is for a time a ventral cavity, or provertebral cavity, even in these solid i48 THE ARTICULATION OF THE BODY " protovcrtebrae" (Fig. 143 uivh). This vesicular condition of the provertebra is of the greatest phylogenetic interest ; wo must, According to the codlom theory, regard it as an hereditary reproduction of the hollow dorsal Somites of the am* phioxus (Figs. 1511 100) and the lower vertebrates (Figs. 161-163). This rudi- ment.iry "proverlehr.il cavity" has no physiological significance whatever in the amniote-embryo ; it soon disappears, being filled up with cells of the muscular plate, The innermost median- part of the divides into two plates, which grow round the chorda, and thus form the foundation of the body of the vertebra (■wk). The upper plate presses between the chorda and the medullary tube, the lower between the chorda and the alimentary canal (Fig. 137 C). As the plates of two opposite prevertebral pieces unite from the right and left, a circular sheath is formed round this part of the chorda. From this developes the body of a vertebra — that is to say, the massive lower or ventral half of the bony ring, which is called the " vertebra" Frontal nasal process Eye Mouth inlet Arch of tongue First branchial arc Middle brain Fore brain Olfactory pit Arch of upper jaw Arch of 16\ver jaw Spout-hole (first gill-cleft) Gill* Last branchial arch -•■• Fig. 167— Head of a shark embryo < Prist 'turns ("rom Parker.) Seen from the ventral side. primitive segment plates, which lies I immediately on the chorda (Fig. 145 ch) and the medullary tube ( >nj, forms the vertebral column in all the higher verte- brates (it is wauling in the lowest) ; hence it may be called the skeleton plate. In each of the provertebra it is called the " sclerotome "(in opposition to the out- lying muscular plate, the "myotome"). From the phylogenetic point of view the myotomes are much older than the sclerotomes. The lower or ventral part of each sclerotome (the inner and lower edge of the cube-shaped provertebra) ), one-third of an inch long, magnified tWCflt) times. proper and surrounds the medullary tube (Figs. 164-166). The upper or dorsal half of this bony ring, the vertebral arch (Fig. 145 wb), arises in just the same way from the upper part of the skeletal plate, and therefore from the innei and upper edge oi the cube-shaped primitive verte- bra. As the upper edges of two opposing somites grow together over the medullary tube from right and left, the vertebral arch becomes closed The whole of the secondary vertebra, which is thus formed from the union of the skeletal plates oi t\\ o prevertebral pieces THE ARTICULATION OF THE BODY MO and encloses a part of the chorda in its body, consists at first of a rather soft mass of cells ; this afterwards passes into a firmer, cartilaginous stage, and finall) into a third, permanent, bony Stage. These three stages can generally be dis- tinguished in the greater part oi tin- skeleton, of the higher vertebrates ; al first most parts of the skeleton arc soft, tender, and membranous; they then become cartilaginous in the course ol their development, and finally bony. At the head part oi the embryo in the amniotes there is not generally a cleavage o( the middle germinal layer into pro- vertebraland lateral plates, but the dorsal and ventral somites are blended from the first, and form what are called the " head- plate^ " (big. I48 £). From these are 168 Fig. 169. Figs, i 68 and 169— Head of a chick embryo, ol the third day. Fig. 168 from the front. Fig- 169 from the right, n rudimentary nose, (olfactory pit). / rudi- mentary eye (optic pit. lens-cavity), g rudimentary ear (auditory pit), «.• fore-brain. £-/ c\c-cleft. Of the three pair- of gill-arches the first has passed into a process of the upper jaw (o) and of the lower jaw ( uj. (From Kdlliker. ) formed the skull, the bony case 'oi' the brain, and the muscles and corium of the body. The skull developes in the same era) as the membranous vertebra] column. The right and left halves of the head Cur\e over the cerebral vesicle, enclose the foremost part of the chorda below, and thus finally form a simple, soft, membranous capsule about the brain. This, is afterwards converted into a cartila- ginous primitive skull, such as we find permanently in many of the tishes. Much later this cartilaginous skull becomes the permanent bony skull with its various parts. The bony skull in man and all the other amniotes is more highly differen- tiated and modified than that of the lower vertebrates, the- amphibia and fishes. But as the one has arisen phylogene- tically from the other, we must assume thai in the former no Jess than the latter the skull was originally formed from the sclerotomes oi a number of (at least nine) head-somites. While the .0 tii ulation oi the vertebrate body is always obvious in the episotna or dorsal body, and is clearly expressed in the segmentation oi the mu plates and vertebrae, it is more lateni the hypotonia or ventral body. Neverthe- less, the hyposomites oi the vegetal half ol the body are not less important than the episOmiteS oi the animal hall. The segmentation in the ventral cavity affect: the following principal systems oi organs : 1, the gonads ov sex-glands (gonoiomes); 2, the nepniidia or kidneys (nephro- Fig. 170— Head of a dog embryo, seen from the Iront. a the two lateral halves ol the Foremost e( rebral vesicle, b rudimentary eye, c middle cerebral vei dt- first pair ol gill-arches(f upper-jav wer- jaw process), f, / , / second, third, and fourth pa.rs of gill-arches, g hi k heart {g right, h left auricle: 1 left, k right ventricle), / origin of the aorta with three pairs of arches, which go to the gilt-arches. (From Bhc/ioff.) tomes); and 3, the head-gut with its gill-clefts (branchiotomi The metamerism of the hyposoma is less 'conspicuous because in all the craniotes tlu cavities of the ventral seg- ments, in the walls of which the sexual products are developed, have long since coalesced, and formed a single large body- cavity, owing 10 the disappearance of the partition Tin- cenogenetic process is so old that the cavity seems to be un seg- mented Irom the first in all the cranioies, and the rudiment of the gonads alAJ is almost always unsegmented. It is the more interesting to learn that, according to the important discovery of Ruckert, this sexual structure is at first segmcnlal even in the actual selachii, and the several i5o THE . 1 A" //<• ULA rn )N OF THE FODV gonotomes only blend into a simple sexual cavities, formed from the hyposomites gland oi\ either side secondarily. Amphioxus, the solo surviving repre- sentative of the acrania, once more yields u> most interesting information ; in this case the sexual glands remain segmented Rudimoni ol cai (labyrinthic vesicles) of the trunk. The gonads are the most importanl segmental organs of the hyposoma, in the sense that they are phylogenctically the oldest. We and sexual glands (as pouch- Pncumoirastric nerve X. Vagus / Terminal nerve XI Accessor/us Point df develop- mini of arm (or fore-leg) True spinal nerve Point of develop- ment ol the hind- Twenticth spinal nerve Fig. 171.— Human embryo Of the fourth week (twenty-six day's old), one-fourth of an inch in length magnified twenty times. (From Moll.) The rudiments ol the cerebral nerves and the roots of the spinal nereis are especially marked. Underneath the four gill-arches (left side) is the heart (with auricle, / ', and ventricle, K ), under this again the liver ( Lj. throughout life. The sexually mature lancelet has, on the right and left of the gut, a series of metamerous sacs, which are filled with ova in the female and sperm in the male. These segmental gonads arc originally nothing else than the real gonotomes, separate body- like appendages of the gastro-canal System) in most of the lower animals, even in the medusa , et< ., which have no kidneys. The latter appear first (.h a pair of excretory tubes) in the platodes (turhcllaria), and have probably been inherited from these by the articulates THE ARTICULATION ()/■' THE /!()/)) '5* (annelids) on tho one hand and the unarticulated prochordonia on the othcY, and from these passed to tin- articulated vertebrates. The oldesl form of the kidney system in this stem are the seg- mental pronephridia or prorenal canals, in the same arrangement as Boveri found them in the amphioxus. They are small canals that lie in the frontal plane, o\) each side of the chorda, between the episoma and hyposoma (Fig. 102a); their internal funnel-shaped opening leads 1 n t i> the various body-cavities, their outer opening is the lateral furrow ol the epidermis. Originally they must have h.ul a double function, the carrying awaj of the urine from the episomites and the release of the sexual i ills from the hypo- somites The recenl investigations of Rtidkerl and Van Wijhe ow the mesodermic seg- ments of the trunk and the excretOT) system of the selachii show th.it these "primitive fishes" we closely related to the amphioxus in this further respect. Tin transverse section of the shark-embryo in Fig. 161 shows this \ei\ clearly. In other higher vertebrates, also, the kidneys develop (though very differently formed later oi\) from similar structures, which have been secondarily derived from thi segmental pronephridia of (he acrania. The parts of the mesoderm at which the first traces of them are found are usually called the middle or mesenteric plates. As the firs! traces of the gonads make their appearance in the lining of these middle plates nearer inward (or the middle) from the inner funnels of the nephro-canals, it is better to count this part of the mesoderm with the hyposoma. The chief and oldest organ of the verte- brate hyposoma, the alimentary canal, is general!} described as an unsegmented organ. But we could just as well say that it is the oldest of all the segmented organs of the vertebrate ; (he double row ol the culom-poiu lies grows out of the dorsal wall of the gut, on either side of the chorda. In the brief period during which these segmental ccelom-pouchcs are still openly connected with the gut, they look just like a double chain of segmented ■ I glands. But apart from this, we have originally in all vertebrates an important articulation of the fore-gut, that is wanting in the lower gut, the segmentation of the branchial (gill) gut. The gill-clefts, which originally in the older acrania pierced the wall of the fbre- gut, and the gill-arches that separated them, were presumably also segmental, and distributed among the various meta- mera of the chain, like the gonads in the after-gut and the ruphridi.i. In the amphioxus, too, they are still segmentally formed. Probabl) there was a division oi labour of the hypubomites in th« older (and long extinct] acrania, in such wise that those of the lore-gul took o\ei the function of breathing and those of the after-gul that of it proaiM tion. The- former developed into gill-pouches, the latter into Frc. 172.— Transverse section of the shoulder and fore-limb (wing) o( a chic k-cmbryo of the fourth day, magnified aboul twentj times. Beside the medul- lary tube «i can see on ca« li side three clear streaks in the dark dorsal wall, which advance into the rudimen- tary fore-limb or wing (f). The uppermost ol them is the muscular plate; the middle is the hind and the lowest the fore root of a spinal nerve. 1'iuler the chorda 'in the middle is tin- single aorta, at each side ol it a cardinal vein, and lulow these the primitive kidneys. The gut Is atmOSl closed. The ventral wall advances into trie amnion, which encloses the embryo. (From Rcmak.) sex - pouches. There may have been primitive kidneys in both. Though the gills have lost their function in the higher animals, certain parts of them have been generally maintained in the embryo by a tenacious heredity. At a very early stage we notice hi the embryo of man and the other amniotes, at each side of the head, the remarkable and important structures which we call the gill-arches and gill-clefts (Figs. 167-ifo /). They belong to the characteristic and inalien- able -organs of the amniotc - embryo, and are found always in the same '5-' THE ARTICULATION of- Till. BODY spot and with the same arrangement and Structure. Fheie are formed to the lii^ht and left in the lateral wall of the fore-gut cavity, in its roremosi part, first a pan and then several pairs of sac- shaped inlets, din pierce the whole thick- ness of the lateral wall of the head. The} are thus converted into clefts, through which one can penetrate freely from with- out into the gullet. The wall thickens between these bran :hial folds, and changes into an* arch-like 01 sickle-shaped piece the gill, or gullet-arch. In this ihe muscles aw\ skeletal parts of the branchial Fig. 175.— Transverse section of the pelvic region and hind logs of a chick-embryo of the fourth day, magnified about fort\ times, h horn-plate, w medullary tube, n canal of the tube. u primitive kidney-., .t chorda, e hind legs, b allantoic canal in the ventral wall. / aorta, 0 cardinal \eins. a gut, d gut-gland layer, /"gut-fibre layer, g embryonic epithelium, r dorsal muscles, c body- cavity or cceloma. _(From Wuldcyer.) gut separate ; a blood-vessel arch rises afterwards on their inner side (Fig. 98 ka). The number of the branchial arches and the clefts that alternate- with them is four or five on each side in the higher verte- brates (Fig. 170 <<',/,/'./"). In some of the fishes (selachii) and in the cyclostoma we find six or. seven of them permanently. These remarkable structures had origi- nally the function of respiratory organs — gills. In the fishes the water that serves for breathing, and is taken in at the mouth, still always passes out by the branchial clefts a< the sides of the gullet. In the higher vertebrates they afterwards dis- appear. The branchial arches are con- verted partl\ into the jaws, partly into the bones of the tongue and the ear. From the first gill-cleft is formed the tympanic cavity of the ear. There are few parts o\ the vertebrate organism that, like the outer covering or Integument of the both, are not subject to metamerism. Theoutei s\lit\ ( epidermis ) is unsegmented from the first, and pro- ceeds from the continuous horny plate. Moreover, the underlying cutis is also not met amerous, although it developes from the segmental structure of the cutis-plaU-s (I"'igs. 101, 162 Cp). The vertebrates are strikingly and profoundly different from the articulates in these respects also. Further, most of the verte- brates still have a number of un- articulated organs, which have arisen locally, by adaptation of particular parts of the body to certain special functions. Of this character are the sense-organs in the episoma, and the limbs, the heart, the spleen, and the large visceral glands — lungs, liver, pancreas, etc. — in the hyposoma. The heart is originally only a local spindle-shaped enlargement of the large ventral blood-vessel or principal vein, at the point where the subintestinal passes into the branchial artery; at the limit of the head and trunk (Fig . 170, 171). The three higher SI organs — nose, eye, and ear — were originally developed in the same form in all the craniotes, as three pairs of small depressions in the skin at the side of the head. The organ of smell, the nose, has the appearance of a pair of small pits above the mouth- aperture, in front of the head (Fig. 169 ti). The organ of sight, the eye, is found at the side of the head, also in the shape of a depression (Figs. 16a /, 170, £), to which corresponds a large out- growth of the foremost cerebral vesicle on each side. Farther behind, at each side of the head, there is a third depres- sion, the first trace of the organ of hearing (Fig. 169^). As yet we can see nothing of the later elaborate structure of these organs, nor of the characteristic build ol the face. When the human embryo has reached THE ARTlCt LATtON OF THE Honv «5t this stage ol development, it can still scarcely be distinguished from that <>i any other higher vertebrate. All the chief parts ol ihc body are now laid down. tin. Inad with the primitive skull, the-rudiments ol the three higher sensc-oi gansand ihe five cerebral vesicles, and the gill-arches and cleAs, the trunk significance. From it we can gathei the most important phylogcnelu «.<"u lusions. There is still no trace of the limbs. Although head and trunk are separated and all the principal internal orguns are laid down, inert is no indication whatever ol the "extremities" al this stage, they arc formed later on. line again \\c FlC. 174.— Development Of the lizard's legs ( Lacerta agilis), with special relation to their blood-vessels. I. 7. 5. 7, O. J J right fore-leg ; ij. 15 left tore-leg ; 2. 4, 6, #, /ft jj right hind-leg , 14, lb left hind-leg ; SRV lateral veins ot the trunk, H/-imoilical vein, (From F. Hochstetter.) with the spinal cord, the rudiment of the vertebral column, the chain of metamcra, the heart and chief blood-vessels, and the kidneys. At this stage man is a higher vertebrate, but shows no essential morpho- logical difference from the embryos of the mammals, the birds, the reptiles, etc. This is an ontogenetic fact of the utmost have a fact of the utmost interest. It proves that the older vertebrates had no feet, as we find to be the case in the lowest living vertebrates (amphioxus and the cyclostoma). The descendants of these ancient footless vertebrates only acquired extremities — two fore-legs and two hind- legs — at a much later stage of development. "54 THE ARTICCLATIOS OF THE BODY These were at first all alike, though they afterwards vary considerably in structure becoming tins (of breast and belly) in the fishes, wings and legs in the birds, fore and hind legs in the creeping which represent at first simple roundish knobs o\- plates. Gradually each of these plates becomes B large projection, in which we can distinguish a small inner part and a broader outer part. The latter Fig 175.— Human embryo, five weeks old, Half an Inch loner, seen from (lie right, magnified ten Hate*. (From RhsmtI Bardem and Harmon Lewis.) In the undirected head we sec the eye, mouth, and ear. In ibe trunk the skin and part of the muscles have been removed, so that the cartilaginous vertebral column is free; tin. dor;,;d root of a spinal nerve goes OUt from each vertebra (towards the skin of the back), in the middle of the lower half of the figuri part o' the ribs and intercostal muscles are visible. The skin and muscles have also been r.-moved Ironi the right limbs ; the internal rudiments of the five fingers of the hand, and five toes of the foot, are clearly seen within the fin-shaped plate, and also the strong network of nerves that goes from the spinal cord to thfi extremities. The tail projects under the loot, and to the right of it is the first part of the umbilical cord. Animals, arms and legs in the apes and man. All these parts develop from the same simple original structure, which forms secondarily from the trunk-wall (Figs. 172, 173). They have always the appearance of two pairs of small buds, is the rudiment of the foot or hand, the former that of the leg or arm. The similarity of the original rudiment of the limbs in dilTcrcnt groups of vertebrates is very striking. How the five fingers or toes with their THE ARTICULATION OF THE />'"/>)' '55 blood - vessels gradually differentiate within the simple fm-liUe structure of the limbs can be seen in the instance ot' the li/.irJ in Fig. 174 They are formed in just the same waj in man: in the human embryo of the weeks the the fingers cm clearly he distinguished within the fin- plate (Fig. 175). The careful study and comparison of Fkj. 170. P» .77. human cmbiyos with those of other vertebrates at this stage of development is very instructive, and reveals more mysteries to the impartial student than all the religions in the world put together. For instance, if we compare attentively he three successive stages of develop- ment thai arc represented, in twenty different amniotcs we find a remarkable likcrw •. When we sec that as a fact twenty different amniotes of such diver- gent characters develop from the same embryonic form, we can easily under- stand that they may all descend from a common ancestor. In the fust stage of development, in which the head with the the eerchral vesicles is alread) clearly indicated, hut there are no limbs, tin- embryos of all the verlehrates, from the fish toman, are only incidentally Or not at all different from each other. In the second stage, which show-, the limbs, we begin to see dif- ferences between the embryos of the lower and higher vertebrates; but the human Frc. 178. Fics. 1 76-8. -Embryos of the bat ( Vtsf*rtilu> murinus) at three different stages. (From Oscar Schnltze.) Fig. 176: Rudimentary limbs (v fore-leg, h hind-leg). / lenticular depression, r olfactory pit, ok upper jaw, 14! lower jaw, >fr2, jfr3, kt first, second, and third gill-arches, a amnion, n umbilical vessel, d yelk-sac. Fig. 177: Rudiment of flyiny membrane membranous fold between fore and hind leg1, n um- bilical vessel, o car-opening, f flying membrane. Fig. 178: The flying membrane developed and stretched across the fingers of the hands, which cover the face. embryo is still hardly distinguishable from that of jhe higher mammals. In the third stage, in which the gill-arches have disappeared and the face is formed, the differences become more pronounced. These arc facts of a significance that cannot be exaggerated.1 ' Because they show how the most diverse structures may be developed from a common form. As we actually M-i 1 )i >' s in the case of the embryos, we have a right to assume it in tli.it of the stem-forms Nevertheless, this resemblance, however neat, is never a real identity. Even the embryos of the different individuals of one spcties are usually not really identical. If the reader can consult the complete edition of this work at a library, he will find six plates illustrating these twenty embryos. »5<> FQSTAL MEMBRANES .JAY) CIRCULATION If there is an intimate causal connection between the processes of embryology and stem-history, as we must assume in virtue of the laws of heredity, several important fihylogenetic conclusions follow at once rom these ontogenetic facts. The pro- found and remarkable similarity in the embryonic development of man and the other vertebrates can only be explained when wc admit their descent from a common ancestor. As a fact, this common descent is now accepted by all competent scientists; they have sub- stituted the natural evolution for the supernatural creation of organisms. Chapter XV. FCETAL MEMBRANES AND CIRCULATION AMONG the many interesting phenomena that we have encountered in the course of human embryology, there is an especial the other viviparous mammals. As a fact, all the embryonic peculiarities that distinguish the mammals from other Fig. 179- Human embryos from the second to the fifteenth week, natural size, seen JYom the left. the curved back turned towards the right, (Mostly from Hiker.) II. of fourteen du>v III. of three weeks IV. of four weeks. V. of five weeks. VI. of six weeks. VI). of seven weeks. VIII of eight weeks XII. of twelve weeks. XV. of fifteen weeks. importance in the fact that the develop- ment of the human body follows from the beginning just the same lines as that of animals are found also in man , even the ovum with its distinctive membrane (zona prilucida, Fig. 1^) shows the same typical FU-ITAL M EM n RAX IS AND CIRCULATION »57 structure in all mammals (apart from the older oviparous monotremes). It lias long since been deduced from the Structure of the developed man thai his natural place in the animal kingdom is among the mammals: Linne* (1715 placed him In this class with the apes, in one and the same order (primates J y in his Systema A^ on this remarkable agreement in the chief embryonic features in ntUUI and the other animals. We shall make use of it later on for our monophyletic theory of descent — the hypothesis of a common descent of man and all the mela/oa from the gasl I The first rudiments of the principal parts *« *■■ Fig. 180.— Very young human embryo of the fourth week. Ode-fourth of an inch long I taken from the nromb of a suicide eight hours after death). (From RabL) n nasal pits, a eye, u lower jaw. z arch of h void bone. k:, and td third and fourth gill-arch, h heart, s primitive segments, vg fore-limb (arm). A^hind-limh (leg), between the two the ventral pedicle. of the body, especially the oldest organ, the alimentary canal, are the same every- where ; they have always the same extremely simple form. All the pecu- liarities that distinguish the various groups of animals from each other only appear gradually in the course of embryonic development ; and the close! the relation of the various groups, the later they are found. We may formulate this phenomenon in a definite law, which may in a sense be regarded as an appendix to our biogenetic law. This is the law of the ontogenetic connection of related animal forms. It runs : The closer the iS8 FCETAL MEMBRA VES AND CIRCULATION relation of two fully-developed animals in respect of their whole bodily structure, and the nearei the) are connected in the classification of the animal kingdom, the longer do their embryonic tonus retain their identity, and the longer is it impos- sible (01 onK possible oi\ the ground of subordinate features) to distinguish between theirembryos. This law applies bo all animals whose embryonic develop- ment I-, in the main, an hereditary summar) of (heir ancestral history, or in which the original form of development h.is been faithfully preserved by heredity. When, on the other hand, it has been altered by cenogenesis, or disturbance Fig. iSr.— Human embryo of the middle of the fifth week, one-third of an inch long. (From Rabl.) Letters as in Fig. 180, except sk curve of skull, ok upper jaw. kb neck-indentation. of development, we find a limitation of the law, which increases in proportion to the introduction of new features by adap- tation (cf. Chapter I., pp. 4-6). Thus the apparent exceptions to the law can always be traced to cenogenesis. When we apply to man this law of the ontogenetic connection of related forms, and run rapidly over the earliest stages of human development with an eye to it, we notice first of all the structural identity of the ovum in man and the other mammals at the very beginning (Figs. 1, 14). The human ovum possesses all the distinctive features of the ovum of the viviparous mammals, especially the characteristic formation of its membrane I 'ii,i pellncida), which clearly distin- guishes it from the ovum of all other .mini. lis. When the hum. in foetus has attained the age of fourteen days, it forms a round vesi< le (or "embryonic vesicle") about a quarter of an inch in diameter. A thicker part of its border forms a simple iole-shaped embryonk shield one-twelfth of. in inch long (rig. 133). On its dorsal side we find in the middle line the straight medullary furrow, bordered by the two parallel dorsal or medullary swellings. Behind, it p;h^> by the neurenteric canal into the primitive gut or primitive groove. From this the folding of the two civlom- pouches proceeds in the same way as in the other mammals (of. Figs. 96, 97). in the middle of the sole-shaped embryonic shield the first primitive segments immediately begin to make their appearance. At this age -the human embryo cannot be dis- tinguished from that of other mammals, such as the hare or dog. A week later (or after the* twenty-firsi day) the human em- bryo has doubled its length ; it is now about one-fifth of an inch long, and, when seen from the side, shows the characteristic bend of the back, the swelling of the head-end, the first outline of the three higher sense-organs, and the rudiments of the gill- clefts, which pierce the sides of the neck (Fig. 179, III.). The allantois has grown out of the gut behind. The embryo is already entirely enclosed in the amnion, and is only connected in the middle of the belly by the vitelline duct with the embryonic vesicle, which changes into the yelk-sac. There are no extremities or limbs at this stage, no trace of arms or legs. The head-end has been strongly differentiated from the tail-end ; and the first outlines of the cerebral vesicles in front, and the heart below, under the fore-arm, are already more or less clearly seen. There is as yet no real face. Moreover, we seek in vain at this stage a special character that may distinguish the human embryo from that of other mammals. A week later (after the fourth week, on the twenty-eighth to thirtieth day of development) the human embryo has FCETAL !// i;/>7,\l.\7 s i\/> C1RCUL \TION •59 reached a length of about one-third <>t an , inch (Fig. 17Q, IV.). We can now clearly distinguish the head with its various parts ; inside it the five primitive cerebral vesicles (fore-brain, middle-brain, inter- mediate-brain, hind-brain, and after- inain); under the head the gill-arches, which divide the gill-< lefts ; at the sides of the head the rudiments of the eyes, a couple of pits In the outer .skin, with a head bends over t In- trunk, almost at a rifjht nng'c. The latter is still connected iii the mil die of its ventral tide with the embryonic vesicle . but the embryo Ins still furthi I itself from it, so that ii already hangs out as the yelk-sac. The hind part oi the body is also \ery much curved, so that the pointed tail-end is directed toward, the head. The head and face-part are sunk entirely on the Fir. 182— Median longitudinal section of the tail of a human embryo, two-thirds of an inch long. (From ftoss GratiTille Harrison.) Med medullary tube, Ca.fil. tauJal filament, ch chorda, ao caudal artery, I'.c.i. caudal vein, an anus, S.itg sinus urogcuitalis. pair of corresponding simple vesicles growing out of the lateral wall of the fore-brain (Figs. 180, 181 a). Far behind the eyes, over the last gill-arches, we see the vesicular rudiment o\ the auscultory organ. The rudimentary limbs are now clearly outlined — four simple buds of the shape of round plates, a pair of fore (vgj and a pair of hind legs (hg), the former a little larger than the latter. The large still open breast. The bend soon increases so much that the tail almost touches the forehead (Fig. 179, V.; Fig. 181). We may then distinguish three or four special curves on the round dorsal surface — namely, a skull-curve in the region of the second cerebral vesicle, a neck-curve at the beginning of the spinal cord, and a tail-curve at the fore-end. This pro- nounced curve is only shared by man and ino FCETAL i//-:i//.7,> I VES AND CIRCULATION the higher classes of vertebrates (the amniotes); it is tnuch slighter, or not found at all, in the*lower vertebrates. At this age (four weeks) man has .1 con- siderable tail, twice as lorn; as his k-^s. A vertical longitudinal section through ihe middle plane o\ Oiis tail (Fig. 18a) shows thai me hinder end of the spinal marrow extends to the point of the tail, Fig. 103.— Human embryo, four weeks old, opened on the ventral side. Ventral and dorsal walls are cut away, so as to show' the contents of the pectoral and abdominal cavities. All the appendages are also removed (amnion, allantois, yelk-sac), and the middle part of the gut. n eye, j nose, 4 upper jaw. 5 lower jaw, 6 second, 6" third gill-arch, ov heart (0 right, o' left auricle ; v right, v left ventricle), b origin of the aorta,/" liver (u umbilical vein), e gut (with vitelline artery, cut off at a), j' vitelline vein, m primitive kidneys,- t rudimentary sexual glands, r terminal gut (cut off at the mesentery z), n umbilical artery _ « umbilical vein, 9 fore-leg, 9' hind-leg. (From Coste.\ as also does the underlying chorda (ch), the terminal continuation of the vertebral column. Of the latter, the rudiments of the seven coccygeal (or lowest) vertebra; are visible — thirty-two indicates the third ar.d thirty-six the seventh of these. Under the vertebral column we see the hindmost ends of the two large blood-vessels of the tail, the principal artery (aot/a-cauda/is or atleria saeralis media, Ao), and the principal vein (vena caudalis or sarra/is media ). Underneath is the opening of the anus (an ) and the urogenital sinus ( -^"g)- From this anatomic structure of the human tail it is perfectly clear thai Fig. 184. — Human evbryo, five weeks old, opened from the vent al side (as in Fig. 183). Breast and belly-wall and liver arc removed. J outer nasal process, 4 upper jaw, 5 lower jaw, z tongue, V right, '<'' left ventricle of heart, o' left auricle, b origin of aorta, bl , b", b'" first, second, and third aorta-arches, c, {'). (From Allen Thomson.) 1. Not opened, natural si /i- 2. Opened and magnified. Within the outer chorion the tiny curved foetus lies on the large embryonic reside, to the left above, rudiments of caudal vertebrae. They attain a length o\ eight to ten inches and more. Granville Harrison has very care- fully studied one of these cases of " pig- tail," which he removed by operation from a six months' old child in 1901. The tail moved briskly when the child cried or was excited, and was drawn up when at rest. In the opinion of some travellers and anthropologists, the atavistic tail-forma- tion is hereditary in certain isolated tubes (especially in south-eastern Asia and the 1 10 187 I ... 187. Human ovum of ten days. (From Allen Thomson.) Natural use, opened; the small fcetus m the right half, above. Fig. i88.-Human tcetus o( ten days, i..k.n From the preceding ovujn, magnified ten time*, a yak b neck (the medullary groove already dosed), Ik.. I (with open medullars groove), d hind pari (with open medullary groove), e a shred >■! tin .union. Fig. 189.— Human ovum of twenty to twenty-tvo days. (From Allen Thomson.) Natural si/e, opened. The chorion forms a spacious vesicle, to the inner wall of which the small FcetUS (to the right above) is attached by a short umbilical cord FlG. 190.- Human foet-US of twenty to twenty-iwo days, taken from the preceding ovum, magnified. a amnion, b yelk-sac, c lower-jaw process of the first gill-arch, d upper-jaw process oi Bame, e second gill- arch (two smaller ones behind). Three giil-clefts are clearly seen. f rudimentary fore-leg, g auditory vesicle, h eye, /heart. archipelago), so that we might speak of a special race or "species" of tailed men r6a / (/■: r. 1 1. membr. i \ /• s .1 \n circula tiojv (Homo eaudatusj. Bartels has "no large, and almost fills the whole of the doubt that these tailed men "will be di>- pectoral cavity (Fie. 183 ov). Behind it covered in the advance of our geogra- I are the very small rudimentary lungs. phic.il and ethnographical knowledge of The primitive kidneys (mj are very large; the I uids in question * {Archio fur Anihro- they till the greater pari o( the abdominal polofit, Band XV., p. 120). ' cavity, and extend from the liver (/) to When we open a human embryo of one I the pelvic gut* Thus at the end of the Fig iqi — Human embryo of sixteen to eighteen days. (From Coste) Magnified. The embryo is surrounded by (ho amnion ( a ). and lie>- tree with this in the opened embryonic vesicle. The belly is drawn up by the large yelk-sac ( d ). and fastened to the inner w^ll of the embryonic membrane by the short and thick pedicle (b ). Hence the normal convex curve of the back (Fig. 100) is here changed into ,tn abnormal concave surface. h heart, m parietal mesoderm. The spots on the outer wall of the serolemma are the roots of the branching chorion-villi, which are free at the border month (Fig. 183), we find the alimentary canal formed in the body-cavity, and for the most part cut off from the em- bryonic vesicle. There are both mouth and anus apertures. But the mouth- cavity is not yet separated from the nasal cavity, and the face not yet shaped. The J heart shows all its four sections ; it is \vi . first month all the chief organs are already outlined. But there are at this stage no features by which the human embryo materially differs from that of the dog, the hare, the ox, or the horse — in a word, of any other higher mammal. All these embryos have the same, or at least a very similar, form ; they can at the most be FCETAL MEM B RAXES A. YD CfRCUL XT/ON '63 distinguished from the human embryo by the total size of the body or some other insignificant difference in si/e. Thus, for instance, in man the head i- larger in proportion to the trunk than in t lie ox. The features by means of which we dis- tinguish between them are not clear until latei on. Even at a much more advanced stage ol development, when wt tan dis- tinguish the human tutus from that of Scrolc Umbilical vesicle 1 \ elk-sac) Umbilical cord __ .(pedicle) Serolcmma Amnion , Chorion 1 Fig. 19a.— Human embryo of the fourth week, one-third of an inch long, l)ing in the dissected chorion. The tail is rather longer in the dog than in man. These are all negligible dif- , the face of the long-nosed ape with that of abnormally ape-like human beings (such as the as in external form. It is also expressed in the construction of the envelopes and appendages that we find surrounding the ftetus externally, and that we will now consider more closely. Two of these appendages —the amnion and the allantois — are only found ill the three higher classes of vertebrates, while the third, the yelk-sac, is found in most of the verte- brates. This is a circumstance of great importance, and it gives us valuable data for constructing man's genealogical tree. As regards the external membrane that encloses the ovum in the mammal womb, FOETAL MEMBRANES AND CIRCULATION "<>S we find it just the same in man as in the higher mammals. The ovum is, the reader will remember, first surrounded by the transparent structureless ovolemma or sona pellucida (Figs, i, 14). But very soon, even in the first week of development, this is replaced b\ the permanent chorion. TJiis is formed from the external layer of the amnion, the s, ">.'< >urn of the mammal embryo. This is the EtllantOlS or " primi- tive Urinary sac," an important embryonic organ, only found in the three higher classes of vertebrates, [n all the amni- otes the allantois quickly appears ai the hinder end of the alimentary canal, grow- ing out oi the cavity of the pelvic gut (Kg. 147 r, k, Fig. 195 .»/.<"). Tne further development oi the allantois varies considerably in the three suh-elasses of the mammals. The two lower sub- classes, monotremes and marsupials, retain the simpler structure of their ancestors, the reptiles. The wall of the Fig. 196.— Diagrammatic frontal section of the pregnant human WOmb. (From Longet.) The embryo hangs by the umbilical cord, which e>icJo5^;s the pedicle of the allantois ( al J. nb umbilical vessel, am amnion, eh chorion, ds decidua serotina. dv decidua vera, dr decidua reflexa, z villi of the placenta, c cervix uteri, u uterus. allantois and the enveloping serolemma remains smooth and without villi, as in the birds. But in the third sub-class of the mammals the serolemma forms, by invagination at its outer surface, a number of hollow tufts or villi, from which it takes the name of the chorion or mallochorion. The gut-fibre layer of the allantois, richly supplied with branches of the umbilical vessel, presses into these tufts of the primary chorion, and forms the " secondary chorion." Its embryonic blood - vessels are closely correlated to the contiguous maternal blood-vessels of the environing womb, and thus is formed the important nutritive apparatus of the embryo which we call the placenta. The pedicle of the allantois, which connects the embryo with the placenta and conducts the strong umbilical vessels from the former to the latter, is covered by the amnion, and, with this amniotic sheath and the pedicle of the yelk-sac. forms what is called the umbilical cord Fig. 196 al). As the large and blood- tilled vascular network of the fcetal allantois attaches itself closely to the mucous lining o\ the maternal womb, and the partition between the blood- vessels of mother and child becomes much thinner, we get that remarkable nutritive apparatus of the fcetal body which is characteristic of the placentalia (or choriata). We shall return afterwards to the closer consideration of this (cf. Chapter XXIII.). In the various orders of mam- mals the placenta undergoes many modifications, and these are in part of great evolutionary importance and useful in classi- fication. There is only one of these that need be specially men- tioned— the important fact, estab- lished by Selenka in 1890, that the distinctive human placenta- tion is confined to the anthro- poids. In this most advanced group of the mammals the allan- tois is very small, soon loses its cavity, and then, in common with the amnion, undergoes certain peculiar changes. The umbilical cord developes in this case from what is called the "ventral pedicle." Until very recently this was regarded as a structure peculiar to man. We now know from Selenka that much-discussed ventral pedicle is the merely the pedicle of the allantois, combined with the pedicle of the amnion and the rudimentary pedicle of the yelk-sac. It has just the same struc- ture in the orang and gibbon (Fig. 197), and very probably in the chimpanzee and gorilla, as in man ; it is, therefore^ not a disproof, but a striking fresh proof, of the blood-relationship of man and the anthropoid apes. We find only in the anthropoid apes — the gibbon and orang of Asia and the chimpanzee and gorilla of Africa — the peculiar and elaborate formation of the placenta that characterises man (Fig. 198). FCET. 1 1. hfEMBR I \7. \ . I YD l 7AV 7 7. •/ 7/aV 167 In this case there is at an early stage an intimate blending of the chorion of the embryo and the part of the mucous lining o( the womb to which it attaches. The villi of the chorion with the bloodvessels they contain grow SO completely into the tissue of the Litems, which is rich in blood, that it becomes impossible to separate them, and they form together a soil oi cake. This comes away as the "after- birth " at parturition; at the same time, the part of the mucous lining of the womb that has united inseparably with the chorion is torn away; hence it is called the deeidua (" falhng-away mern- utennaj — namely, that part of the mucous lining of the womb which unites intimately with the chorion-villi of the foetal placenta. The internal or false deeidua (interna or reflexa. Fig. 196 , Fig. 199 /y is that part o\ the mUCOUS lining oi the womb whichencloses the remaining surface of the ovum, the smooth chorion (chorion lint), in the shape of a special thin membrane. The origin o( these three different- deciduous membranes, in regard to which Quite erroneous \ iews (still retained in their names) formerly prevailed, is now quite clear. The externa] deeidua vera is the specially modified and subsequently Fig. 107.— Male embryo Of the Stamang-griDDon ( HylobaUs siamanga) of Sumatra, two-thirds natural size; to the kit the dissected uterus, of which only the dorsal half is given. The embryo has been taken out, and the limbs folded together ; it is still connected by the umbilical cord with the centre of the circular placenta which is attached to the- inside of the womb. This embryo takes the head-position in the womb, and this is normal in man also. brane"), and also the "sieve-membrane," because it is perforated like a sieve. We find a deeidua of this kind in most of the higher placentals ; but it is only in man and the anthropoid apes that it divides into three parts -the outer, inner, and placental deeidua. The external or true deeidua (Fig. 196 du, Fig. 199 g) is the part of the mucous lining of the womb that clothes the inner surface of the uterine cavity wherever it is not connected with the placenta. The placental or spongy deeidua ( placcntalis or serotina, Fig. 196 ds, Fig. 199 d) is really the placenta itself, or the maternal part of it (placenta detachable superficial stratum of the original mucous lining of the womb. The placental deeidua serotina is that part of the preceding which is completelv transformed by the ingrowth of the chorion-villi, and is used for constructing the placenta. The inner deeidua reflexa is formed by the rise of a circular fold of the mucous lining (at the border of the deeidua vera and serotina), which grows over the foetus (like the amnion) to t he- end. The peculiar anatomic features that characterise the human fcetal membranes are found in just the same way in the higher i68 F(ETAL MEMBRANES AND CIRCULATION apes. Untilreccntlyitwas (bought that the human embryo was distinguished by its peculiar construction of a solid allantois and ,i special ventral pedicle, and thai the umbilical cord developed from (his in a different way than in the other mammals. The opponents of the unwelcome "ape- theory" laid great stress on this, and thought they had at last discovered an important indication that separated man from all the other placentals. But the described the amnion has no blood-vessels at any moment of its existence. But the other two vesicles, the yelk-sac and the allantois, are equipped with large blood- vessels, and these effect the nourishment of the embryonic body! We may take the opportunity to make a few general obser- vations on the first circulation in the embryo and its central organ, the heart. The first blood-vessels, the heart, and the first blood itself, are formed from the Oviduct- Mouth of che" uterus Fie. 1^8— Frontal section Of the pregnant human Womb. (From Turner.) The embryo (a month old) hangs in the middle of the amniotic cavity by the ventral pedicle or umbilical cord, which connects it with the placenta (above). remarkable discoveries published by the distinguished zoologist Selenka in 1890 proved that man shares these peculiarities of placentation with tlie anthropoid apes, though they are not found in the other apes. Thus the very feature which was advanced by our critics as a disproof became a most important piece of evidence in favour of our pithecoid origin. Of the three vesicular appendages of I the amniote embryo which we have now ! gut-fibre layer. Hence it was called by earlier embryologists the "vascular layer." In a sense the term is quite correct. But it must not be understood as if all the blood-vessels in the body came from this layer, or as if the whole of this layer were taken up only with the formation of blood- vessels. Neither of these suppositions is true. Blood-vessels may be formed inde- pendently in other parts, especially in the various products of the skin-fibre layer. F(ETAL MEMBRANES AND ( 7A'( ( I. A //OX .09 I'il.. i c/RCl'l.ATIOX 171 are called oinphaJo-mesenteric Or vitelline arteries. The) represent the first begin- ning of a foetal circulation. Thus, the first blood-ves wis pass o\ ei the embryonic body and reach as far as the edge of the germinative area. At first they are confined to the dark or " vascular ana. But the) afterwards extend over the whole surface of the embryonic vesicle. In the (.iid, the whole of the yelk-sac is covered with a vascular net-work. These vessels have to gather food from the contents of the yelk-sac and convey it to the em- bryonic body. This is done by the veins, which pass first from the germinative area, and afterwards from the yelk-sac, to the farther end of the heart. They are called vitelline, or, frequently, omphalo- mesenteric, \ eins. These vessels naturally atrophy with the degeneration of the umbilical vesicle, and the vitelline circulation is replaced by a second, that of the allantois. Large blood-vessels are developed in the wall of the urinary sac or the allantois, as before, from the gut-fibre layer. These vessels grow larger and larger, and are very closely connected with the vessels that develop in the body of the embryo itself. Thus, the secondary, allantoic circulation gradually takes the place of the original vitelline circulation. When the allantois has attached itself to the inner wall of the chorion and been converted into the placenta, its blood-vessels alone effect the nourishment of the embryo. They are called umbilical vessels, and are origin- ally double — a pair of umbilical arteries and a pair of umbilical Veins. The two umbilical veins (Fig. [83 w), which convey blood from the placenta to the heart, open at first into the united vitelline veins ' The latter then disappear, and the right umbilical vein t,roes wuh them, so that henceforth a single large vein, the left umbilical vein, conducts all the blood from the placenta to the heart of the embryo. The two arteries of the allantois, 'or the umbilical arteries ( Fit^s. 1S3 n, 1X4 ;/), are merely the ultimate terminations of the primitive aortas, which are strongly) developed afterwards. This umbilical circulation is retained until the nine months of embryonic life are over, and the human embryo enters into the world j as an independent individual. The um- : bilical cord (Fig. 196 a/), in which these large blood-vessels pass from the embryo to the placenta, comes .'.way, together ' with the latter, in the after-birth, and 1 with the use of the lungs begins an entirely new form of circulation, which is Confined to the body of the infant. There is a great phylogenetic signi- ficance in the perfeel agreement which we find between man and the anthropoid ape. in these important features of em- bryonic circulation, and the special ctn\- sti LICtion of the placenta and the umbilical COrd. We must infer- from it a close blood-relationship of man and the anthro- pomorphic apes a common descent of them from one and the .same extinct l> Flu. j, j Boat-shaped embryo of the doe, from the ventral side, magnified about ten times. In front under the forehead ue can see the first pair of nil-arches ; underneath is the S-shaped heart, at the sides of which are the auditory vesicles. The heart divides behind into the two vitelline veins, which expand in the germinative area (which is torn off all round). On the floor of the open belly lie, between the protovertebne. the primitive aortas, from which five pairs of vitelline arteries arc given off. (From Bitchoff.) group of lower apes, Huxley's " pitheco- metra-pi inciple " applies to these onto- genetic features as much as to any other morphological relations: "The differences in construction of any part of the body are less between man and the anthropoid than between the latter and the lower apes." This important Huxleian law, the chief consequence of which is " the descent of man from the ape," has lately been con- firmed in an interesting and unexpected way from the side of the experimental '-■ / ■■(/■; /■. / / ME MB R. i NE$ A ND i 7/,v 1 1..\ TtON physiology of the Mood. The experi- ments of H.ms Friedenthal at Berlin nave shown thai human blood, mixed with the blood ol lower apes, has a poisonous As we know from man v other experiments that the mixture of two different kinds of blood is only possible without injury in the case o! two closclv related animals of FlG. 203.— Lar Or White-handed gibbon < Hvhbates lar or albimanus), from the Indian mainland Brehm.) (Fror effect on the latter ; the serum of the one destroys the blood-cells of the other. But this does not happen when human blood is mixed with that of the anthropoid ape. the same family, we have another proof of the close blood-relationship, in the literal sense of the word, of man and the anthropoid ape. FCETAl. MEMBRANES A. YD CIRCULATION The existing anthropoid apes are only a small remnant of a larva family of eastern apes (or Catarrh ma), from which man was evolved about the on J of the Tertiary period. They fall into two geo- '73' eight to twelve species of it in th* East Indies. I made observations of four o( them during my voyage in the East Indies (1901), and had a specimen of the ash- grey gibbon ( Hylobatcs Ifuciscus) living FlG. 204. — Young Orang ( Satyrus orang). asleep. graphical groups — the Asiatic and the African anthropoids. In each group we can distinguish two genera. The oldest of these four genera is the gibbon (Hylobates, Fig. 203); there are from for several months in the garden of my house in Java. I have described the interesting habits of this ape (regarded by the Malays as the wild descendant o\ men who had lost theii waj | in my Maiayiscktn '74 FCETAL MEAfliR. I M:S ,1V/) CIRCULATION Reisebriefen (chap. xi.). Psychologically, he showed a good deal of resemblance to the children of my Malay hosts, with whom he played and formed a very close- friendship. The second, larger and stronger, genus of Asiatic anthropoid ape is the orang ( Satyrus ); he is now found only in i he- islands of Borneo and Sumatra. Selenka, liar and salient cheek-pads in the elderly male ; these are wanting in the other group, the ordinary orang-outang [Eusatyrus). Several species have lately been distin- guished in the two genera of the black African anthropoid apes (chimpanzee and gorilla). In the genus Anthropitlucus ( or . I nlhropopilliccus, formerly Troglodytes), Z& &i /.Sf Fig. 205. — Wild orang ( Dyssatyms auritus). (From R. Fick and Leutemann.) who has published a very thorough Study of the Development a?id Cranial Structure of the Anthropoid Apes (1899), distin- guishes ten races of the orang, which may, however, also be regarded as "local varieties or species." They fall into two sub-genera or genera : one group, Dis- satyrus (orang - bentang, Fig. 205), is distinguished for the strength of it's limbs, and the formation of very pecu- the bald-headed chimpanzee, A. calvus (Fig. 206), and the gorilla-like. A. mafuca differ very strikingly from the ordi- nary Anthropithecus niger (Fig. 207), not only in the size and proportion of many parts of the body, but also in the peculiar shape of the head, especially the ears and. lips, and in the hair and colour. The controversy that still continues as to whether these different forms of F(ETAL MF.MIIRANES AND ( 7/.Y V 'LA T/O.V •75 Fie. 206. — The bald-headed chimpanzee ( Anthrojrithecus calvus). Female. This fresh species, described by Frank Beddard iri 1897 as Trcgludytes ca.'i us, differ! considerably from the Ordinary A. nigtr (Fig. icq) in the structure of the head, the colopring, and the absence of hair in parts. i 70 FOETAL MEMBRANES AND CIRCULATION chimpanzee and orang are " merely local varieties" or "true species " is .m idle one ; as in all such disputes of classifiers there is an utter absence of clear ideas as to what a species really is. Of the largest and most famous of all the anthropoid apes, the gorilla, Paschen has lately discovered a grant-form in the interior of the Cameroons, which scorns to differ from the ordinary species (Gorilla to that of man, but it is substantially the same. " The same 200 bones, arranged in the same way, form our internal skeleton ; the same 300 muscles effect our movements; the same, hair covers our skin ; the same groups of ganglionic cells compose the ingenious mechanism of our brain ; the same four-chambered heart is the central pump of our circulation." The really existing differences in the \y\^ Fig. 307.— Female Chimpanzee ( Anthropithecus nigerj. (From Brehm.) gina Fig. 208), not only by its unusual size and strength, but also by a special formation of the skull. This giant gorilla {Gorilla gigas, Fig. 209) is six feet eight inches long ; the span of its great arms is about nine feet ; its powerful chest is twice as broad as that of a strong man. The whole structure of this huge anthropoid ape is not merely very similar shape and size of the various parts are explained by differences in their growth, due to adaptation to different habits of life and unequal use of the various organs. This of itself proves morphologically the descent of man from the ape. We will return to the point in the twenty-third chapter. But I wanted to point already to this important solution of " the ques- tion of questions," because that agreement FCETAL MEMBRANES AND CIRCULATION '77 in the formation of the embryonic mem- branes and m foetal circulation whii li I have described affords a particularly weighty proof of it. It is the nunc instructive a-> even cenogenetlc structures may in certain circumstaiu es have a high phylogenetic value. In conjunction with the outer facts, it affords a striking con- firmation of Our biogenetic law. >S5l^ Fig. ao&— Female gorilla. (From Brehm) Fig. 3oq.— MalO giant-gorilla ( Gorilla gig-as), from Yaunde, in the interior of the Cameroons. Killed by H. Paschrn. stufiid by I'mlauff. ClIAPTKR XVI. STRUCTURE OF THK LANCKLKT AND Till- SEA-SOUIRT Is turning from the embryology to the phylogenj of man from the development of the individual to that of the species we must hear in mind the direct causal connection that exists between these two main branches of the science of human evolution. This import. mi causal nexus finds its simplest expression in " the tun J, i mental law of organic development," the content and purport of which we have fully considered in the first chapter. According to this biogenetic law, mito- gen v is a brief .i\\d condensed recapitula- tion of phylogeny If this compendious reproduction were complete in all eases, it would he very easj to construct the whole story of evolution on an embryonic basis. When we wanted to know the ancestors of any higher organism, and, therefore, o( man to know from what forms the r.ue as a whole has been evolved —we should merely have to follow the series of forms in the development of the individual from the mum ; we could then regard each of the successive forms as the representative of an extinct ancestral form. However, this direct application of ontogenetic facts to phylogenetic ideas is possible, without limitations, only in a very small section of the animal kingdom There are, it is true, still a number of lower invertebrates (for instance, some of the Zoophyta and Vermalia) in which we are justified in tgnising at once each embryonic form as the historical reproduction, or silhouette, as it were, of an extinct ancestor. Hut m the great majority of the animals, and in the case of man, this i- impossible, because the embryonic forms themselves have been modified through the change of the conditions of existence, and have lost their original character to some extent. During the immeasurable course of organic history, the many millions of years during which life was developing on our planet, secon- dary changes of the embryonic forma have taken place in most animals. The young of animals (not only detached larva? , hut also the embryos enclosed ill the womb) may he modified by Lhe influence of the environment, just as well as the mature organisms are by adapta- tion to the conditions of life ; even Spe« ies are altered during the embryonic develop- ment. Moreover, it is an advantage for all higher organisms (and the advantage is greater the more advanced they arc) to curtail and simplify the original course of development, and thus to obliterate the traces of their ancestors. The higher the individual organism is in the animal kingdom, the less completely does it reproduce in its embryonic development the series of its ancestors, for reasons that are as yet only partly known to us. The fact is easily proved by comparing the different developments of higher and ■lower animals in any single stein. In order to appreciate this important feature, we have distributed the embryo- logical phenomena in two groups, pal in- genetic and cenogenetic. Cinder palin- genesis we count those fails of em bryo- logy that we can directly regard as a faithful synopsis of the corresponding stem-history. By ccnogencsis we under- stand those ernbryonic processes which we cannot directly correlate with correspond- ing evolutionary processes, but must regard as modifications Or falsifications of them. With this careful discrimination between pal ill genetic and cenogenetic phenomena, our biogenetic law assumes i the following more precise shape : The rapid and brief development of the indi- vidual (ontogeny) is a condensed synopsis of the long and slow hisloi") of the stem (phylogeny): this synopsis is the more faithful and complete in proportion as lhe original features have been preserved by heredity, and modifications have not been introduced by adaptation. 79 i8o STRUCTURE OF THE LANCELET AND THE SEASOU/RT In order to distinguish correctly be- tween palingenetic andcenogcnetic pheno- mena in embryology, and deduce sound conclusions in connection with stem- history, we must especially make a com- parative Much ol the former. In doing this it is tx-si to emplo) the methods that have long been used In geologists for the purpose of establishing the succession of the sedimentary rocks in the crust of the earth. This solid crust, which encloses the glowing central mass like a thin shell, is composed of different kinds of rocks . there are, firstly, the volcanic rocks which were formed directly by the codling at the surface of the molten mass of the earth ; secondly, there are the sedimentary rocks, that have been made out of the former by the action of water, and have been laid in successive strata at the bottom of the sea. Each of these sedimentary strata was at first a soft layer of mud ; but in the course of thousands of years it condensed into a solid, hard mass of stone (sandstone, limestone, marl, etc.), and at the same time permanently preserved the solid and imperishable bodies that had chanced to fall into the soft mud. Among these bodies, which were either fossilised or left characteristic impressions of their forms in the soft slime, we have especially the more solid parts of the animals and plants that lived and died during the deposit of the slimy strata. Hence each of the sedimentary strata has its characteristic fossils, the remains of the animals and plants that lived during that particular period of the earth's history. When we make a comparative study of these strata, we can survey the whole series of such periods. All geo- logists are now agreed that we can de- monstrate a definite historial succession in the strata, and that the lowest of them were deposited ii> very remote, and the uppermost in comparatively recent, times. However, there is no part of the earth where we find the series of strata in its entirety, or even approximately complete. The succession of strata and of corres- ponding historical periods generally given in geology is an ideal construc- tion, formed by piecing together the various partial discoveries of the succes- sion of strata that have been made at different points of the earth's surface (cf. Chapter XVI II.). We must act in this way ill constructing the phytogeny of man. We must try to piece together a fairly complete picture of the sei ies ofoui ancestors from the various phylogenetic fragments that we find in the differenl groups of the animal king- dom. We shall see that we are really in a position to form an approximate picture of the evolution of man and the mammals by .1 proper comparison of the embryology of vciv differenl annuals— a picture that we could never have framed from the ontogeny of the mammals alone. As a result oi the above-mentioned cenogenetic processes those of disturbed and curtailed heredity — whole series of lower stages have dropped out in the embryonic de- velopment of man and the other mammals, especially from the earliest periods, or been falsified by modification. Hut we find these lower stages in their original purity in the lower vertebrates and their invertebrate ancestors. Especially in the lowest of all the vertebrates, the lancelet or Amphioxus, we have the oldest stem-forms completely preserved in the embryonic development. We also find important evidence in the fishes, which stand between the lower and higher vertebrates, and throw further light on the course of evolution in certain periods. Next to the fishes come the amphibia, from the embryology of which we can also draw instructive conclusions. They represent the transition to the higher vertebrates, in which the middle and older stages of ancestral development have been either distorted or curtailed, but in which we find the more recent stages of the phylogenetic process well preserved in ontogeny. We are thus in a position to form a fairly complete idea of the past development of man's ancestors within the vertebrate stem by putting together and comparing the embryo- logical developments of the various groups of vertebrates. And when we go below the lowest vertebrates and compare their embryology with that of their invertebrate relatives, .we can follow the genealogical tree of our animal ancestors much farther, down to the very lowest groups of animals. In entering the obscure paths of this phylogenetic labyrinth, clinging to the Ariadne-thread of the biogenetic law and guided by the light of comparative anatomy, we will first, in accordance with the methods we have adopted, discover and arrange those fragments from the manifold embryonic developments ot very different animals from which the stem- historv of man can be composed. 1 would call attention particularly to the fact that STRUCTURE OF THE LANCELET AND THE SEA-SQUIRT iXi «i can employ this method with the same confidence and right as. iho geologist No geologist has ever had oculai proof thai the vast rocks thai compose out Carboniferous oi Jurassic 01 Cretaceous strata were really deposited in watei Vet no one doubts the fact. Further, no geo- logist has evei learned by direct observa- tion that these various sedimentary forma- tions wore deposited in a certain ordei . vet all are agreed as to this ordei This is because the nature and origin oi these rocks cannot be rationally understood unless we assume that the) were so deposited. These hypotheses are univer- sally received as s.iic and indispensable "geological theories," because they alone [jiM' a rational explanation oi the strata, Oui evolutionary hypotheses can claim the same value, for the same reasons. In formulating them we are acting on the same inductive and deductive methods, and with almost equal confidence, as the geologist. We hold them to be correct, and claim the status of "biological theories'1 foi them, because we cannot understand the nature and origin oi man and the othei organisms without them, and because they alone satisfy our demand tor a knowledge of causes. And just as the geological hypotheses that were ridi- culed as dreams at (lie beginning y^i the nineteenth century are now universally admitted, so our phylogenetic hypotheses, which are still regarded as fantastic in certain quarters, will soonei or later be generally received li is true that, as will soon appear, our task is not so simple as that oi the geologist It is just as much more difficult and complex as man's organisation is more elaborate than the structure oi the rocks. When we approach this task, we find an auxiliary of the utmost importance in the comparative anatomy and embryology of two lower animal-forms. One of these animals is the lancelet ( AmpkiOXttfJ, the other the sea-squirt (AscidiaJ. Both of these animals are very instructive. Both are at the horder between the two chief divisions oi the animal kingdom — the vertebrates and invertebrates. Ihe verte- brates comprise the ahead) mentioned classes, from the Amphioxus to man (acrania, lampreys, fishes, dipneusts, amphibia, reptiles, birds, and mammals). Following the example of Lamarck, it is usual to put all the other animals together under the head oi invertebrates. But, as I have often mentioned already, the group is composed of a number of very different stems, Oi these we have no interest just now in the echmodernis, molluSCS, and articulates,, as they are independent branches ol the animal-tree, and have nothing to do with the vertebrates. On the othei hand, we are greatly concerned with a very interesting group 'hat has only recently been carefully studied, and that lias a most important relation to the ancestral tree ol the vertebrates This is the stem of the Tuincaies One member ot this group, the sea-squirt, very closely approaches the lowest vertebrate, the Amphioxus, in its essential internal struc- ture and embryonic development Until 1806 no one had any idea oi ilu- 1 lose con- nection ol these apparently vers different ammais, it was a \en fortunate accident that the embryology of these related forms was discovered just at the tune when the question of the descent of the vertebrates from the invertebrates came to the front In order to understand it properly, we must first COnsidet these remarkable animals in their fully-developed forms and compare their anatomy We begin with the lancelet— aftei man the most important and interesting of all animals. Man is at the highest summit, the lancelet at the lowest root, of the verte- brate stem. It lives on the Rat, sandy parts of the Mediterranean coast, partly buried in the sand, and is apparently found in a number oi seas. It has been found m the North Sea (on the British and Scandinavian coasts and in Heligoland) and at various places on the Mediterranean (for instance, at Nice, Naples, and Messina) It is also found on the coast of Brazil and in the most distant parts of the Pacific Ocean (the coast of Peru. Borneo, China, Australia, etc.) Recently eight to ten species of the amphioxus have been determined, distri- buted in two or three genera. Johannes Muller classed the lancelet with the fishes, although he pointed out that the differences between this simple vertebrate and the lowest fishes are much greater than between the fishes and the amphibia. But this was far from expres- sing the real significance of the animal. We may confidently lay down the follow- ing principle : The Amphioxus differs more from the fishes than the fishes do from ' See the ample monograph b> Arthur Willcv, Amphioxus and the Ancestry 0/ the Vertebrates, Boston, 1894. ■8a S TRl '( 7V A7-. OF THE I . ! Nt EI.ET A .\7> THE SEA-SQUIRT man and the other vertebrates. As a matter o\ fact, it is so1 different from all the oilier vertebrates in its whole organi- thisstcm i, tin- \crania (Amphioxus and its extinct relatives), and i, the Craniota (m. in and the oilier vertebrates). The \ \ ~ao Fig. 210. Fig, 3ii. FlC. 210. — The lancelet (Amphioxus lanreolatus), twice natural size, left view. Tin- long a*is is vertical ; the mouth-end is above, the tail-end belivv ; a mouth, surrounded by threads of beard ; b anus, c gill-opening (porta branchialis J, d gill-crate. »• stomach, /liver, g small intestine, h branchial cavity, i chorda (axial rod), underneath it the aorta ; k aortic arches, /trunk of the branchial artery, »i swellings on its branches, n vena cava, o visceral vein. Fig. in— Transverse section of the head of the Amphioxus. (From Boveri.) Above the branchial gut (kd) is the chorda, above this the neural tube (in which we can distinguish the inner grey and the outer white matter) ; above again is the dorsal fin ( fh ). To the right and left above (in the episoma) are the thick muscular plates (m) ; below (in the hyposoma) the gonads (g). ao aorta (here double). '< curium. A endostyl, /fasoc, gl glomerulus ol the kidneys. X- branchial vessel, hi partition between the cceloma (si ) and atrium ( pj, ml trans- verse ventral muscle, n renal canals, of upper and uf lower canals in the mantle-folds, p peribronchial cavity, (atrium), sc cceloma (subchordal body-cavity), si principal (or subintestinal) vein, sk perichorda (skeletal layer). sation that the laws of logical classification Compel us to distinguish two divisions vl~ first and lower division comprises the vertebrates that have no vertebra? or skull STRUCTURE OF THE LANCE LET AND THE SEA-SQUIRT 183 (cranium ). Ot these the only living representatives are the Amphioxus and Paramphioxus, though there musl have been a number of different spec les .u an e n U penod ot the earth's histoi \ . Opposed to the Acrama is the second division ot tin- vertebrates, which com- prises dl the other members of the stem, from the tishes up to man. All these vertebrates have a head quite distinct from the trunk, with a skull (cranium) .mo\ brain , all have a centralised heart, lully- formed kidneys, etc. Hence the) are called the Crunidta. These Craniotes are, however, without a skull in their earlier period. As we already know from embryo- logy, even man, like every oilier mammal, passes in the earlier course of his develop- ment through the important static which we call the chordula ; at this lowei stage the animal has neither vertebra- nor skull nor limbs (Figs. 83 86). And even after the formation of the primitive veiu has begun, the segmented foetus of the amniotes still has for a long time tile simple form of a lyre-shaped disk or a sandal, without limbs or extremities. When we compare this embryonic condi- tion, the sandal-shaped foetus, with the developed lancelet, we may say that the amphioxus is, in a certain sense, a perma- nent sandal-embryo, or a permanent em- bryonic form o\' tlie Acrania ; it never rises 'above a low grade of development which we have long since passed. The fully-developed lancelet (Fig. 210) is about two inches long, is colourless or of a light red tint, and has the shape of a narrow lancet-formed leaf. The body is pointed at both ends, but much com- pressed at the sides. There is no trace of limbs. The outer skin is very thin and delicate, naked, transparent, and composed of two different layers, a simple external Stratum of cells, the epidermis, and a thin underlying cutis-layer. Along the middle line of the back runs a narrow fin-fringe which expands behind into an oval tail-fin, and is continued below in a short anus-fin. The fin-fringe is supported by a number of square elastic fin-plates. In the middle of the body we find a thin String of cartilage, which goes the whole length of the body from front to h,n Ic, and is pointed at both ends (Fig. 2101). This straight, cylindrical rod (somewhat compressed for a time) is the axial rod or the chorda Jorsalis^ in the lancelet this i^ the only trace of a vertebral column. The chorda developes no further, but retains its original simplicity throughout life. It 1- enclosed by a firm membrane, the chorda-sheath or penchorda. The real features of -this and of its dependent loi mations are best seen in the transverse section of the Amphioxus (Fig. 211). The perichorda forms a cylindrical lube imme- diately ovei the chorda, and the central nervous system, the medullar) lube, is enclosed in it. This import. mi psychic organ also remains in its simplest shape throughout life, as a ivliiiclik.il tube, terminating with almost equal plainness at either end, and enclosing a narrow cai\.i\ in Us thick wall. However, the fore end is a little rounder, and contains a small, almost imperceptible bullions swel- ling of the canal. This must beregarded as the beginning of a rudinuntaiv brain. At the foremost end of it there is a small black pigment-spot, a rudimentary eye ; and a narrow canal leads to a superficial sense-organ. In the vicinity of this optic spot we find at the left side a small ciliated depression, the single olfactory organ. There is no organ of hearing. This defective development of the higher sense-organs IS probably, in the main, not an Original feature, but a result of degene- ration. Underneath the axial rod or chorda runs a very simple alimentary canal, a tube that opens on the ventral side of the animal by a mouth in front and anus behind. The oval mouth is surrounded by a ring of cartilage, on which there are twenty to thirty cartilaginous threads (organs of touch, Fig. 210 a). The alimentary canal divides into sections of about equal length by a constriction in the middle. The f a mal t, this itrium (commonly called the peri branchial cavity) is a secondary structure formed by the development >>t 1 1 ouplc of lateral mantle- folds 01 j.;iil-> o\ 1 1 5 (Afu UJ. The real body-cavity ( l.h ) is very narrow and entirely closed, lined with epithelium. The peribranchial cavity ( \ ) is full o, water, -o\<\ its walls art lined with the skin-sense layer; it opens outwards in the reai through the respiratory pore 210 <)■ On the inner surface of these mantle- folds f M ,), in the ventral half of the wide mantle cavity (atrium), we hud the sex-organs of the Amphioxus. At each Jut •1 Fie. 214- Fie. 315 Fk;. 214. -Transverse section of a young Amphioxus. immediately after metamorphosis, through the hindertnosl third (between tin- atrium-cavity and the anus) l'n. »i$.- Diagram of preceding. (From Hatschrb.) A epidermis. B medullary tube. C chorda, /> aorta. £ visceral epithelium, r subtntestinal vein. / corium-plate. _> muscle-plate. ; Encie-plate. 4 outer chorda-sheath, .? myoseptum, (i skm-fibrc plate, 7 gut- fibre plate. / mvoeei'l. // spl.uu hnoeivl. / dorsal fin. //, anus-fin. into a double row of primitive segments side of the branchial put there are (Fig. 124), and each t>f these subdivides, between twenty and thirty roundish four- bv a frontal or lateral constriction, into an upper (dorsal) and lower (ventral) pouch. These' important structures are seen very clearly in the trunk of the amphi- oxus (1 he latter third, Pigs. 212-215), but it is otherwise in the head, the foremost third (Fig. 2it>). Here we find a number of complicated structures that cannot be understood until we have studied them on the embryological side in the next chapter (c( Fig 81). The branchial put lies fire, in ■■ spacious cavity filled with water, which was wrongly thought formerly to be the body -cavity (Fig. corned sacs, which can clearly be seen from without with the naked eye, as they shine through the thin transparent body- wall. These sacs are the sexual glands , they are the same si/e and shape in both sexes, only differing in contents. In the female they contain a quantity of simple Ova (Fig. 2igjg)', in the male a number of much smaller cells that change into mobile ciliated cells (sperm-cells). Both sacs lie on the inner wall of the atrium, and have tio special outlets When the ova of the female and the sperm oT the male ate ripe, they fall into the atrium, p.iss through the gill-clefts into the fore- i s, , s TR I r< ' Ti TRE < )/■' THE I ■ I «VH ELE T . I ND THE SK. I sni 1KT gut, and are ejected through the mouth. Above the sexual glands, at the dorsal angle of the atrium, we find the kidneys. These important excretor) organs could not be found in the Araphioxus foi a long other vertebrates (Fig. 218 />'). Their internal aperture (Fig. -'17 B) opens into the body-cavity; their outer aperture into the atrium (('). The prerenal canals lie in the middle ol the line ot the head, out wards Fig. 216 — TranSVCrSC section Of the lancclet, in the fore half. (From Ralf>h.) The outer covering is the simple cell-layer ol the epidermis (/■'). Under this is th<- thin coriurn, the subcutaneous tissue ol which is thickened; it sends connective-tissue partitions between the muscles (Mi) and to the chorda-sheath. TV medul- l.irv tube, Ch chorda, Lh body-cavity, A atrium, /. upper wall of same, /u inner wall, r?« outer wall, J.h\ ventral remnant of same, Kst gill-rods, M ventral muscles, A' scam of the joining of the ventral folds (gill-covers), (, sexual glands. time, on account of 1 heir remote position and their smallncss; they were discovered in i.S<)o by Theodor Boveri (Fig. 217 .1). They are short segmented canals, corres- ponding to the primitive kidneys of the from (he uppermost section of the gill- arches, and have important relations to the branchial vessels (II). For this reason, and in their whole arrangement, the primitive kidneys of the Amphioxus STRUCTURE OF THE LANCELET AND THE SEA-SQUIRT 187 show i learl) lhal the) arc ( quivali ill lo the prerenal canals of the Cran iotas (Fig. 218 /•')• The prerenal duel of I he latter (Fig, _ms C) corresponds to the branchial cavity or allium of the former (Fig. ai7C) It we sum up the results of our anatomic study of the Amphioxus, and l i... -17. Transverse section through the middle of the Amphioxus. (From Boteri.) On the left a kri"-rixl has been struck, and on the ngb la gin-deft : consequently on the left ire see the whole i usually attached, often by means o\~ regular roots. The dorsal and ventral sides differ a good deal internally, but frequently cannot be distin- guished externally. If we open the thick tunic or mantle in order to examine the internal organisation, we first find a spacious cavity tilled with water — the mantle-cavity or respiratory cavity (Fig. 220 cl). It is also called the branchial cavity and the cloaca, because it receives the exilements and sexual products as well as the respirator) water. The greater part oi the respiratory cavity is Occupied by the large grated branchial Sai fbrj. This is so like the gill-crate of the Amphioxus in its whole arrange- ment that the icscmbl.unc was pointed out by the English naturalist Goodsir, years ago, before anything was known "t the relationship oi the two animals. As .1 fact, even in tin- Asi idia the mouth fa) openf fust into this wide branchial Fig. i2o— Organisation of an Ascidia (left view); the dorsal side is turned to the right and the ventral side to the left, the mouth (o) above ; the ascidia is attached at the tail end. The branchial gut (br). which is pierced by a number of clefts, continues below in the visceral gut. The rectum opens through the anusr'ay into the atrium (cl), from which the excre- ments are ejected with the respirators water through the mantle-hole or cloaca (a) ; m mantle. (From Geernbaur.) sac. The respiratory water passes through the lattice-work of the branchial sac into the branchial cavity, and is ejected from this by the respiratory pore (a J. Along the ventral side of the branchial sac runs a ciliated groove — the hypobranchial groove which we have previously found at the same spot in the Amphioxus. The food oi the Ascidia also ioo STRUCTURE OF THE LANCELET AND THE SEA-SQUIRT consists of tiny organisms, infusoria, diatoms, parts of decomposed marine plants and animals, etc. These pas-, with the water into the gill-crate and the digestive part of the gut at the end of it, at first into an enlargement of it that represents the stomach. The adjoining small intestine usually forms a loop, bends forward, and opens by an anus (Fig. 220 a), not directly outwards, but first into the mantle cavity ; from this the excrements are ejected by a common outlet (a' ) together with the used-up Fig. 221— Organisation of an Ascidia (as in Fig. aao, seen from the left) sb branchial sac, v stomach, * small intestine, c heart, t testicle, vd -.perm-duct, o ovary, o' ripe ova in the branchial cavity. The two small arrows indicate the entrance and exit of the water through the openings of the mantle. (From Milne-Edwards. ) water and the sexual products. The outlet is sometimes called the branchial pore, and sometimes the cloaca or ejection- aperture. In many of the Ascidiae a glandular mass opens into the gut, and this represents the liver. In some there is another gland besides the liver, and this is taken to represent the kidneys. The body -cavity proper, or cceloma, which is filled with blood and encloses the hepatic put, is very narrow in the Ascidia, as in th^ Amphioxus, and is here also usually confounded with the wide atrium, or- peribronchial cavity, full of water. There is wo trace in the fully-developed Ascidia of a chorda dorsalis, or internal axial skeleton. It is the. more interesting that the young animal that emerges from the ovum has a chorda, and that there is a rudimentary medullary tube above it. The latter is wholly atrophied in the developed Ascidia, and looks like a small nerve-ganglion in front above the gill- irate. It corresponds to the upper "gullet-ganglion" or " primitive brain" in other vermalia. Special sense-organs are either wanting altogether or are only found in a very rudimentary form, as simple optic spots and touch-corpuscles or tentacles that surround the mouth. The muscular system is very slightly and irregularly developed. Immediately under the thin corium, and closely con- nected with it, we find a thin muscle tube, as in the worms. On the other hand, the Ascidia has a centralised heart, and in this respect it seems to be more advanced than the Amphioxus. On the ventral side of the gut, some distance behind the gill-crate, there is a spindle- shaped heart. It retains permanently the simple tubular form that we find tem- porarily as the first structure of the heart in the vertebrates. This simple heart of the Ascidia has, however, a remark- able peculiarity. It contracts in alter- nate directions. In all other animals the beat of the heart is always in the same direction (generally from rear to front) ; it changes in the Ascidia to the reverse direction. The heart contracts first from the rear to the front, stands still for a minute, and then begins to beat the opposite way, now driving the blood from front to rear ; the two large vessels that start from either end of the heart act alternately as arteries and veins. This feature is found in the Tunicates alone. Of the other chief organs we have still to mention the sexual glands, which lie right behind in the body-cavity. All the Ascidia? are hermaphrodites. Each indi- vidual has a male and a female gland, and so is able to fertilise itself. The ripe ova (Fig. 221 0') fall directly from the ovary (o) into the mantle-cavity. The male sperm is conducted into this cavity from the testicle (I) by a special duct (vd). Fertilisation is accomplished here, and in many of the Ascidiae developed embryos are found. These are then EMBRYOLOGY OF THE LANCELET AND /'///■: SEA-SQUIRT 191 ejected with the breathing-water through the cloaca (q), and bo " born alive." If we now glance at the entire struc- ture of the simple Ascidia (especially Phallusia, Cynthia, etc.) and compare it with that of the Amphioxus, we shall find that the tWO have few points of contact. It is true that the fully-developed Ascidia resembles the Amphioxus iii several important features of its internal struc- ture, and especially in the peculiar character of the gill-crate and gut. But iii most other features of organisation it is so far removed from it, and is so unlike it in external appearance, that the really close relationship of the two was pot discovered until their embryology •*. is studied. We will now compare the embryonic development of the two animals, and find to our great astonish- ment thai the same embryonic form developes from the ovum of (he Amphioxus as from that of the Ascidia a typical ihoniula. Chapter XVII. EMBRYOLOGY OF THE LANCELET AND THE SEA-SOUIRT The structural features that distinguish the vertebrates from the invertebrates are so prominent that there was the greatest difficulty in the earlier stages of classifica- tion in determining the affinity of these two great groups. When scientists began to speak of the affinity of the various animal groups in more than a figurative - in a genealogical — sense, this question came at once to the front, and seemed to constitute one of the chief obstacles to the carrying-out of the evolutionary theory. Even earlier, when they had studied the relations of the chief groups, without any idea of real genealogical connection, they believed they had found here and there among the invertebrates points of contact with the vertebrates : some of the worms, especially, seemed to approach the vertebrates in structure, such as the marine arrow- worm (Sag itta ). But on closer study the analogies proved untenable. When Darwin gave an im- pulse to the construction of a real stem- history of the animal kingdom by his reform of the theory of evolution, the solution of this problem was found to be particularly difficult. When I made the first attempt in my General Morphology (1866) to work out the theory and apply it to classification, I found no problem of phytogeny that gave me so much trouble as the linking of the vertebrates with the invertebrates. But just at this time the true link was discovered, and at a point where it was least expected. Towards the end of 1866 two works of the Russian zoologist, Kowalevsky, who had lived for some time at Naples, and studied the embryo- logy of the lower animals, were issued in the publications ol the St. Petersburg Academy. A fortunate accident had directed the attention of this able observer almost simultaneously to the embryology of the lowest vertebrate, the Amphioxus, and that of an invertebrate, the close affinity of which to the Amphi- oxus had been least suspected, the Ascidia. To the extreme astonishment of all zoologists who were interested in this important question, there turned out to be the utmost resemblance in structure from the commencement of development between these two very different animals — the lowest vertebrate and the mis- shaped, sessile invertebrate. With this undeniable identity ol "ontogenesis, which can be demonstrated to an astounding extent, we had, in virtue of the bio- genetic law, discovered the long-sought genealogical link, and definitely identified the invertebrate group that represents the nearest blood-relatives of the vertebrates. i.,j EMBRYOLOGY OF THE LANCELET AND THE SEA-SQUIRT The discovery was confirmed In other zoologists, and there can no longer be an) doubt thai of all the classes of inver- tebrates that of the Tunicates is most closely related to the vertebrates, and of the Tunicates the nearest are the Ascidia;. We cannot mu that the vertebrates are descended from the Ascidia; and still less the reverse but we can say that of all the invertebrates it is the Tunicates, and, within this group, the Ascidia;, that arc the nearest blood-relatives of the ancient Stem-form of the vertebrates. We must assume as the common ancestral group of both stems an extinct family of the extensh e \ ei malia-stem, the Prochordonia 04 Prockordata ("primitive chorda-ani- mals "). In order to appreciate fully this remark- able fact, and especially to secure the sound basis we seek for the genealogical tree of the vertebrates, it is necessary to .study thoroughly the embryology of both these animals, and compare the individual development of the Amphioxus step by step with that of the Ascidia. We begin with tlie ontogeny of the Amphioxus. From the concordant observations of Kowalevsky at Naples and Hatschek at Messina, it follows, firstly, that the ovum- segmentation and gastrulation of the Amphioxus are of the simplest character. They take place in the same way as we find them in many of the lower animals ot different invertebrate stems, which we have already described as original or primordial ; the development of the Ascidia is of the same type. Sexually- mature specimens of the Amphioxus, which are found in great quantities at Messina from April or May onwards, begin as a rule to eject their sexual products in the evening ; if you catch them about the middle of a warm night and put them in a glass vessel with sea- water, they immediately eject through the mouth their accumulated sexual pro- ducts, in consequence of the disturbance. The males give out masses of sperm, and the females discharge ova in such quan- tity that many of them stick to the fibrils about their mouths. Both kinds of cells pass first into the mantle-cavity after the opening of the gonads, proceed through the gill-clefts into the branchial gut, and are discharged from this through the mouth. The ova are simply round cells. They are only 5 5ff of an inch in diameter, and thus are only half the size of the mammal and have no distinctive features. The clear protoplasm of the mature ovum is made so turbid by the numbers of dark granules of food-yelk or deutoplasm scattered in it that it is difficult to follow (he process of fecundation and the behaviour of the two nuclei during it (p. 51). The active elements of the male sperm, the cone-shaped spermato- zoa, are similar to those of most other animals (^f. Fig. 20). Fecundation takes place when these lively ciliated cells of the sperm approach the ovum, and seek to penetrate into the \ elk-matter or the cellular substance of the OVUm with their head-part— the thicker part of the cell that encloses the nucleus. Only one spermatozoon can bore its way into the yelk at one pole of the ovum-axis ; its head or nucleus coalesces with the female nucleus, which remains after the extru- sion of the directive bodies from the germinal vesicle, Thiw is formed the "stem-nucleus," ot the nucleus of the "stem-cell" (cylula, Fig. 2). This now undergoes total segmentation, dividing into two, four, eight, sixteen, thirty-two cells, and so on. In this way we get the sphorical, mulberry-shaped body, which we call the morn la. The segmentation of the Amphioxus is not entirely regular, as was supposed after the first observations of Kowalevsky (1866). It is not completely equal, but a little unequal. As Hatschek afterwards found (1879), the segmentation-cells only remain equal up to the morula-stage, the spherical body of which consists of thirty- two cells. Then, as always happens in unequal segmentation, the more sluggish vegetal cells are outstripped in the cleavage. At the lower or vegetal pole of the ovum a crown of eight large ento- dermic cells remains for a long time unchanged, while the other cells divide, owing to the formation of a series of horizontal circles, into an increasing number of crowns of sixteen cells each. Afterwards the segmentation-cells get more or less irregularly displaced, while the segmentation-cavity enlarges in the centre of the morula ; in the end the former all lie on the surface of the latter, so that the foetus attains the familiar blastula shape and forms a hollow ball, the wall of which consists of a single Stratum of cells (Fig. 38 A ('). This layer is the blastoderm, the simple epi- thelium from the cells of which all the tissues of the body proceed. EM BRYOLOGY OF THE /.. \ \( 7. LE T . I ND THE SEASQl 7RT 193 These important early embryonic pro- cesses take place SO quickly in the Amphi- oxus that four or five hour- alter fecunda- tion, or about midnight, the spherical blastula is completed. A pit-like depres- sion is then formed at the vegetal pole of it, and in consequence oi this the hollow sphere doubles on itself (Fig. 38 />>. This pit becomes deeper and deeper (Fig. 38 E, /• i ; at last the invagination (or doubling is complete, and the inner or folded part oi the blastula-wall lies on the inside of the outer wall. We thus gel a hollow hemisphere, the thin wall oi which is made up oi two layers of cells (Fig. 38 E). From hemispheric.il the body soon becomes almost spherical once more, and then oval, the internal oavib enlarging considerably and its mouth growing narrower (Fig. 213). The form which the Amphioxus-embryo has thus leached is a real " cup-!ar\ a " or gastrula, of the original simple type that we have previously described as the " bell-gas- trula " or atthigastrula (Figs. 29-35). As in all the other animals that form an archigastnila, the whole body is nothing but a simple gastric sac or stomach ; its internal cavity is the primitive gut ( pro- ifdsftr or anhcntcroji. Fig. 38 g, 35 dj, and its aperture the primitive mouth ( prostoma ox blastoporus, o). The wall is at once gut-wall and body-wall. It is composed oi two simple cell-layers, the familiar primary germinal layers. The inner layer or the invaginated part of the blastoderm, which immediately en- closes the gut-cavity is the entoderm, the inner or vegetal germ-layer, from which develop the wall oi the alimentary canal and all its appendages, the ccelom- pouches, etc. (Figs. 35, 36 i). The outer stratum of cells, or the non-invagi- nated part of the blastoderm, is the ecto- derm, the outer or animal germ-layer, which provides the outer skin (epidermis) and the nervous system ffj. The cells oi the entoderm are much larger, darker, and more fatty than those oi the ectoderm, which are clearer and less rich in fatty particles. Hence before and during in- vagination there is an increasing differen- tiation oi the inner from the outer layer. The animal cells of the outer layer soon develop vibratorv hairs ; the vegetal cells of the inner layer d>> SO nuuli later. A thread-like process grows out oi each cell, and effects continuous vibratory movements. By the vibrations of these blender hairs the t^astrula of the Amphi- OXUS swims about in the sea, when U has pierced the thin ovolcmma, like the gas- trula o\' many other animals (Fig. 36). As in many other lower animals, the cells have only one whip-like hair each, and so are called flagetlat* (whip) cells (in contrast with the ciliated cells, which have a number of short lashes or cilia). In the further course oi its rapid development the roundish bell-gastrula becomes elongated, and begins to flatten on one side, parallel to the long axis. The flattened side is the subsequent dorsal side ; the opposite or ventral side remains curved. The latter grows more quickly than the former, with the result that the primitive mouth is forced to the dorsal side (Fig. 39). In the middle of the dorsal surface a shallow longitudinal groove or fuiTOW is formed (Fig. 79), and the edges oi the body rise up on cat. h side oi this groove in the shape of two parallel swellings. This groove is, of course, the dorsal furrow, and the swell- ings are the dorsal or medullary swellings ; they form the first structure of the central nervous system, the medullary tube. The medullary swellings now rise higher; the groove between them becomes deeper and deeper. The edges of the parallel swell- ings curve towards each other, and at last unite, and the medullary tube is formed (Figs. 83 m, 84 m). Hence the formation of a medullary tube out of the outer skin takes place in the naked dorsal surface of the free-swimming larva of the Amphioxus in just the same way as we have found in the embryo of man and the higher animals within the fcetal mem- branes. Simultaneously with the construction oi the medullary tube we have in the Amphioxus-embryo the formation of the chorda, the ccelom-pouches, and the meso- derm proceeding from their wall. These processes also take place with charac- teristic simplicity and clearness, so that they are very instructive to compare with the vermalia on the one hand and with the higher vertebrates on the other. While the medullary groove is sinking in the middle line of the flat dorsal side of the oval embryo, and its parallel edges unite 10 form the ectodermic neural tube, the single chorda is formed directly under- neath them, and on each side of this a parallel longitudinal fold, from the dorsal wall oi the primitive gut. These longi- tudinal folds oi the entoderm proceed from the primitive mouth, or from its lower 194 EMBRYOLOGY OF THE LANCELET AND THE SEA-SQUIRT and hinder edge. Here we see at an early stage .1 couple of large entodermtc cells, which are distinguished from all the others by their great sue, round form, and fine- grained protoplasm ; they are the two promesoblasts, o\- polar coils oi' the meso- derm (Fig. 83 p). They indicate the Original starting-point oi the two CCelom- pouches, which grow from this spot between the inner and outer germinal layers, sever themselves from the primi- tive gut, and provide the cellular material lor the middle layer. Immediately after their formation the two ccelom-pouches of the Amphioxus are divided into several parts by longitudinal and transverse folds. Each of the primary pouches is divided into an upper dorsal and a lower ventral section by a couple of lateral longitudinal folds (Fig. 82). But these are again divided by several parallel transverse folds into a number of succes- sive sacs, the primitive segments or somites (formerly called by the unsuitable name of " primitive vertebras"). They have a differ- ent future above and below. The upper or dorsal segments, the episomites, lose their cavity later on, and form with their cells the muscular plates of the trunk. The lower or ventral segments, the hyposomites, corresponding to the lateral plates of the craniote-embryo, fuse together in the upper part owing to the disappearance of their lateral walls, and thus form the later body-cavity (metaccel) ; in the lower part they remain separate, and afterwards form the segmental gonads. In the middle, between the two lateral coelom-folds of the primitive gut, a single central organ detaches from this at an early stage in the middle line of its dorsal wall. This is the dorsal chorda (Figs. 83, 84 ch). This axial rod, which is the first foundation of the later vertebral column in all the vertebrates, and is the only representative of.it in the Amphioxus, originates from the entoderm. In consequence of these important folding-processes in the primitive gut, the simple entodermic tube divides into four different sections : — I., underneath, at the ventral side, the permanent alimen- tary canal or permanent gut ; II., above, at the dorsal side, the axial rod or chorda ; and III., the two ccelom-sacs, which immediately sub-divide into two struc- tures— IIIa., above, on the dorsal side, the episomites, the double row of primitive or muscular segments ; and IIIb., below, on each side of the gut, the hyposomites, the two lateral plates th.it give rise to the sex-glands, and the cavities of which partly unite to form the body-cavity. At the same time, the neural or medullary tube is formed above the chorda, on the dorsal surface, by the closing of the parallel medullary swellings. All these processes, which outline the typical structure of the vertebrate, take place with astonishing rapidity in the embryo of the Amphioxus ; in the afternoon of the first day, or twenty- four hours after fertilisation, the young vertebrate, the typical embryo, is formed ; it then has, as a rule, six to eight somites. The chief occurrence on the second day of development is the construction of the two permanent openings of the gut — the mouth and anus. In the earlier stages the alimentary tube is found to be entirely closed, after the closing of the primitive mouth ; it only communi- cates behind by the neurenteric canal with the medullary tube. The permanent mouth is a secondary formation, at the opposite end. Here, at the end of the second day, we find a pit-like depression in the outer skin, which penetrates inwards into the closed gut. The anus is formed behind in the same way a few hours later (in the vicinity of the additional gastrula- mouth). In man and the higher verte- brates also the mouth and anus are formed, as we have seen, as flat pits in the outer skin ; they then penetrate inwards, gradually becoming connected with the blind ends of the closed gut-tube. During the second day the Amphioxus- embryo undergoes few other changes. The number of primitive segments in- creases, and generally amounts to four- teen, some forty-eight to fifty hours after impregnation. Almost simultaneously with the forma- tion of the mouth the first gill-cleft breaks through in the fore section of the Amphi- oxus-embryo (generally forty hours after the commencement of development). It now begins to nourish itself independently, as the food material stored up in the ovum is completelyusedup. The furtherdevelop- ment of the free larvae takes place very slowly, and extends over several months. The body becomes much longer, and is compressed at the sides, the head-end being broadened in a sort, of triangle. Two rudimentary sense-organs 'are deve- loped in it. Inside we find the first blood- vessels, an upper or dorsal vessel, corre- sponding to the aorta, between the gut and the dorsal cord, and a lower or ventral EMBRYOLOGY OF THE LANCE LET AND THE SEASQUiRT 195 vessel, corresponding to the subintestinal vein, at the lower border of the gut. Now, the gills of respirator* organs also are formed .it the fore-end of the alimen- tary canal. The whole of the anterior ov respiratory section of the guj is converted into a gil(-crate, which is pierced trellis- wise by numbers of branchial-holes, as in the ascidia This is done by the foremost' part of the gut-wall joining star-wise with the outer skin, and the. formation ofclefts Fig. 222 Fig. 223. at the point of connection, piercing the wall and leading into the gut from with- out. At first there are very few of these branchial clefts ; but there are soon a number of them — first in one, then in two, rows. The foremost gill-cleft is the oldest. In the end we have a sort of lattice work of fine gill-clefts, supported on a number of stiff branchial rods; these arc connected in pairs by transverse rods. At an early stage of embryonic develop- ment the sini. turcofthe rVmphioausrlarva is substantial!) the same as the Weal picture we have previously formed of the "Primitive Vertebrate" ( Figs. 98-102). Bui the bod) aftei w mis undergoes various modifications, especially in the fore-part. These modifications do not concern us, as they depend on special adaptations, and do 'not affect tlie- hereditary vertebrate type. When the free-swimming Amphi- Figs. 222-224. -Transverse sections of youngr Amphioxus-larVSB (diagrammatic, from Ralph.) (Cf. also Fig. 216.) In Fig. izi there is free communication from without with the gut-cavity ( />) through the gill- defUfATjL In Fig. 223 the lateral folds of the body- wall, or the gill-covers, which grow downwards, arc formed. In Fig. 224 these lateral folds have united underneath and joined their edges in the middle line of the Ventral side (R seam). The respiratory water now pencil from the gut-cavity ( D) into the mantle-cavity (A). The letters have the same meaning throughout : A' medullary tube, Ch chorda, M lateral muscles, Lh body-Cavity, G part of the body-cavity in which the sexual organs are subsequently formed. D gut-cavity, clothed with the gut-gland layer, a >. A mantle-cavity, A'gill-clefls. b = E epidermis, Ei the same as visceral epithelium of the mantle-cavity, £j as parietal epithe- lium of the mantle-cavity. oxus-larva is three months old, it abandons its pelagic habits and changes into the young animal that lives in the sand. In spite of its smallness (one-eighth of an inch), it has substantially the same struc- ture as the adult. As regards the remain- ing organs of the Amphioxus, we need only mention that the gonads or sexual glands are developed very late, imme- diately out of the inner cell-layer of the io6 EMBRYOLOGY OF '/'///■: LANCELET AND THE SEA-SQUIRT body-cavity. Although we can find after- wards no continuation oi the body-cavitv (Fig. 216 (') in the Lateral walls of the mantle -cavity, in the gill-covers 01 mantle-folds (Fig. 224 c'), there is one present in the beginning (Fig. 224 /.//). The sexual cells are formed below, at the bottom of tliis continuation (Fig. 224 S). For the rest; the subsequent development into the adult Amphioxus o( the larva we have followed is so simple that we need not go further into it here. We may now turn to the embryology of the Aseidia, an animal that seems to stand so much lower and to be so much more simply organised, remaining for the greater part of its life attached to the bottom of the sea like a shapeless lump. It was a fortunate accident that Kowa- levsky first examined just those larger specimens of the Aseidia? that show most clearly the relationship of the vertebrates to the invertebrates, and the larvae of which behave exactly like those of the Amphioxus in the first stages of develop- ment. This resemblance is so close in the main features that we have only to repeat what we have already said of the ontogenesis of the Amphioxus. The ovum of the larger Aseidia {Phal- lusia, Cynthia, etc.) is a simple round cell of via to rhz of an inch in diameter. In the thick fine-grained yelk we find a clear round germinal vesicle of about rW of an inch in diameter, and this encloses a small embryonic spot or nucleolus. Inside the membrane that surrounds the ovum, the stem-cell of the Aseidia, after fecundation, passes through just the same metamorphoses as the stem-cell of the Amphioxus. It undergoes total seg- mentation ; it divides into two, four, eight, sixteen, thirty-two cells, and so on. By continued total cleavage the morula, or mulberry-shaped cluster of cells, is formed. Fluid gathers inside it, and thus we get once more a globular vesicle (the blastula) ; the wall of this is a single stratum of cells, the blastoderm. A real gastrula (a simple bell-gastrula) is formed from the blastula by invagination, in the same way as in the amphioxus. Up to this there is no definite ground in the embryology of the Ascidiae for bringing them into close relationship with the Vertebrates ; the same gastrula is formed in the same way in many other animals of different stems. But we now find an embryonic process that is peculiar to the Vertebrates, and that proves irre- fragabl) the affinity of the Ascidiae to the Vertebrates. From the epidermis of the gastrula a medullary tube is formed on the dorsal side, and, between this and the primitive gut, a chorda, these are the Organs that are otherwise only found in Vertebrates. The formation of these very important organs takes place in the Ascidia-gastrula in precisely the same way as in that of the Amphioxus. In the Aseidia (as in the other case) the oval 1 gastrula is first flattened on one side — the Subsequent dorsal side. A groove or furrow (the medullary groove) is sunk in the middle line of the Hat surface, and two parallel longitudinal swellings arise on either side from the skin layer. These medullary swellings join together over the furrow, and form a tube; in this case, again, the neural or medullary tube is at first open in front, and connected with the primitive gut behind by the neuren- teric canal. Further, in the Ascidia-larva also the two permanent apertures of the alimentary canal only appear later, as independent and new formations. The permanent mouth does not develop from the primitive mouth of the gastrula ; this primitive mouth closes up, and the later anus is formed near it by invagination from without, on the hinder end of the body, opposite to the aperture of the medullary tube. During these important processes, that take place in just the same way in the Amphioxus, a tail-like projection grows out of the posterior end of the larv; -hody, and the larva folds itself up within the round ovolemma in such a way that the dorsal side is curved and the tail is forced on to the ventral side. In this tail is developed — starting from the primitive gut — a cylindrical string of cells, the fore end of which pushes into the body of the larva, between the alimentary canal and the neural canal, and is no other than the chorda dorsalis. This important organ had hitherto been found only in the Vertebrates, not a single trace of it being discoverable in the Invertebrates. At first the chorda only consists of a single row of large entodermic cells. It is afterwards composed of several rows of cells. In the Ascidia-larva, also, the chorda developes from the dorsal middle part of the primitive gut, while the two coelom-pouches detach themselves from it on both sides. The simple body-Cavity is formed by the coalescence of the two. When the Ascidia-larva has attained EMBRYOLOGY OF THE LANCE LET AND THE SEA-SQUIRT 197 this stage of development it begins to move about in the ovolemma. This causes the membrane to burst. The" larva emerges From it, and swims about in the sea In means of its oar-like tail. These free-swimming larva' of the Ascidia have been known lor a long time. They were first observed h\ Dai win during his voyage round the world in 1833. They resemble tadpoles in outward appearance, and use their tails as 08X8, as the tadpoles do. However, this livelv and highly-developed condition does not last long. At first there is a progressive development; the foremost pari of the medullary tube enlarges into a brain, and inside this two single sense-OTgans are developed, a dorsal audilorv vesii le and a ventral Bye. Then a heart is formed on the ventral side of the animal, Of the lower wall of the gut, in tin. same simple form and at the same spot at which the heart is developed in man and all the other vertebrates. In the lower muscular wall of the gut we find a weal- like thickening, a solid, spindle-shaped String of cells, which becomes hollow in the centre ; it begins to contract in different directions, now forward and now- backward, as is the case with the adult Ascidia. In this way the sanguineous fluid accumulated in the hollow muscular tube is driven in alternate directions into the blood-vessels, which develop at both ends of the cardiac tube. One principal vessel runs along the dorsal side of the gut, another along its ventral side. The former corresponds to the aorta and the dorsal vessel in the worms. The other corresponds to the subintestinal vein and the ventral vessel of the worms. With the formation of these organs the progressive development of the Ascidia comes to an end, and degeneration sets in. The free-swimming larva sinks to the floor of the sea, abandons its locomotive habits, and attaches itself to stones, marine plants, mussel-shells, corals, and other objects ; this is done with the part of the body that was foremost in move- ment. The attachment is effected by a number of out-growths, usually three, which can be seen even in the free- swimming larva. The tail is lost, as there is no further use for it. It under- goes a fatty degeneration, and disappears with the chorda dorsalis. The tailless body changes into an unshapely lube, and, by the atrophy of some parts and the modification of others, gradually assumes the appe.uaiue we have already deS4 i ibed. Among the living Tunicates there is a verv interesting group of small animals thai remain throughout life at tin- stage oi development of tlu- tailed, free Ascicha- larva, and swim about briskly in the sea In means of their broad oar-tail. These are the rental kahle Lopvlala (. l/>/>cn n^iunn. iuiaju r KJn inn u/vi\^nun t sai\ij inn jo.imui / n t branchial clefts. These instructive Cope- i.u.i, comparable to permanent Ascidia- larv.i1, come next to the extinct Prochor- donia, those ancient worms which we must regard as the common ancestors of i lu- [untcates and Vertebrates. Thechorda of the Appendicaria is a lonj*-, cylindrical string ( K i t^" . 22C < "), and serves as an attachment for the muscles that work the Bat oar-tail. Among the various modifications which the Ascidia - larva undergoes after its establishment at the sea-fioor, the most interesting (after the loss of the axial rod) is the atrophy oi one of its chief organs, the medullary tube. In the Amphioxus the spinal marrow continues* to develop, but in the Ascidia the tube soon shrinks into a small and insignificant nervous ganglion that lies above the mouth and the gill-crate, and is in accord with the extremely slight mental power of the animal. This insignificant relic of the medullary tube seems to be quite beyond comparison with the nervous centre of the vertebrate, yet it started from the same structure as the spinal cord of the Amphioxus. The sense-organs that had been developed in the fore part of the neural tube are also lost ; no trace of them can be found in the adult Ascidia. On the other hand, the alimentary canal becomes a most extensive organ. It divides presently into two sections — a wide fore or branchial gut that serves for respiration, and a narrower hind or hepatic gut that accomplishes digestion. The branchial or head-gut of the Ascidia is small at' first, and opens directly out- wards only by a couple of lateral ducts or gill-clefts — a permanent arrangement in the Copelata. The gill-clefts are developed in the same way as in the Amphioxus. As their number greatly increases we get a large gill-crate, pierced like lattice work. . In the middle line of its ventral side we find the hypobranchial groove. The mantle or cloaca-cavity (the atrium) that surrounds the gill-crate is also formed in the same way in the Ascidia as in the Amphioxus. The eject ion-opening Of this penbranchial cavity corresponds to the branchial pore of the Amphioxus. In the adult Ascidia the branchial gut and the heart on its ventral side are almost the oulv organs that recall the original affinity with the vertebrates. The further development of the Ascidia in detail has no particular interest for us, and we will not go into it. The chief result that we obtain from its embryo- logy is the complete agreement with that of the Amphioxus in the earliest and most important embryonic stages. They do not begin to diverge until after the medullary tube and alimentary canal, and the axial rod with the muscles between the two, have been formed. The Amphioxus continues to advance, and resembles the embryonic forms of the higher vertebrates ; the Ascidia degene- rates more and more, and at last, in its adult condition, has the appearance of a very imperfect invertebrate. v If we now look back on all the remark- able features we have encountered in the structure and the embryonic development of the Amphioxus and the Ascidia, and compare them with the features of man's embryonic development which we have previously studied, it will be clear that I have not exaggerated the importance of these very interesting animals. It is evident that the Amphioxus from the vertebrate side and the Ascidia from the invertebrate form the bridge by which we can span the deep gulf that separates the two great divisionsof the animal kingdom. The radical agreement of the lancelet and the sea-squirt in the first and most impor- tant stages of development shows some- thing more than their close anatomic affinity and their proximity in classifica- tion ; it shows also their real blood-relation- ship and their common origin from one and the same stem-form. In this way, it throws considerable light on the oldest roots of man's genealogical tree. DURATION OF THE HISTORY OF OUR STEM '99 Chapter XVI 1 1. DURATION OF THE HISTORY OF OUR STEM Our comparative investigation of the I anatomy and ontogeny of the Amphioxus and Ascidia has given ns invaluable assistance. We have, in the Brsl place, bridged the wide gulf that has existed up , to the present between the Vertebrates and Invertebrates ; and, in the second place, we have discovered in the embryo- logy of the Amphioxus a number of ancient evolutionary stages that have long since disappeared from human embryology, and have been lost, in virtue of the law of curtailed heredity. The chief of these stages are the spherical blastula (in its simplest primary form) and the succeeding archigastrula, the pure, original form of the gaslrula which the Amphioxus has preserved to this day, and which we find in the same form in a numberof Invertebrates of various classes. Not less important are the later embryonic forms of the ccelomula, the chordula, etc. Thus the embryology of the Amphioxus and the Ascidia has so much increased our knowledge of man's stem-history that, although our empirical information is still very incomplete, there is now no defect of any great consequence in it. We may now, therefore, approach our proper task, and reconstruct the phylo- geny of man in its chief lines with the aid of this evidence of comparative anatomy and ontogeny. In this the reader will soon see the immense importance of the direct application of the biogenetic law. But before we enter upon the work it will be useful to make a few general observa- tions that are necessary to understand the processes aright. We must say a few words with regard to the period in which the human race was evolved from the animal kingdom. The first thought that occurs to one in this connection is the vast difference between the duration of man's ontogeny and _ phylogeny. The individual man needs only nine months for his complete development, from the fecundation of the ovum to the moment when he leaves the maternal womb. The human embryo runs its whole course in the brief space of forty weeks (as a rule, 280 days). In many other mammals the time of the embryonic development is much the same as in man --for instance, in the cow. In the horse and ass it takes a little longer, forty-three to forty-five weeks ; in the camel, thirteen months. In the largest mammals, the embryo needs a much longer period for its development in the womb- a year and a half in the rhinoceros, and ninety weeks in the elephant. In these cases pregnancy lasts twice as long as in the case of man, or one and three- quarter years. In the smaller mammals the embryonic period is much shorter. The smallest mammals, the dwarf-mice, develop in three weeks ; liares in four weeks, rats and marmots in five weeks, the dog in nine, the pig in seventeen, the sheep in twenty-one and the goat in thirty-six. Birds develop still more quickly. The chick only needs, in normal circumstances, three weeks for its full development. The duck needs twenty- five days, the turkey twenty-seven, the peacock thirty-one, the swan forty-two, and the cassowary sixty-five. The smallest bird, the humming-bird, leaves the egg after twelve days. Hence the duration of individual development within the foetal membranes is, in the mammals and birds, clearly related to the absolute size of the body of the animal in question. But this is not the only determining feature. There are a number of other circumstances that have an influence on the period of embryonic development. In the Amphioxus the earliest and most important embryonic processes take place so rapidly that the blastula is formed in four hours, the gastrula in six, and the typical vertebrate form in twenty-four. In every case the duration of ontogeny shrinks into insignificance when we com- pare it with the enormous period that has been necessary for phylogeny, or the gradual development _of the ancestral series. This period is not measured by years or centuries, but by thousands and millions of years. Many millions of years had to pass before the* most advanced OCR AT/ON OF THE //IS TORY OF OUR STEM vertebrate, man, was evolved, step bj step, from his ancient unicellularances,tors. I'hi- opponents of evolution, who declare that this gradual development of the human form from lower animal forms, and ultimately from a unicellular organism, is an incredible miracle, forget that the same miracle lakes place within the space oi nine months in the embryonic development o( every human being. Each of us has, in the forty weeks properly speaking, in the first four weeks o( his development in the IVOmb, passed through the same series o\ transformations that our animal ancestors underwent in the course of millions of years. It is impossible to determine even approximately, in hundreds or even thou- sands of years, the real and absolute dura- tion of the phylogenetic period. But for some time now we have, through the research of geologists, been in a position to assign the relative length of the various sections of the organic history of the earth. The immediate data for determining this relative length of the geological periods are found in the thickness of the sedi- mentary strata — the strata that have been formed at the bottom of the sea or in fresh water from the mud or slime deposited there. These successive layers of limestone, sandstone, slate, marl, etc., which make up the greater part of the rocks, and are often several thousand feet thick, give us a standard for computing the relative length of the various periods. To make the point quite clear, I must say a word about the evolution of the earth in general, and point out briefly the chief features of the story. In the first place, we encounter the principle that on our planet organic life began to exist at a definite period. That statement is no longer disputed by any competent geolo- gist or biologist. The organic history of the earth could not commence until it was possible for water to settle on our planet in fluid condition. Every organism, without exception, needs fluid water as a condi- tion of existence, and contains a consider- able quantity of it. Our own body, when fully formed, contains sixty to seventy per cent, of water in its tissues, and only thirty to forty per cent, of solid matter. There is even more water in the body of the child, and still more in the embryo. In the earlier stages of development the human foetus contains more than ninety per cent, of water, and not ten per cent, of solids. In the lower marine animals, especially certain medusa1, the body con- sists to tlie extent of more than ninety- nine per cent. o\' sea-water, and has not one per cent, of solid matter. No organism can exist or discharge its functions with- out water. No water, no life ! But tluid water, on which the existence of life primarily depends, could not exist in\ our planet until the temperature of the surface of the incandescent sphere had sunk to a certain point. Up to that time it remained in the form of steam. But as soon as the first fluid water could be con- densed from the envelope of steam, it began its geological action, and has con- tinued down to the present day to modify the solid crust of the earth. The final outcome of this incessant action of the water — wearing down and dissolving the rocks in the form of rain, hail, snow, and ice, as running stream or boiling surge — is the formation of mud. As Huxley says in his admirable Lectures on the Causes of Phenomena in Organic Nature, the chief document as to the past history of our " earth is mud ; the question of the history of past ages resolves itself into a question about the formation of mud. As I have said, it is possible to form an approximate idea of the relative age of the various strata by comparing them at different parts of the earth's surface. Geologists have long been agreed that there is a definite historical succession of the different strata. The various super- imposed layers correspond to successive periods in the organic history of the earth, in which they were deposited in the form of mud at the bottom of the sea. The mud was gradually converted into stone. This was lifted out of the water owing to variations in the earth's surface, and formed the mountains. As a rule, four or five great divisions are distinguished in the organic history of the earth, corre- sponding to the larger and smaller groups of the sedimentarv strata. The larger periods are then sub-divided into a series of smaller ones, which usually number from twelve to fifteen. The comparative thickness of the groups of strata enables us to make an approximate calculation of the relative length of these various periods of time. We cannot say, it is true, " In a century a stratum of a certain thickness (about two feet) is formed on the average ; therefore, a layer 1,000 feet thick must be 500,000 years old." Different strata of the same thickness may need very different periods for their formation. But from DCRAT/OX OF THE HISTORY OF OCR STFAf 20 1 the thickness or size of the stratum we tiin draw some conclusion as to t Lie relative length of the period. The first and oldest oi the four or five chief divisions of the organic history of the earth is called the primordial, archaic, or archeozoic period. It' wo compute the total a\ a age thickness of the sedimentary Strata at about 130,000 feet, this first (7 : (>) indicate possibly 0 : <>. Of late years the thickness of the archaic rocks has been put at i>o,txx> feet. The primordial period falls into three subordinate sections the Laurentian, Huronian, and Cambrian, corresponding to the three chief groups of rocks that comprise the archaic formation. The immense period during which these rocks SYNOPSIS OF THE PALEONTOLOGICAL FORMA- TIONS, OR THE FOSSILIFEROUS STRATA OF THE CRUST Groups. Systems. Formations. Synonyms of Formations. V. Anthropolithic fjroupv or anthroposoic (quaternary) gTOirps of strata. I\". Cenolithic groups, or cenosoic (tertiary) groups of strata. III. Mesolithic groups. or mesoxoic ndary) groups of strata. II. Paleolithic- groups, or paleozoic (primary) groups of strata. I. Archeolithio groups, or archeo/oic (primordial) groups oi strata. XIV. Recent (alluvium). XIII. Pleistocene (diluvium). XII. Pliocene (neo-tertiary I. XI. Miocene (middle tertiary) Xb. Oligocene (old tertiary). Xa. Eocene (primitive tertian '• IX. Chalk (cretaceous). VIII. Jurassic. VII. Triassic VIb. Permian. Via. Carboniferous (coal-measures). V Devonian. IV Silurian. III. Cambrian. II. Huronian. I. Laurentian. ,i.v 3+ 33- { I A. Present K event. Post-glacial. Glacial. Arverne. Subapcnninc. Falun. Limbourg. Aquitaine. Ligurium. Gypsum. Coarse chalk. London c Li \ . White chalk. Green sand. Neoconian. Wealden. Portland. Oxford. Hath. Lias. Keuper. Muschelkalk. Buntcr. Zechstein. Neurot sand. Carboniferous sandstone. Carboniferous limestone. Pilton. Ilfracombe. Linton. Ludlow. Wenlock. Llandeilo. Potsdam. Longrnj nd. Labrador. Ottawa. t'pper alluvial. Lower alluvial. Upper diluvial. Lower diluvial. t'pper pKocene. Lower pliocene. L'pper tniocene. Lower miocene. Upper oligocene. Lower ohgocene. l'pper eocene. Middle eocene. Lower eocene. l'pper cretaceous. Middle cretaceous. Lower cretaceous. Weald-tormation. Upper oolithic. Middle oolithic. Lower oolithic. Liassic Upper triassic. Middle triassic. Lower triassic. l'pper permian. Lower permian. Upper carboniferous. Lower carboniferous. Upper Middle Lower l'pper Middle- Lower l'pper Lou er l'pper Lower devonian, devonian, devonian. silurian. silurian. silurian. Cambrian. Cambrian, laurentian. laurentian. period comprises 70,000 feet, or the greater part of the whole. For this and other reasons we may at once conclude that the corresponding primordial or archeolithic period must have been in itself much longer than the whole of the remaining periods together, from its close to the {>resent day. It was probably much onger than the figures I have, quoted were forming in the primitive ocean probably comprises more than 50,000,000 wars. At the commencement of it the oldest and simplest organisms were formed by spontaneous generation— the Monera, with which the history of life on our planet opened. From these were first developed unicellular organisms of the simplest character, the Frotophyta 2o: DURATION OF THE HISTORY OF OUR STEM and Protozoa (paulotomea, amoebae, rhizo- pods, infusoria, and Other PrOtlSts). During this period the whole of the invertebrate ancestors of the human race .were evolved from the unicellular organ- isms. We can deduce this from the fact that we already find remains of fossilised fishes (Selachii and Ganoids) towards the close of the following Silurian period. These are much more advanced and much younger than the lowest vertebrate, the Ampllioxus, and the numerous skull-less vertebrates, related to the Amphioxus, that must have lived at that time. The whole oi the invertebrate ancestors of the human race must have preceded these. The primordial age is followed by a much shorter division, the paleozoic or Primary age. It is divided into four long periods, the Silurian, Devonian, Carboni- ferous, and Permian. The Silurian strata are particularly interesting because they contain the first fossil traces of vertebrates — teeth and scales of Selachii ( Paheodus) in the lower, and Ganoids ( Pteraspis) in the upper Silurian. During the Devonian period the "old red sandstone" was formed ; during the Carboniferous period were deposited the vast coal-measures that yield us ourchiefcombustive material; in the Permian (or the Dyas), in fine, the new red sandstone, the Zechstein (mag- nesian limestone), and the Kupferschiefer (marl-slate) were formed. The collective depth of these strata is put at 40,000 to 45,000 feet. In any case, the paleozoic age, taken as a whole, was much shorter than the preceding and much longer than the subsequent periods. The strata that were deposited during this primary epoch contain a large number of fossils ; besides the invertebrate species there are a good many vertebrates, and the fishes prepon- derate. There were so many fishes, '•specially primitive fishes (of the shark type) ahd plated fishes, during the Devonian, and also during the Carboni- lerous and Permian periods, that we may describe the' whole paleozoic period as " the age oFfishes." Among the paleozoic plated fishes or Ganoids the Crossoptcrygii and the Ctenodipterina (dipneusts) arc of great importance. During this period some of the fishes began to adapt themselves to living on land, and so gave rise to the class of the amphibia. We find in the Carboniferous period fossilised remains of five-toed amphibia, the oldest terrestrial, air- breathing vertebrates. These amphibia increase in variety in the Permian epoch. Towards the close oi it we liiv.1 the first Am motes, I ho ancestors oi the three higher classes oi Vertebrates. These are lizard- like animals; the first to be discovered was the ProUrosaurus, from the marl at Eisenach. The rise oi the earliest Amniotes, among which must have been the common ancestor of the reptiles, birds, and mammals, is put back towards the close of the paleozoic age by the dis- covery of these reptile remains. The ancestors of our race during this period were at first represented by true fishes, then by dipneusts and amphibia, and finally by the earliest Amniotes, or the Protamniotes. The third chief section of the organic history of the earth is the Mesozoic or Secondary period. This again is sub- divided into three divisions : Triassic, Jurassic, and Cretaceous. The thickness of the strata that were deposited in this period, from the beginning of the Triassic to the end of the Cretaceous period, is altogether about 15,000 feet, or not half as much as the paleozoic deposits. During this period there was a very brisk and manifold development in all branches of the animal kingdom. There were especially a number of new and interest- ing forms evolved in the vertebrate stem. Bony fishes ( ' Teleostei) make their first appearance. Reptiles are found in extra- ordinary variety and number ; the extinct giant - serpents (dinosauria), the sea- serpents (halisauria), and the flying lizards (pterosauria)are the most remark- able and best known of these. On account of this predominance of the reptile-class, the period is called "the age oi reptiles." But the bird-class was also evolved during this period ; they certainlv originatell from some division of the lizard-like reptiles. This is proved by the embryological identity of the birds and reptiles and their comparative anatomy, and, among other features, from the circumstance that in this period there were birds with teeth in their jaws and with tails like lizards (Archajopteryx, Odontornis). Finally, the most advanced and (for us) the most important class of the verte- brates, the mammals, made their appear- ance during the mesozoic period. The earliest fossil remains of them were found in the latest Triassic strata- lower jaws of small ungulates and marsupials. More numerous remains are found a little later DURATION OF TIIF HISTORY OF OCR STEM *° 5 in the Jurassic, and 9ome in the Creta- ceous. All the mammal remains thai we have from this section belong tothe lowei promammals and marsupials ; among these were most certainly the ancestors Of the human race. On the other hand, we have not found a single indisputahle fossil of ant higher mammal (a placental) in the whole of this period. This division of tfie mammals, which includes man, was not developed until later, towards the Close of this ov in the following period. The fourth section of the organic history of the earth, the Tertiary or CenotOtC age, was much shorter than the pre- ceding. The strata that .were deposited during this period have a collective thick- ness of only about 3,000 feet. It is sub- divided into four sections— the Eocene, Oligocene, Miocene, and Pliocene. During these periods there was a very varied development of higher plant and animal forms ; the fauna and flora of our planet a'pproached nearer and nearer to the character that they bear to-day. In particular, the most advanced class, the mammals, began to preponderate. Hence the Tertiary period may be called "the age of mammals." The highest section of this class, the placentals, now made their appearance ; to this group the human race belongs. The first appearance of man, or, to be more precise, the develop- ment of man from some closely-related group of apes, probably falls in either the miocene or the pliocene period, the middle or the last section of the Tertiary period. Others believe that man properly so-called — man endowed with speech— was not evolved from the non-vspeaking ape-man (Pithecanthropus ) until the following, the anthropozoic, age. In this fifth and last section of the organic history of the earth we have the full development and dispersion of the various races of men, and so it is called the Anthropozoic as well as the Quaternary period. In the imperfect condition of paleontological and ethno- graphical science we cannot as yet give a confident answer to the question whether the evolution of the human race from some extinct ape or lemur took place at the beginning of this or towards the middle or the end of the Tertiary period. However, this much is certain: the development of civilisation falls in the anthropozoic age, and this is merely an insignificant fraction of the vast period of the whole history of life. When we VOL. J I remember this, it seems ridii ulous to restrict the word "history" to the civilised period. If we divide into a hundred equal parts the whole period of the history of life, from the spontaneous generation of the fust Monera to the present day, and if we then represent the relative duration of the five chief sect ions or ages, as calculated from the average thickness of the strata they contain, as percentages of this, we get something like the follow- ing relation : — I. Archeolithic or archeozoic (prim- ordial) age 53 6 II. Paleolithic or paleozoic (primary) age 32 1 III. Mesolithic or mesozoic (secon- dary) age ii 5 IV. Cenolithic or cenozoic (tertiary) age 2 3 V. Anthropolithic or anthropozoic (quaternary) age o 5 100 o In any case, the "historical period " is an insignificant quantity compared with the vast length of the preceding ages, in which there was no question of human existence on our planet. Even the im- portant Cenozoic or Tertiary period, in which the first placentals or higher mammals appear, probably amounts to little over two per cent, of the whole organic age. Before we approach our proper task, and, with the aid of our ontogenetic acquirements and the biogenetic law, follow step by step the paleontological development of our animal ancestors, let us glance for a moment at another, and apparently quite remote, branch of science, ' a general consideration of which will help us in the solving of a difficult problem. I mean the science of comparative philo- logy. Since Darwin gave new life to biology by his theory of selection, and raised the question of evolution on all sides, it has often been pointed out that there is a remarkable analogy between the development of languages and the evolution of species. The comparison is perfectly just and very instructive. We could hardly find a better analogy when we are dealing with some of the difficult and obscure features of the evolution of Species. In both cases we find the action of the same natural laws. All philologists of any competence in their science now agree that all human languages have been gradually evolved from very rudimentary beginnings. '1 he a/■/»• i //ox or THE /i/sroh'v or ocr stem 205 same posterity tint the stem-form reaFlj produced thousands ol years ago. It is certain ili.a man has descended from some extincl mammal ; and we should jusi .is certainh class this in the order of apes if we had it before us. It is equally certain tint this primitive ape descended in turn from an unknown lemur, and this from an extinct marsu- pial. But it is just .is clear that all these extinct ancestral forms can only be claimed as belonging to the living order of mammals in virtue of their essential internal structure and their resemblance in the decisive anatomic characteristics of each order. In external appearance* in the characteristics of thegeitus or spcries, they would differ more or less, perhaps \er\ considerably, from all living repre- sentatives of those orders. It is a uni- versal and natural procedure in phvlo- genetic development that the stem-forms themselves, with their specific peculiarities, have been extinct for some time. The forms that approach nearest to them among the living species are more or less perhaps \ci v substantially — different from them. Hence in our phylogenetic inquiry and in the comparative study of the living, divergent descendants, there can only be a question of determining- the greater or less remoteness of the latter from the ancestral form. Not a single one of the older stem-forms has continued unchanged down to our time. We find just the same thing in com- paring the various dead and living languages that have developed from a common primitive tongue. Ifweexamine our genealogical treeofthelndo-Germanfc languages m this light, we see at once that all the older or parent tongues, of which we regard the living varieties of the stem as divergent daughter or grand- daughter languages, have been extinct for some time. The Aryo-Romanic and the Slavo-Germanic tongues have com- pletely disappeared ; so also the Aryan, the Greco-Roman, the Slavo-Lettic, and the ancient Germanic. Even their daughters and grand-daughters have been lost ; all the living Indo-Germanic languages are only related in the sense that they are divergent descendants of common stem- forms. Some forms have diverged more, and some less, from the original stem- form. This easily demonstrable fact illustrates very well the analogous case of the origin of the vertebrate species. Phylogenetic VOL. II. comparative philology here viclds .» strong support to phylogenetic compara- tive ZOOlogy. Hut the one c.w\ adduce more direct evidence than the other, as the paleontological material of philology — the old monuments ^^f the extinct tongue — have been preserved much better than the paleontological material of /oology, the fossilised bones and imprints of verte- brates. We may, however, trace man's genea* logical tree not only as far as the lower mammals, but much further — to the amphibia, to the shark-like primitive fishes, and, in tine, to the skull-less verte- brates that closely resembled the Am- phioxus. Hut this must not be under- stood in the sense that the existing Amphioxus, or the sharks or amphibia of tO-day, can give us any idea of the ex- ternal appearance of these remote stem- forms. Still less must it be thought that the Amphioxus or any actual shark, or any living species of amphibia, is a real ancestral form of the higher vertebrates and man. The statement can only rationally mean that the living forms I have referred to are collateral lines that are much more closely related to the extinct stem-forms, and have retained the resemblance much better, than any other animals we know. They are still so like them in regard to their distinctive internal structure that we should put them in the same class with the extinct forms if we had these before us. But no direct descendants of these earlier forms have remained unchanged. Hence we must entirely abandon the idea of finding direct ancestors of the human race in their characteristic external form among the living species of animals. The essen- tial and distinctive features that still connect living forms more or less closely with the extinct common stem-forms lie in the internal structure, not the external appearance. The latter has been much modified by adaptation. The former has been more or less preserved by heredity. Comparative anatomy and ontogeny prove beyond question that man is a true vertebrate, and, therefore, man's special genealogical tree must be connected with that of the other Vertebrates, whk h spring from a common root with him. But we have also many important grounds in comparative anatomy and ontogeny for assuming a common origin for all the Vertebrates. If the general theory of 3o6 DURATION OF THE history OF OIK STEM evolution is correct, all the Vertebrates, including man, come from a single * oinmon an< estor, a long-extinct " Pi iml- live Vertebrate." 1 lence the genealogical tree of the Vertebrates is at the same time thai of the human race. Our task, therefore, of constructing mnn's genealog) becomes the larger aim of discovering the genealogy of the entire vertebrate stem. As we now know from the comparative anatomy and ontogeny of the Amphioxus and the Ascidia, this is in turn connected with the genealogical tree of the Invertebrates (directly with that of the Vermalia), but has no direct connec- tion with tlie independent stems of the Articulates, Molluscs, and fcchinoderms. If we do thus follow our ancestral tree through various stages down to the lowest worms, we come inevitably to the Gas/ma, that most instructive form that gives the clearest possible picture of an animal with two germinal layers. The Gastraea itself has originated from the simple multi- cellular vesicle, the Blasteea, and this in turn must have been evolved from the lowest circle of unicellular animals, to which we give the name of Protozoa. We have already considered the most im- portant primitive type of these, the uni- cellular Amoeba, which is extremely in- structive when compared with the human ovum.' With this we reach the lowest of the solid data to which we are to apply our biogenetic law, and by which we may deduce the extinct ancestor from the embryonic form. The amoeboid nature of the young ovum and the unicellular condition in which (as stem-cell or cytula) every human being begins its existence justify us in affirming that the earliest ancestors of the human race were simple amoeboid cells. Hut the further question now arises: " Whence came these first amoebae with which the history of life began at the commencement of the Laurentian epoch?" There is only one answer to this. The earliest unicellular organisms can only have been evolved from the simplest organisms we know, the Moncra. These are the simplest living things that we can conceive. Their whole body is nothing but a particle of plasm, a granule of living albuminous matter, discharging of itself ail the essential vital functions that form the materia! basis of life. Thus we come to the last, or, if you prefer, the first, question in connection with evolution — the question of the origin of the Monera. This is the real question of the origin of life, or of spontaneous generation. We have neither space nor occasion to go further in this Chapter into the ques- tion of spontaneous generation. For this I must refer the reader to the fifteenth chapter of the History of Creation, and especially to the second book of the General morphology, or to the essay o\\ " The Monera and Spontaneous Generation " in my Studies of the Monera and otherProtists.1 I have given there fully my own view of this important question. The famous botanist Nageli afterwards (1884) de- veloped the same ideas. I will only Say a few words here about this obscure question of the origin of life, in so far as our main subject, organic evolution in general, is affected by it. Spontaneous' generation, in the definite and restricted sense in which I maintain it, and claim that it is a necessary hypothesis in ex- plaining the origin of life, refers solely to the evolution of the Monera from inor- ganic carbon-compounds. When living things made their first appearance on our planet, the very complex nitrogenous compound of carbon that we call plasson, which is the earliest material embodiment of vital action, must have been formed in a purely chemical way from inorganic carbon-compounds. The first Monera were formed in the sea by spontaneous generation, as crystals are formed in the mother-water. Our demand for a know- ledge of causes compels us to assume this. If we believe that the whole inorganic history of the earth has proceeded on mechanical principles without an\ inter- vention of a Creator, and that the history of life also has been determined by the same mechanical laws ; if we see that there is no need to admit creative action to explain the origin of the various groups of organisms ; it is utterly irrational to assume such creative action in dealing with the first appearance of organic life on the earth. This much-disputed question of "spon- taneous generation " seems so obscure, because people have associated with the term a mass of very different, and often very absurd, ideas, and have attempted to solve the difficulty by the crudest experiments. The real doctrine of the spontaneous generation of life cannot possibly be refuted by experiments. ' Tin- Englinh reader « ill fin J a luminous and up-to- date chapter on the subject in HaeckeTs recently* written and translated Wonder* 0/ Life, - Trans. OUR PROTIST ANCESTORS 207 Every experiment that has a nega- tive result only proves that no organ- ism lias boon formed out of inorganic matter in the conditions- highly artificial conditions we have established. On the other hand, .it would be exceedingly diffi- cult to prove the theory by way 01 experi- ment ; and even if Monera were Mill formed dailj by spontaneous generation (which 'in quite possible), it would be very difficult, if not impossible, to find a solid proof of it. Those who will not admit the spontaneous generation of the first living things in our sense must have recourse to a supernatural miracle ; and this is, as a matter of fact, the desperate resource to which our " exact " scientists are driven, to the complete abdication of reason. A famous English physicist, Lord Kelvin (then Sir VV. Thomson), attempted to di>peiise with the hypothesis of spon- taneous generation by assuming that the organic inhabitants of the earth were developed from germs that came from the inhabitants of other planets, and that chanced to fall on our planet on fragments of their original home, or meteorites. This hypothesis found many supporters, among others the distinguished German physicist, Helmholtz. However, it was refuted in 1872 by the able physicist, Friedrich Zollner, of Leipzig, in his work, O/i tin- Nature of Comets. He showed i leai ly how unscientific this hypothesis is ; firstly in point of logic, and secondly in point of scientific content At the same time lie pointed out t h.it our hypothesis of spontaneous generation is " a necessary condition for understanding nature accord- ing to the law of causality.*1 1 repeat that we must tail in the aid of the In pothesis only as regards the Monera, the structureless K organisms without organs." Ever} complex organism must have been evolved from some lower organism. We must not assume the spon- taneous generation of even the simplest cell, for this itself consists of at least two parts tlie internal, firm nuclear sub- stance, and the external, softer cellular substance or the protoplasm of the cell- body. These two parts must have been formed by differentiation from the indif- ferent plasson ofa'moneron, or a cvtode. For this reason the natural history of the Monera is of great interest ; here alone can we find the means to overcome the chief difficulties of the problem of spon- taneous generation. The actual living Monera are specimens of such organless or structureless organisms, as they must have been formed by spontaneous genera- tion at the commencement of the history , of life. Chapter XIX. OUR PROTIST ANCESTORS Under the guidance of the biogenetic law, and on the basis of the evidence we have obtained, we now turn to the in- teresting task of determining the series of man's animal ancestors. 1'hvlogenv as a whole is an inductivescience. From the totality of the biological processes in the life {■•f plants, animals, and man we have gathered a confident inductive idea that the whole organic population of our plane! has been moulded on a harmonious law of evolution. All the interesting phenomena that we meet in ontogeny and paleontology, comparative anatomy and dysteleology, the distribution and habits of organisms- all the important general laws that we abstract from the phenomena of these sciences, and combine in har- monious unity — are the broad bases of our great biological induction. But when we come to (he application of this law, and seek to determine with ils aid the origin of the various species of organisms, we are compelled to frame aoS 0(/K /'A'Of/ST ANCESTORS hypotheses that have essentially a deduc- )ivc character, and are inferences from the genera] law to particular cases. But these special deductions are just as much justified and necessitated by the rigorous laws of logic as the inductive conclusions on which the whole theory of evolution is built. The doctrine of the animal ancestry of the human race is a special deduction of this kind, and follows with logical necessity from the general induc- tive law of evolution. 1 must point out at once, however, that the certainty of these evolutionary hypo- theses, which rest. on clear special de- ductions, is not always equally strong. Some of these inferences are now beyond question ; in the case of others it depends on the knowledge and the competence of the inquirer what degree of certainty he attributes to them. In any case, we must distinguish between the absolute -certainty of the general (inductive) theory of descent and the relative certainty of special (deductive) evolutionary hypo- theses. We can never determine the whole ancestral series of an organism with the same confidence with which we hold the general theory of evolution as the sole scientific explanation of organic modifications. The special indication of stem-forms in detail will always be more or less incomplete and hypothetical. This is quite natural. The evidence on which we build is imperfect, and always will be imperfect ; just as in comparative philo- logy. The first of our documents, paleon- tology, is exceedingly incomplete. We know that all the fossils yet discovered are only an insignificant fraction of the plants and animals that have lived on our planet. For every single species that has been preserved for us in the rocks there are probablyhundreds, perhaps thousands, of extinct species that have left no trace behind them. This extreme and very un- fortunate incompleteness of the paleonto- logical evidence, which cannot be pointed out too often, is easily explained. It is absolutely inevitable in the circumstances of the fossilisation of organisms. It is also due in part to the incompleteness of our knowledge in this branch. It must be borne in mind that the great majority of the stratified rocks that compose the crust of the earth have not yet been opened. We have only a few specimens of the innumerable fossils that are buried in the vast mountain ranges of Asia and Africa. Onl\ a | .ut o\ Europe and North America has been investigated carefully.. Tha whole of the fossHs known to us certainly do not amount to a hundredth part of the remains that are really buried in the ci ust of the earth. We may, therefore, look forward to a rich harvest in the future as regards this science. However, our paleontological evidence will (for reasons that I have fully explained In the sixteenth chapter of the History of Creation) always be defective. The second chief source of evidence, ontogeny, is not less incomplete. It is the most important source of all for special phytogeny ; but it has great defects, and often fails n\ We must, above all, clearly distinguish between palingenetic and cenogenetic phenomena. We must never forget that the laws of curtailed and disturbed heredity often make the original course of development almost unrecog- nisable. The recapitulation of phytogeny by ontogeny is only fairly complete in a few cases, and is never whole complete. As a rule, it is precisely the earliest and most important embryonic stages that suffer most from alteration and condensa- tion. The earlier embryonic forms have had to adapt themselves to new circum- stances, and so have been modified. The struggle for existence has had just as profound an influence on the freely moving and still immature young forms as on the adult forms. Hence in the embryology of the higher animals, especially, palin- genesis is much restricted by cenogenesis ; it is to-day, as a rule, only a faded and much altered picture of the original evolution of the animal's ancestors. We can only draw conclusions from the em- bryonic forms to the stem-history with the greatest caution and discrimination. Moreover, the embryonic development itself has only been fully studied in a few species. Finally, the third and most valuable source of evidence, comparative anatomy, is also, unfortunately, very imperfect ; for the simple reason that the whole of the living species of animals are a mere fraction, of the vast population that has dwelt on our planet since the be^innin^ of life. We may confidently put the total number of these at more than a million species. The number of animals whose organisation has been studied up to the present in comparative anatomy is propor- tionately very small. Here, again, future research will yield incalculable treasures. OCR PROTIST ANCESTORS J09 Bui, for the prc->».Mii, in view of this patent incompleteness of our chief sources of evidence, we must naturally be careful not to lay too much stress in human phytogeny on the particular animals we nave studied, or regard .ill the various siam- of development with equal confi- dence is stem-forms. In my first efforts to construct the of man's ancestors I drew up a list of, ai first ten, afterwards twenty to thirty, forms that may be regarded more or loss certainl) as animal ancestors of the human race, or as stages that in a sense mark off the chief sections in the long story of evolution from the unicellular organism to man. Of these twenty to thirty stages, ten to twelve belong to the older gTOUp of the Invertebrates and eighteen to twenty to the younger division of the Vertebrates. In approaching, now, the difficult task of establishing the evolutionary succes- sion of these thirty ancestors of humanity since the beginning of life, and in ventur- ing to lift the veil that covers the earliest secrets of the earth's history, we must undoubtedly look for the first living things among the wonderful organ- isms that we call the Monera ; they are the simplest organ- isms known to us — in fact, the simplest we can con- ceive. Their whole body con- sist > merely of a simple par- ticle or globule of structure- It 5S plasm or plasson. The discoveries of the last four decades have led us to believe with increasing certainty that wherever a natural body exhibits the vital processes of nutrition, reproduction, voluntary movement, and sensation, we the action o( a nitrogenous carbon- compound of the chemical group of the albuminoids ; this plasm (or protoplasm) is the material ha-is of all vital functions. Whether we regarded the function, in the monistic sense, as the direct action of the material substratum, or whether we take matter and force to be distinct things in the dualist ic sense, it is certain that we have not as vet found any living organism in whkh the exercise of the vital func- tions is not inseparably bound up with plasm. The SOfi slimy plasson of the bodv of the moncron is generally called " proto- plasm," and identified with the cellular matter ol the ordinary plant and animal cells. But we must, to be accurate, distinguish between the plasson of the cytodes and the protoplasm of the Cells. This distinction is of the utmost importance for the purposes of evolution. As I have often said, we must recognise two different Stages of development in these "elementary or- ganisms," or plastids ("builders"), that represent the ultimate units of organic individuality. The earlier and lower stage are the unnucleated cvtodes, the bodv of which consists of only one kind of albu- minous matter the homogeneous plasson or "formative matter." The later and higher stage are the nucleated cells, in which we find a differentiation of the original plasson into two different forma- tive substances the caryoplasm of the nucleus and the cytoplasm of the body of the cell (cf. pp. 37 and 42). The Monera are permanent- cytodes. Their whole body consists of soft, struc- tureless plasson. However carefully we examine it with our finest chemical reagents and most powerful microscopes, Fit;. 326 — Chroococcus minor f Xa 'gelt j. magnified 1.500 times. A phytomoneron. the globular plastids of which secrete a gelatinous struc- tureless membrane. The unnucleated globule of plasm (bluish-green in colour) increases by simple cleavage ( 11-ii ). we can find no definite parts or no anatomic structure in it. Hence, the .Monera are literally organisms without organs ; in fact, from the philosophic point of view they are not organisms at all, since they have no organs. They can only be called organisms in the sense that they are capable of the vital functions of nutrition, reproduction, sensation, and movement. If we were to try to imagine the simplest possible organism, we should frame some- thing like the moncron. The Monera that we find to-day in various forms fall into two groups accord- ing to the nature of their nutrition— the Pnytomonera and the Zoomonera ; from the physiological point of view, the former are (he simplest specimens of the plant (phvton) kingdom, and the latter of the animal (zoon) world. The Phvtomonci a, especially in their simplest form, the Chromacea ( Pkycochromacea or Cyano- phyctaji are the most primitive and the 2IO OCR PROTIST ANCESTORS oldest of living organisms. The typical genus ( 'JitWKoccus (Fig. 226) is represented m several fresh-water species, and often forms a very delicate bluish-green deposit on stones and wood In ponds and ditches. It consists of round, light green particles, from 7-nV, to TFVxr of an inch in diameter. The whole life of these homogeneous globule^ Of plasm consists of simple growth and reproduction by cleavage. When the tiny particle has reached a certain size by t ho continuous assimilation of inorganic matter, it divides into two equal halves, by a constriction in the middle. The two daughter-monera that are thus formed immediately begin a Ftc. 227. — Aphanocapsa primordialis (NSgeli), magnified 1.000 times. A phytomoneron, the round plas- ties of which (bluish-green in colour) secrete a shapeless gelatinous mas-. ; in this the unnucleated cytodes increase continually by simple cleavage, similar vital process. It is the same with the brown ProcytcUa primordialis (formerly called the Protococcus mannus) ; it forms large masses of floating matter in the arctic seas. The tiny plasma-globules of this species are of a greenish-brown colour, and have a diameter of i^^n to su.™ of an inch. There is no membrane discoverable in the simplest Chroococcacea, but we find one in othei members of the same family , in Aphanocapsa (Fig. 227) the enveloping membranes ol the social plastids combine ; in Glarocapsa they are retained through several generations, so that the little plasma-globules are enfolded in many layers of membrane. Next to the Chromacea come the Bac- teria, which have been evolved from them by the remarkable change in nutrition which gives us the simple explanation of the differentiation of plant and animal in the protist kingdom. The Chromacea build up their plasm directly from inor- ganic matter; the Bacteria feed on organic matter. Hence, if we logically divide the protist kingdom into plasma-forming Protophyta and plasma-consuming Pro- tozoa, we must class the Bacteria with the latter ; it is quite illogical to describe them — as is still often done — as Schizomy- cetes, and class them with the true fungi. The Bacteria, like the Chromacea, have no nucleus. As is well known, they play an important part in modern biology as the causes of fermentation and putre- faction, and of tuberculosis, typhus, cholera, and other infectious diseases, and as parasites, etc. But we cannot linger now to deal with these very interesting features ; the Bacteria have no relation to man's genealogical tree. We may now turn to consider the remarkable Protamceba, or unnucleated Amoeba. I have, in the first volume, pointed out the great importance of the ordinary Amoeba in connection with several weighty questions of general biology. The tiny Protamoeba?, which are found both in fresh and salt water, have the same unshapely form and irregular movements of their simple naked body as the real Amoebae ; but they differ from them very materially in having no nucleus in their cell-body. The short, blunt, finger-like processes that are thrust out at the surface of the creeping Protamceba serve for get- ting food as well as for locomotion. They multiply In simple cleavage (Fig. 228). The next stage to the simple cytode- forms of the Monera in the genealogy of mankind (and all' other animals) is the simple cell, or the most rudimentary form of the cell which we find living indepen- dently to-dav as the Amoeba. The earliest process of inorganic differentiation in the structureless body of the Monera led to its division into two different substances — the caryoplasm and the cytoplasm. The caryoplasm is the inner and tinner part of the cell, the substance of the nucleus. The cytoplasm is the outer and softer pari, the substance of the bods ol the cell, by this import. mi differentiation of the plasson into nucleus and cell-body, the OUR PR or l sr ANCESTORS 211 organised cell was evolved from the structureless cytode, the nucleated from the unnucleated plastid. Thai the first cells toappear on the eat ih were formed from the Monera by such a differentiation seems to u^ the only possible view in the present condition of science. We have a direct in- stance of this earliest process of differentia- tion to-day in the ontogeny of many of the lower Protists (such .is the Gregarinae). rhe unicellular form that we have in the ovum has already been described as the reproduction of a corresponding uni- cellular stem-form, and to this we have ascribed the organisation of an Amoeba (cf. Chapter VI.). The irregular-shaped Amoeba, which we find living independently to-day in our fresh and salt water, is the least definite and the most primitive of all the unicellular Protozoa (rig. 16). As the unripe ova (the protova that we find in the ovaries of animals) cannot be distinguished from the common Amoebae, we must regard the Amoeba as the primitive form that is repro- duced in the embryonic stage of the amoeboid ovum to-day, in accor- dance with the biogenetic law. 1 have already pointed out, in proof of the striking resemblance of the two cells, that the ova of many of the sponges were formerly regarded as parasitic Amoebae ( Fig. iX). Large unicellular organisms like the Amoebae were found creeping about inside the body of the sponge, and were thought to be parasites. It was afterwards discovered that thev were real 1 y t lie ova of the sponge from which the embryos were developed. As a matter of fact, these sponge-ova are so much like many of the Amoebae in size, shape, the character of their nucleus, and move- ment of the pseudopodia, that it is impos- sible to distinguish them without knowing their subsequent development. Our phylogenettc interpretation of the ovum, and the reduction of it to some ancient amoeboid ancestral form, supply the answer to the old problem : " Which was first, the egg or the chick?" We can now give a very plain answer to this riddle, with which our opponents have often tried to drive us into a corner. The egg came a long time before the chick. We do not mean, of course, that the egg existed from the first as a bird's <.-i^^, but as an indifferent amoeboid cell of the simplest character. The v^^; lived for thousands of years as an independent unicellular organism, the Amoeba. The egg, in the modern physiological sense of the word, did not make its appearance until the descendants of the unicellular PrOtOZOOn had developed into multicellular animals, and these had undergone sexual differentiation. Even then the v^;^ was first a gastraea-egg, then a platode-egg, then a vei malia-egg, and chordonia-egg ; later still acrania-egg, then fish-egg, amphibia-egg, reptile-egg, and finally bird's egg. The bird's egg we have ex- perience of daily is a highly complicated historical product, the result of countless hereditary processes that have taken pl.n c in the course of millions of years. The earliest ancestors of our race were simple Protophyta, and from these our protozoic ancestors were developed after- wards. From the morphological point of view both the vegetal and the animal Protists were simple organisms, indi- Kr<.. u8.— A moneron (Protamceba) in the act of repro- duction. A The whole moneron, moving like an ordinary amceba by thrusting out changeable processes, // It divides into two halves by a constriction in the middle. C The two halves separate, and each becomes an independent individual. (Highly magnified.) vidualities of the first order, or plastids. All our later ancestors are complex organ- isms, or individualities of a higher order — social aggregations of a plurality of cells. The earliest of these, the Moneada, which represent the third stage in our genealogy, are very simple associations of homogeneous, indifferent cells — undif- ferentiated colonies of social Amoebae or Infusoria. To understand the nature and origin of these protozoa-colonies we need only follow step by step the first embryonic products of the stem-cell. < In all the Metazoa the first embryonic process is the repeated cleavage of the stem-cell, or first segmentation-cell (Fig- 229). We have already fully considered this process, and found that all the different forms of it maybe reduced to one type, the original equal or primordial segmentation (cf. Chapter VIII.). In the genealogical tree or A' PROTIST i.v. 'ESTORS of the Vertebrates this palingenetic form of segmentation has been preserved in the Ampnioxus alone, all die other Vertebrates having cenogenetically modified forms oi cleavage. In any case, the Litter were developed from the former, and so the segmentation of the ovum in the Amphi- oxus lias a great interest for us cf. rig. ,VS). The outcome o\ this repeated cleavage is the formation oi u round small communities of Amoeba arose by the side of these eremitical Protozoa, the sister-cells produced by cleavage remain- ing joined together. The advantages in the Struggle for life which these commu- nities had over the isolated cells favoured their formation and their further develop- ment. We lind plenty of these cell-colonies or communities to-day in both fresh and salt water. They belong to various groups Fio. 229- Original OP primordial OVUm-cleavage. The stem-cell or cvtuU. formeJ by fecundation of the >>\"um. divides by repeated regular cleavage first into two (A), then four ( B ), 'hen eight (C), and finally a large number of segmentation-cells ( D). cluster of cells, composed of homogeneous, indifferent cells of the simplest character (Fig. 230). This is called the morula (= mulberry-embryo) on account of its resemblance to a mulberry or blackberry. It is clear that this morula reproduces for us to-day the simple structure of the multicellular animal that succeeded the unicellular amoeboid form in the early Laurentian period. In accordance with the biogenetic law, the morula recalls the ancestral form of the Morcea, or simple colony of Protozoa. The first cell-com- Fic. 230.— Morula, or mulberry-shaped embryo. munities to be formed, which laid the early foundation of the higher multicellular body, must have consisted of homogeneous and simple amoeboid cells. The oldest Amoeba; lived isolated lives, and even the amoeboid cells that were formed by the segmentation of these unicellular organ- isms must have continued to live inde- pendently for a long time. But gradually both of the Protophyta and Protozoa. To have some idea of those ancestors of our race that succeeded phylogenetically to the Moraeada, we have only to follow the further embryonic development of the morula. We then see that the social cells of the round cluster secrete a sort of jelly or a watery fluid inside their globular body, and they themselves rise to the surface of it (Fig. 29 F, G). In this way the solid mulberry-embryo becomes a hollow sphere, the wall of which is com- posed of a single layer of cells. We call this layer the blastoderm, and the sphere itself the blastula, or embryonic vesicle. This interesting blastula is very impor- tant. The conversion of the morula into a hollow ball proceeds on the same lines originally in the most diverse stems — as, for instance, in many of the zoophytes and worms, the ascidia, many of the echinoderms and molluscs, and in the amphioxus. Moreover, in the animals in which we do not find a real palinge- netic blastula the defect is clearly due to cenogenetic causes, such as the formation of food-yelk and other embryonic adapta- tions. We may, therefore, conclude that the ontogenetic blast ula is the reprod net ion of a very early phylogenetic ancestral form, and that all the Metazoa are descended from a common stem-form, which was in the main constructed like the blastula. In many of the lower animals the blastula is not developed OCA' PROTIST ANCESTORS 2"3 within the foetal membranes, but in the open water. In those cases each blasto- dermic cell begins at an early stage to thrust out one ov more mobile hair-like processes; the body swims about by the vlbraton movement o\ these lashes or whips (Fig. 29 /•')• We Mill find, both in the sea and in fresh water, various kinds o\ primitive multicellular organisms that substantial!} resemble the blastula in structure, and may be regarded in a sense as permanent blastula-forms hollow vesicles or gelati- nous ball*; with a wall composed of a single layer of ciliated homogeneous cells. Theie are " blastaads " oi this kind even among the Protophyta the familiar Volvocina, formerly classed with the infusoria. The common Votvox Sftobator is found in the ponds in the spring a small, green, gelatinous globule, Swimming about bv means oi the stroke oi its lashes, which rise in pairs from the cells o[\ its surface. In the similar Halos- phctra viridis also, which we find in the marine plancton (floating matter), a number oi green cells form a simple layer at the surface oi the gelatinous ball ; but in this case there are iu-> cilia. Some oi the infusoria oi the llagellata- class [Signura, Magosphcerax etc.) are similar in structure to these vegetal clusters, but differ in their animal nutri- tion ; they Ibrm the special group of the CaiaUacta. In September, 1869, I studied the development of one of these graceful animals on the island of Gis-Oe, olT the coast oi Norway [Afagosphtera plunula, Figs. >;, i and 232). The fully-formed body is a gelatinous ball, with its wall composed of thirty-two to sixty-four ciliated cells ; it swims about freely in the sea. After reaching maturity the community is dissolved. Each cell then lives inde- pendently for some time, grows, and changes into a creeping amoeba. This afterwards contracts, and clothes itself with a structureless membrane. The cell then looks just like an ordinary animal ovum. When it has been in this con- dition iov some time the cell divides into two, four, eight, sixteen, thirty-two, and si\t\ -four cells. These arrange themselves in a round vesicle, thrust out vibratorv lashes, burst the capsule, and swim about in the same magosphaera-form with which we started. This completes the life-circle of the remarkable and instructive animal. If we compare these permanent blastula." with the free-swimming ciliated larva; or blastulae, with similar construct in, 1, of main of the lower animals, we can confi- dently deduce from them that there was a yen earl) and long-extinct common stem-form of substantiaJlj the same struc- ture as the blastula. We may call it the Blasttea. Its body consisted, when fully formed, of a simple hollow ball, filled with lluid or structureless jelly, with a wall composed of a single stratum oi ciliated cells. There were piobahlv many genera and species oi these blastaads in the Laurentian period, forming a special 1 lass of marine pi otists. It is an interesting fact that in the plant kingdom also the simple hollow sphere is iound to be an elementary form oi the multicellular organism. At the surface and below the surface (down to a depth of 2,000 yards) oi' the sea there are green globules swimming about, with .1 wall composed oi a single layer ot chlorophyll-bearing cells. The botanist Schmitz gave them the name of Halo- spheera viridis in 1S79. The next stage to the BlasUra, and the sixth in our genealogical tree, is the C,,istra>a that is developed from it. As we have already seen, this ancestral form is particularly important. That it once existed is proved with certainty by the gastrula, which we find temporarily in the ontogenesis of all the Metazoa (Fig. 29 /, A'). As we saw, the original, palingenetic form of the gastrula is a round or oval uni-axial body, the simple cavity of which (the primitive gut) lias an aperture at onv pole of its axis (the primi- tive mouth). The wall o( the gut consists of two strata of cells, and these are the primary germinal layers, the animal skin- layer (ectoderm) and vegetal gut-layer (entoderm). The actual ontogenetic development of the gastrula from the blastula furnishes sound evidence as to the phylogenetic origin oi the GastrtBa from the Blastcea. A pit-shaped depression appears at one side of the spherical blastula (Fig. 29 //). In the end this invagination goes so far that the outer or invag'maicd part of the blastoderm lies close on the inner or non-invaginated part (Fig. 29 ./).; In explaining the phylogenetic origin of the gastraa in the light oi this onto- genetic process, we may assume that the one-layered cell-community of the blasta-a began to take in food more largely at one particular part of its surface. Natural selection would gradually lead to the "4 of A' rxor/sr A ivt 'esto&s formation of a depression or pit at tin's alimentary spot on the surface of the ball. The depression would grow deeper and deeper. In time the vegetal function of taking in and digesting food would be eon- fined to the cells that lined this hole ; the Fig. 2;,i.— The Norwegian Magosphaera pla- nula, swimming about by moans of the lashes or cilia at its surface. other cells would see to the animal func- tions of locomotion, sensation, and pro- tection. This was the first division of labour among- the originally homogeneous cells of the blastaea. The effect, then, of this earliest histo- logical differentiation was to produce two different kinds of cells — nutritive cells in the depression and locomotive cells on the surface outside. But this involved the severance of the two primary germinal layers — a most important process. When we remember that even man's body, with all its various parts, and the body of all the other higher animals, are built up originally out of these two simple layers, we cannot lay too much stress on the phylogenetic significance of this gastrula- tion. In the simple primitive gut or gastric cavity of the gastrula and its rudimentary mouth we have the first real organ of the animal frame in the morpho- logical sense ; all the other organs were developed afterwards from these. In reality, the whole body of the gastrula is merely a " primitive gut." I have shown already (Chapters VIII. and IX.) that the two-layered embryos of all the Metazoa can be reduced to this typical gastrula. This important fact justifies us in con- cluding, in accordance with the biogenetic law, that theii ancestors also were phylo- genetically developed from a similar stem- form. This ancient stem -form is the gastraea. The gastraea probably lived in the sea during the Laurentian period, swimming about in the water by means of its ciliary coat much as free ciliated gastruhe do to-day. Probably it differed from the existing gastrula only in one essential point, though extinct millions of years ago. We have reason, from comparative anatomy and ontogeny, to believe that it multiplied by sexual generation, not merely asexually (by cleavage, gemma- tion, and spores), as was no doubt the case with the earlier ancestors. Some of the cells of the primary germ-layers prob- ably became ova and others fertilising sperm. We base these hypotheses on the fact that we do to-day find the simplest form of sexual reproduction in some of the living gastnxads and other lower animals, especially the sponges. The fact that there are still in existence various kinds of gastracads, or lower Metazoa with an organisation little higher than that of the hypothetical gastraea, is a strong point in favour of our theory. There are not very many species of these living gastraeads; but their morphological and phylogenetic interest is so great, and FlG. 23a.— Section Of same, showing how the pear- shaped cells in the centre of the gelatinous ball are connected by a fibrous process. Each cell has a con- tractile vacuole as well as a nucleus. their intermediate position between the Protozoa and Metazoa so instructive, that I proposed long ago (1876) to make a special class of them. 1 distinguished threeorders in this class — theGastremaria, Physemaria, and Cyemaria (or Dicyc- 0 1 7? PRO TIS T. A AY V- A TORS 215 mida). Hut wo might also regard these three orders as so many independent classes in a primitive gastra-ad stem. The Gastremaria and Cyemaria, the chief of these living gastraads, are small Meta/oa that live parasitically inside Other Meta/oa, and are, as a rule, iV to Js of an inch long, often much less (Fig. 233, 1-15). Their soft body, devoid of skeleton, consists of two simple strata of cells, the primary germinal layers ; the outer of these is thickly clothed with long hair-like lashes, by which the parasites swim about in the various cavities of their host. The inner germinal layer furnishes the sexual products. The pure type of the original gastrula (or arcAtgustruia, Fig. 20 /) is seen in the Pemmatodiscus gastntlaceus, which Monticelli discovered in the umbrella of a large medusa ( Pile ma pulmo )\\\ 1805 ! th* convex surface of this gelatinous umbrella was covered with numbers of clear vesicles, of ^ to \ inch in diameter, in the fluid contents of which the little parasites were swimming. The cup-shaped body of the Pemmatodiscus (Fig. 233, 1) is sometimes rather flat, and shaped like a hat or cone, at other times almost curved into a semi-circle. The simple hollow of the cup, the primitive gut (g), has a narrow opening (0). The skin layer (e) consists of long slender cylindrical cells, which bear long vibratory hairs ; it is separated by a thin structure- less, gelatinous plate (J) from the visceral or gut layer ( ij, the prismatic cells of which are much smaller and have no cilia. Pemmatodiscus propagates asexually, by simple longitudinal cleavage ; oil this account it has recently been regarded .is the representative of a special order of gastraads f Mesogastria j. Probably a near relative of the Pemma- todiscus is the Kunstieria Gruveli (Fig. 233, 2). It lives in the body-cavity of Ver- malia (Sipunculida), and differs from the former in having no lashes either on the large ectodermic cells (e) or the small entoderrtuc ( ij ; the germinal layers are separated by a thick, cup-shaped, gelati- nous mass, which has been called the " clear vesicle " ( f ). The primitive mouth is surrounded by a dark ring that bears very strong and long vibratory lashes, and effects the swimming movements. Pemmatodiscus and Kunstieria may be included in the family of the Gastremaria. To these gastraads with open gut arc closely related the Orthonectida [Rltofia- lura, Fig. 23.1, 3-5). They live parasitically inthebody-cav ityofechinoderms(Ophiura) and vermalia ; they are distinguished by the fait that their primitive gut-cavity is not empty, but Idled with entodermtc i ells, from which the sexual (ills are developed. These gastraads are of both sexes, the male (Fig. 3) being smaller and of a somewhat different shape from the oval female (Fig. 4). The somewhat similar DicvemiJa (Fig. 6) are distinguished from the preceding by the fact that their primitive gut-cavity is occupied by a single large entodermic cell instead of a crowded group of sexual cells. This cell does not yield sexual products, but afterwards divides into a number of cells (spores), each of which, without being impregnated, grows into a small embryo. The Dicyemida live para- siticallv in the body-cavity, especially the renal cavities, of the cuttle-fishes. They fall in several genera, some of which are characterised by the possession of special polar cells ; the body is sometimes roundish, oval, or club-shaped, at other times long and cylindrical. The genu's Conocyema (Figs. 7-15) differs from the ordinary Dieyema in having four polar pimples in the form of a cross, whii h may be incipient tentacles. The classification of the Cyemaria is much disputed ; sometimes they are held to be parasitic infusoria (like the Opa/ina), sometimes platodes or vermalia, related to the suctorial worms or rotifers, but having degenerated through parasitism. 1 adhere to the phylogenetically impor- tant theory that I advanced in 1876, that we have here real gastraeads, primitive survivors of the common stem-group of all the Metazoa. In the struggle for life they have found shelter in the body-cavity of other animals. The small Coelenteria attached to the floor of the sea that I have called the Physemaria ( Haliphysema ;uid Gastro- phj'sema ) probably form a third order (or class) of the living gastraads. The genus Haliphysema (Figs. 234, 235) is externally very similar to a large rhizopod (described by the same name in 1862) of the family of the Rhabdamminida, which was at first taken for a sponge. In order to avoid confusion with these, I afterwards gave them the name of Prophysema. The whole mature body of the Prophvsema is a simple cylindrical or oval tube, with a two-layered wall. The hollow of the tube is the gastric cavity, and the Upper Opening of it the mouth (big. 235 m). 2lG OCR PROT/ST ANCESTORS Fig. 233— Modern gastraeads. Fig. 1. Pemmatodiscus gastrulaceus (Monticelli), in longitudinal section. Fig. a. Kunstleria gruvell (Delage), in longitudinal section. (From Kunstler and Gruvel.) Figs. 3-5- Rhopalura Giardl (Julinj: Fig. 3 male, Fig. 4 female, Fig. 5 planula. Fig. 6. Dlcyema maCPOCephala (Van Beneden): Figs. 7-15. Conocyema polymorpna (Van Beneden): Fig. 7 the mature gastra-ad, Figs. 8-15 its gastrulation. d primitive gut, o primitive mouth, e ectoderm, {'entoderm, / gelatinous plate between e and /(supporting plate, blastoccel). OCR PROTIST AXCESTORS 217 TIk- two strata of cells that form the wall of the tube are the primary germinal layers. These rudimentary zoophytes differ from the swimming i^.is- trseads chiefli in being attached a! one end (the end opposite to the mouth) to the floor o\ the sea. In Prophyst-nia the primitive gut is a simple oval cavity, but in the closely related Gastrpphvst»iiJ it is divided into two chambers by a trans- verse constriction ; the hind and smaller chamber above furnishes the sexual products, the anterior one being for diges- tion. The simplest sponges (Olvn- thus,- Fig. 338) have the same organisation. is 1 he Physemaria. The only material difference between them is that in the sponge the thin two-layered body-wall is pierced by numbers of pores. When these are closed they resemble the Physemaria. Possibly the gastr.uads that we call Physemaria are onlyolynthi with the pores closed. The Ammoamtda, or the simple tubular sand-sponges of the deep-sea [Ammofynthus, etc.), do not differ from thegastraeads in any important point when Fir.. rf& Fig. 334. Fig. 235. Figs. 23*and 235— Prophysema prlmordiale, a living- sm&- traead. Fig. 234. The whole of the spindle-shaped animal (attached below to the floor of the sea). Fig. 235. The same irt longitudinal section. The primitive gut (d) opens abo"e at the primitive mouth ( m ). Between the ciliated cells (gj are the amoeboid ova (e). The skin-layer (h) is encrusted with grains of sand below and sponge- spicules above. the pores are closed. In my Mono- graph on the Sponges (with sixty plates) 1 endeavoured to prove analytically that all the species of this class can be traced phylo- genetically to a common stem-form ( Calcolynthus). The lowest form of the Cnidaria is also not far removed from the gas- tra.>ads. In the interesting common fresh-water polyp (Hydra) the whole body is simply an oval tube with a double wall ; only in this case the mouth has a crown of tentacles. Before these develop the hydra re- sembles an ascula (Figs. 236, 237). Afterwards there are slight histo- logical differentiations in its ecto- derm, though the entoderm remains Figs. 236-237.— Ascula of gastrophysema, attached to the floor of the sea. Fig. 236 external view, 237 longitudinal section, g primitive gut. o primitive mouth. 1' visceral layer, r cutaneous layer. (Diagram.) ■iR O ( rR I f 'ORM-L IKE A \( /• s 7 V )R$ a single stratum of cells. We find the first differentiation oi epithelial and Fiq. 238. — OlynthuS, a very rudimentary sponge. A piece cut away in front. Stinging cejls, or of muscular and neural nils, in the thick ectoderm Of thj hydra. In all these rudimentary living coelen- teria the sexual cells o( both kinds ova and sperm cells— are formed by the same individual ; it is possible thai the oldest gastraeads were hermaphroditic. It is clear from comparative anatomy that hermaphrodism the combination of both kinds of sexual cells in one individual is the earliest form of sexual differentiation ; the separation of the sexes (gonochorism) was a much later phenomenon. The sexual cells originally proceeded from the edge of the primitive mouth of the gastraead. Chapter XX. OUR WORM-LIKE ANCESTORS The gastraea theory has now convinced us that all the Metazoa or multicellular animals can be traced to a common stem- form, the Gastnea. In accordance with the biogenetic law, we find solid proof of this in the fact that the two-layered embryos of all the Metazoa can be reduced to a primitive common type, the gastrula. J ust as the countless species of the Metazoa do actually develop in the individual from the simple embryonic form of the gastrula, so they have all descended in past time from the common stem - form of the Gastnea. In this fact, and the fact we have already established that the Gastraea has been evolved from the hollow vesicle of the one-layered Blastaea, and this again from the original unicellular stem-form, we have obtained a solid basis for our study of evolution. The clear path from the stem-cell to the gastrula represents the first section of our human stem-history (Chapters VIII., IX., and XIX.). The second section, that leads from the Gastraea to the Prochordonia, is much more difficult and obscure. By the Prochordonia we mean the ancient and long-extinct animals which the important embryonic form of the chordula proves to have once existed (cf. Figs. 83-86). The nearest of living animals to this embryonic structure are the lowest Tuni- cates, the C'opelata ( Appeiidicaria) and the larvae of the Ascidia. As both the Tunicates and the Vertebrates develop from the same chordula, we may infer that there was a corresponding common ancestor of both stems. We may call this the Chordcea, and the corresponding stem-group the Prochordonia or Prochor- data. From this important stem-group of the unarticulated Prochordonia (or " primitive chorda-animals ") the stems of the Tuni- cates and Vertebrates have been diver- gently evolved. We shall see presently how this conclusion is justified in the present condition of morphological science. We have first, to answer the difficult and much -discussed question of the development of the Chordaea from the Gastraea ; in other words, " How and by what transformations were the character- istic animals, resembling the embryonic chordula, which we regard as the common stem-forms of all the Chordoma, both OCR WORM-I.IKE ANCESTORS 219 Tunicates .mil Vertebrates, evolved from the simples) two-layered Metazoa?" I'lu- desci n( of the Vertebrates From the Articulates has been maintained by a number o\ zoologists during the l.v>t thirty years with more /cjl than discernment; and, .1^ .1 vast amount has been written on the subject, we must deal with it to some extent. All three i lasses of Articu- lates in succession have been awarded the honour o\ being considered the "real ancestors" of the Vertebrates: first, the Annelids (earth-worms, leeches, and the like), then the Crustacea (eiahs, et( . ), and, finally, the Tracheata (spiders, insects, etc.). Tile most popular o\ these hvpo- theses was the annelid theory, which derived the Vertebrates from the Worms. It was almost simultaneously (1X75) formulated h\ Carl Semper, of Wurtzburg, and Anton Dohrn, o\ Naples. The latter advanced this theory originally in favour of the failing degeneration theory, with which I dealt in mv work, Aims ami Methods of Modern F.mbryoioe v. This interesting degeneration theory — much discus-M.d at that time, but almost forgotten now — was formed in 1875 with the aim o( harmonising the results of evolution and ever-advancing Darwinism with religious belief. The spirited struggle that Darwin had occasioned by the re- formation of the theory of descent in 1859, and that lasted for a decade with varying fortunes in every branch of biology, was drawing to a close in 1870- 1872, and soon ended in the complete victory of transformism. To most of the disputants the chief point was not the general question of evolution, but the particular one of " man's pla'ce in nature " — "the question of questions," as Huxley rightly called it. It was soon evident to every clear-headed thinker that this question could only be answered in the sense of our anthropogeny, by admitting that man had descended from a long series of Vertebrates by gradual modifica- tion and improvement. In this way the real affinity of man and the Vertebrates came to be admitted on all hands. Comparative anatomy and ontogeny spoke too clearly for their testimony to be ignored any longer. Hut in order still to save man's unique position, and espet tally the dogma of personal immortality, a number of natural philo- soplkTs and theologians discovered an admirable way of escape in the " thcoi v of degeneration." Granting the affinity, the\ turned the whole e\ olut ionarv theory upside down, and boldly contended that "man is not the most highly developed animal, but the animals are degenerate men." It is true that man is closely related to the ape, and belongs to the vertebrate stem ; but the chain of his ancestry goes upward instead ol' down- ward. In the beginning "God created man in his own image," as the prototype of the perfect vertebrate ; but, in conse- quent e o( original sin, the human race sank so low that the apes branched oil' from it, and afterwards the lower Vertebrates. When this theory ol degeneration was Consistently developed, its supporters were bound to hold that the entire animal kingdom was descended from the debased children of men. This theory was most Strenuously defended by the Catholic priest and natural philosopher, Michehs, in his Hackelogony : .hi Academic Protest against HceckeVs Anthropogeny (1875). Instill more " academic " and somewhat mvstic form the theory was advanced by a natural philosopher of the older Jena school — the mathematician and physicist, Carl Snell. Hut it received its chief support on the zoological side from Anton Dohrn, who maintained the anthropo- centric ideas of Snell with particular ability. The Amphioxus, which modern science now almost unanimously regards as the real Primitive Vertebrate, the ancient model of the original vertebrate structure, is, according to Dohrn, a late, degenerate descendant of the stem, the "prodigal son " of the vertebrate family. It has descended from the Cyclostoma by a profound degeneration, and these in turn from the fishes ; even the Ascidia and the whole of the Tunicates are merely degenerate fishes ! Following out this curious theory, Dohrn came to contest the general belief that the Ccelenterata and Worms are " lower animals " ; he even declared that the unicellular Protozoa Were degenerate Ovlenterata. In his opinion " degeneration is the great prin* ciple that explains the existence of all the lower forms." Ifthis M'u helis-Dohrn theory were true, and all animals were really degenerate descendants of an originally perfect humanity, man would assuredly be the true centre and goal iA' all terrestrial life ; his anlhropocentric position and his immortality would be saved. Unfor- tunately, this trustful theory is in such OCR M 'ORAf- LIKE . I Ni 'ESTORS flagrant contradiction to .ill the known facts ol paleontology .mJ embryology that it is no longer worth serious scien- tific consideration. But the case is no better foi the much- discussed descent of the Vertebrates from the Annelids, which Dolirn afterwards maintained with great zeal. Ol late years this hypothesis, which raised so much dust and controversy, has been entirely abandoned by most competent zoologists, even those who once sup- ported it. Its chief supporter, Dohrn, admitted in 1890 that it is "dead and buried," and made a blushing retracta- tion at the end of his Studies of the Early History of the Vertebrate. Now that the annelid-hypothesis is "dead and buried," and other attempts to derive the Vertebrates from Medusae, Echinoderms, or Molluscs, have been equally unsuccessful, there is only one hypothesis left to answer the question of the origin of the Vertebrates — the hypo- thesis that I advanced thirty-six years ago and called the " chordonia-hypo- thesis." In view of its sound establish- ment and its profound significance, it may very well claim to be a theory, and so should be described as the chordoma or chordaea theory. I first advanced this theory in a series of university lectures in 1867, from which the History of Creation was composed. In the first edition of this work (1868) I endeavoured to prove, on the strength of Kowalevsky's epoch-making discoveries-, that "of all the animals known to us the Tunicates are undoubtedly the nearest blood-relatives of the Vertebrates ; they are the most closely related to the Vermalia, from which the Vertebrates have been evolved. Naturally, I do not mean that the Vertebrates have descended from the Tunicates, but that the two groups have sprung from a common root. It is clear that the real Vertebrates (primarily the Acrania) were evolved in very early times from a group of Worms, from which the degenerate Tunicates also descended in another and retrogressive direction." This common extinct stem- group are the Prochordonia ; we still have a silhouette of them in the chordula- embryo of the Vertebrates and Tunicates ; and they still exist independently, in very modified form, in the class of the Copelata (Appendicaria, Fig. 225). The chordaea-theory received the most valuable and competent support from Carl Gegenbaur. This able comparative 11101 phologist defended it in 1S70, in the second edition of his Elements of Com- parative Anatomy ; at the same time he drew attention to the important relations of the Tunicates to a curious worm, Balanoglossus : he rightly regards thi-> as the representative of a special cla^s of worms, which he called ** gut-breathers " ( ' Enteropneusta). Gegenbaur referred on many other occasions to the close blood-relationship of the Tunicates and Vertebrates, and luminously explained the reasons that justify us in framing the hypothesis of the descent of the two stems from a common ancestor, an unsegmented worm-like animal with an axial chorda between the dorsal nerve-tube and the ventral gut-tube. The theory afterwards received a good deal of support from the research made by a number of distinguished zoologists and anatomists, especially C. Kupffer, 13. Hatschek, F. Balfour, E. Van Beneden, and Julin. Since Hatschek's Studies of the Development of the Amphidxus gave us full information as to the embryology of this lowest vertebrate, it has become so important for our purpose that we must consider it a document of the first rank for answering the question we are dealing with. The ontogenetic facts that we gather from this sole survivor of the Acrania are the more valuable for phylogenetic purposes, as paleontology, unfortunately, throws no light whatever on the origin of the Vertebrates. Their invertebrate ancestors were soft organisms without skeleton, and thus incapable of fossilisa- tion, as is still the case with the lowest vertebrates — the Acrania and Cyclostoma. The same applies to the greater part of the Vermalia or worm-like animals, the various classes and orders of which differ so much in structure. The isolated groups of this rich stem are living brandies of a huge tree, the greater part of which has long been dead, and we have no fossil evidence as to its earlier form. Nevertheless, some of the sur- viving groups are very instructive, and give us clear indications of the way in which the Chordoma were developed from the Vermalia, and these from the Ccelenteria. While we seek the most important of these palingenetic forms among the groups of Ccelenteria and Vermalia, it is understood that not a single one of them ()( A' /I Oh' M -LIKE ANCESTORS 221 tiuiM be regarded a-- an unchanged, oi even little changed, <. * »p> °' the extinct stem-form. One group has retained one feature, another a different feature, of the original organisation, and othei organs have been furthei developed and charac- teristicall) modified. Hence here, more than in any other part of oui genealogical tree, we have to keep before our mind tin.- full f>i(turr oi development, and separate the unessential secondary pheno- mena from the essential and primary. It will he ust. Uil firs! (o point out the chief advances in organisation by which the simple Gastraea gradually became the more developed Chordaea. We find our first solid datum in the gastrula of the Amphioxus (Fig. 38). Its bilateral and tri-axial type indicates that the Gastraeads the common ances- tors oi all the Meta/oa divided at an early stage into two divergent groups. The uni-axial Gastraea became sessile, and gave rise to two steins, the.Sponges and the Cnidaria (the latter all reducible to simple polyps like the hydra). But the tri axial Gastraea assumed a certain pose or direction oi the body on account oi its swimming or creeping movement, and in order to sustain this it was a great advantage to share the burden equally between the two halves of the body (right and left). Thus arose the typical bilateral form, which has three axes. The same bilateral type is found in all our artificial means oi locomotion — carts, ships, etc.; it is b) far the best for the movement of the body in a certain direction and steadv position. Hence natural selection early developed this bilateral type in a section of the Gastraeads, and thus produced the stem-forms of all the bilateral animals. The Gastnea fri/atcralis, oi which we may Conceive the bilateral gastrula of the amphioxus to be a palingenetic reproduc- tion, represented the two-sided organism of the earliest Metazoa in its simplest form. The vegetal entoderm that lined their simple guf-cavity served for nutri- tion ; the ciliated ectoderm that formed the external skin attended to locomotion and sensation ; finally, the two primitive mesodermic cells, that lay to the right and left at the ventral border of the primi- tive mouth, were sexual Cells, and effected reproduction. In order to understand the further development of the gastraea, we must pay particular attention to : (1) the careful study of the embryonic stages of the amphioxus that lie between the gastrula and the chordula; (2) the mor- phological Stud) ot (he simplest Platodes ( Plalodaria and Turbelluria ) and several groups oi unarticulated Vermaha (Gas~ trvfruha, Nemettina, Enteropneusta). We have to consider the Platodes first, because they are on the border between the two principal groups of the Meta/oa, the Ccelenteria and the Ccelomaria. With the former they share the lack of body- cavity, anus, and vascular system ; with the latter they have in common the bilateral type, the possession of a pair of nephridia or renal canals, and the forma- tion oi a vertical brain or cerebral ganglion. It is now usual to distinguish four classes oi Platodes : the two free- living classes oi the primitive worms (Platodaria) and the coiled - worms (TurbeJlaria), and the two parasitic classes of the suctorial worms ( Vn»ia- ioda ) and the tape-worms ( Cstoda ). We have only to consider- the first two of these classes ; the other two are parasites, and have descended from the former by adaptation to parasitic habits and conse- quent degeneration. The primitive worms (Plalodaria ) are very small flat worms oi' simple con- struction, but of great morphological and phylogenetic interest. They have been hitherto, as a rule,. regarded as a special order of the Turbelluria, and associated with the Rhabdocaela; but they differ considerably from these and all the other Platodes (flat worms) in the absence of renal canals and a special central nervous System ; the structure of their tissue is also simpler than in the other Platodes. Most of the Platodes of this group (. \phanostomum, AmpAicJuerus, Convo- lute, Schisoprora, etc.) are very soft and delicate animals, swimming about in the sea by means oi a ciliary cdat, and very small (A to ,\f inch long). Their oval body, without appendages, is sometimes spindle-shaped or cylindrical, sometimes flat and leaf-shaped. Their skin is merely a layer of ciliated ectodermic cells. Under this is a soft medullary substance, which consists oi cntodennic cells with vacuoles. The food passes through the mouth directly into this digestive medul- lary substance, in which we do not generally see any permanent gut-cavity (it may have entirely collapsed) ; hence these primitive Platodes have been called Acorla (without gut-cavity or civlom), or, more correctly, Cryptoccela, or Pseudocaela. The sexual organs of these hermaphroditic 222 OUR M '( MM/ / IKE . I \( 'ESTi )RS Platodaria are very simple two pairs of strings of i ells, the inner of which, (the ovaries, Fig'. 839 <>) produce ova, and the outer (the spermaria, y) sperm-cells. These gonads are not yet independent sexual glands, but sexually differentiated cell-groups in the medullary substance, or, in other words, parts of the gut-wall. Their products, the sex-cells, are conveyed out behind by two pairs of short canals ; Fig. 2tg— Aphanostomum Langii (Pfarckrl). a primitive worm of the platodaria class, of the order of Cryptocoela or Acocla. This new species of the genus Aphanostomum. named after Professor Arnold Lang of Zurich, was found in September. 1899. at A.iareio in Corsica (creeping between fucoidea). It is one-twelfth ol an inch long, one-twenty-fifth of an inch broad, and violet in colour, a mouth, ^auditory vesicle (ecto- derm, ;' entoderm, o ovaries. S spcrmaries. f female aperture, m male aperture the male opening- ( m ) lies just behind the female ( f). Most of the Platodaria have not the muscular pharynx, which is very advanced in the Turbellaria and Trematoda. On the other hand, they have, as a rule, before or behind the mouth, a bulbous sense-organ (auditory vesicle or organ of equilibrium, r). and many of them have also a couple oi simple optic spots. The cell-pit of the ectoderm that lies underneath is rather thick, and represents the first rudiment of a neural ganglion (vertical brain or acroganglion). The Turbellaria, with which the similar Platodaria were formerly classed, differ materially from them in the more advanced structure of their organs, and especially in having a central nervous system (verti- cal brain) and excretory renal canals (nephridia); both originate from the ecto- derm. But between the two germinal layers a mesoderm is developed, a soft mass of connective tissue, in which the organs •are embedded. The Turbellaria are still represented by a number of dif- ferent forms, in both fresh and sea-watet. The oldest of these are the very rudi- mentary Hnd tiny forms that are known as Rhabdocoela on account of the simple construction of their gut ; they are, as a rule, less than a quarter of an inch long, and of a simple oval or lancet shape (Pig. 240). The surface is covered with ciliated epithelium, a stratum of ectodermic cells. The digestive gut is still the simple primi- tive gut of tlie gastraea fdj, with a single aperture that is both mouth and anus (m). There is, however, an in- vagination of the ectoderm at the mouth, which has given rise to a muscular pharynx (sd). It is noteworthy that the mouth of the Turbellaria (like the primi- tive mouth of the Gastraea) may, in this class, change its position considerably in the middle line of the ventral surface ; sometimes it liesbehind ( Qbisthostomum J, sometimes in the middle ( Mesostomum) , sometimes in front ( Prosostomwn ). This displacement of the mouth from front to rear is very interesting, because it corresponds to a phylogenetic displace- ment of the mouth. This probably occurred in the Platode ancestors of m st (or all ?) of the Ccelomaria ; in these the permanent mouth (metastoma) lies at the fore end (oral pole), whereas the primitive mouth (prostotna) lay at the hind end of the bilateral body. In most of the Turbellaria there is a narrow cavity, containing a number Of secondary organs, between the two primary germinal layers, the outer or animal layer of which forms the epidermis and the inner vegetal layer the visceral epithelium. The earliest of these organs are the sexual organs ; they are very variously constructed in the Platode- 1 lasv ; m the simplest case there are merely two pairs of gonads or sexual glands— a pair of testicles (Fig. 241A) OCR WORM-LIKE ANCESTORS 223 and a pair of ovaries ( e ). The) open externally, sometimes bj a common aperture fJIfonogtmoporaJ, sometimes b) separate ones, the female behind the male [Digonopora, Fig. -'41). The sexual glands develop originally from the two promesoblasts or primitive mesodermic cells (Fig. 83 />). As these earliest meso- dermic Structures extended, and became spacious sexual pouches in the later descendants of the Platodes, probably the two cuelom-pouches were formed From them, the first trace o\ the real body- cavity o\ the higher Metazoa (EnUro- caelaj. The gonads ate among the oldest organs, the few other organs that we find in the Platodes between the gut-wall and body-wall being later evolutionary products. One o\' the oldest and most important ol these are the kidneys or tupkridia, which remove unusable matter from the body (Fig. -'40 nc). These urinary or excretory organs were origi- nally enlarged skin-glands — a couple o( canals that run the length of the body, and have a separate or common external aperture ftnnj. They often have a number oi branches. These special ex- cretory organs are not found in the other Ccelenteria (Gastraeads, Sponges, Cni- daria) or the Cryptoccela. They are fust met in the Turbellaria, and have been transmitted direct from these to the Vermalia, and from these to the higher stems. Finally, there is a very important new organ in the Turbellaria, which we do not find in the Cryptoceela (Fig. ->;,()) and their gastr.xad ancestors — the rudi- mentary nervous system. It consists of a couple o( simple cerebral ganglia (Fig. 241 £)and tine nervous fibres that radiate from them ; these are partly voluntary nerves (or motor fibres) that go to the thin muscular layer developing under the skin ; and partly sensory nerves that proceed to the sense-Cells o\ the ciliated epiderm (J). Many o\' the Turbellaria have also special sense-organs; a couple of ciliated smell pits ( na ), rudimentary eyes ( d'< J, and, le-s frequently, auditory vesicles. On fliese principles I assume that the oldest and simplest Turbellaria arose from Platodaria, and these directly from bilateral Gastraeads. The chief advances were the formation of gonads and neph- ridia, and o\ the rudimentary brain. On this hypothesis, which 1 advanced in 1872 in the first sketch of the gastrsea-theory (■Monogmph <>n the Sponger ), there is no direel affinity between the Platodes and the Olid. nia. Next to the ancient stem-group of the Turbellaria come a number of more recent chordoma ancestors, which we class with the Vermalia or ffelminthes, the unai liculated worms. These true Fig. 240. Fie. 241. Fig. 240.— A simple turbellarian ( Khabdoccelum). m mouth, sii gullet Dithelium, sm gullet muck's, ciliated epiderm. (Diagram.) J worms (Vermes, lately also called Scole- eida ) are the difficulty or the" lumber.-room of the zoological classifier, because the various classes have very complicated relations to the lower Platodes on the imic hand and the more advanced animals mi the other. But il we exclude the Platodes and the Annelids from this stem, we find a fairly satisfactory unity of organisation --4 OUR U \ )RAf-LIKE . I NCESTt >RS in the remaining classes. Among these worms we tin J some important forms that show considerable advance in organi- sation from the platode to the chordoma stage. Three o\ these phenomena are particularly instructive : (0 The forma- tion of a true (secondary) body-cavit) (cceloma); (2) the formation oi a second aperture of the gut, the alius ; and (3) the formation o\ a vascular system. The great majority of the Ycrmalia have these of .^n amis at the posterior end (Fig. 242 RS "5 Botli classes have a complete ciliary ^o.n o\\ the epidermis, a heritage from the Tucbellann md theGastraeads ; also, both have two openings of ihe gut, the mouth and mus, like the Gastrotricha. Bui we find also in important organ thai is wanting in the preceding forms the vascular system. In their more ad> an< ed mesoderm we find a few contractile longi- tudinal canals which force the blood through the body by their contractions; these are the first blood-vessels. t an - S Fu.. i44. Fia m - A simple flemertine. m mouth, a* gut a anus, g brain, n nerve*, h ciliary coat, u Kfltory pits thead-v lefts), HKeyes, r dorsal veeaet, /lateral vessels. ( Diagram ) Fig. ns—AyoungEnleropneustf/ialanoglossusj. (Pram Alexander Agassiz.) r acorn-shaped snout, h neck, k gill-clefts and gill-arches of the fore-gut. in long rows on each side. a digestive hind-gut. filling the greater part o( the body-cavity. V intestinal vein or ventral vessel. King between the parallel folds of the skin, it anus. Fig 245. 226 OUR WORM-LIKE ANCESTORS The Nemertina were formerly classed with the much less advanced Turbellaria. But thej differ essentially from them in having an anus and blood-vessels, and several other marks of higher organi- sation. They have generally long and narrow bodies, like a more or less flattened COrd ; there are, besides several small species, giant-forms w ith a width of J to \ inch and a length of several yards (even ten to fifteen). Most of them live in the sea, but some in fresh water and moist earth. In their internal structure they approach the Turbellaria on the one hand and the higher Vermalia (especially the Enteropneusta) on the other. They have a good deal 01 interest as the lowest and oldest of all animals with blood. In them we find blood-vessels for the first time, distributing real blood through the body. . Fie. 246.— Tiansverse section of the branchial gut. A of Balanoglotsus. B of Atridia. r branchial gut. n pharyngeal gTOOve, central folds between the two. Diagrammatic illustration from Gegenbaur,\f> show the relation of the dorsal branchial-gut ca\ it y < >■) to the pharyngeal or hypobranchial groove (n). The blood is red, and the red colouring- matter i-, haemoglobin, connected with elliptic discoid blood-cells, as in the Verte- brate-. Most of them have two or three parallel blood-canals, which run the whole- length of the body, and are connected in front and behind by loops, and often by a number of ring-shaped pieces. The chief of these primitive blood-vessels is the one that lies a"bove the gut in the middle line of the back (Fig. 244 r) ; it may be compared to either the dorsal vessel of the Articulates or the aorta of the Verte- brates. To the right and left are the two serpentine lateral vessels (Fig. 244 /). After the Nemertina, 1 take (as distant relatives) the Enteropneusta; thev may be classed together with them as Fronton ia or Rhyncocccla (snout-worms). There is now only one genus of this class, with several species ( Balanoghssus ) ; but it is very remarkable, and may be regarded as the last survivor of an ancienl ,.?.i long-extinct class of Vermalia. They are related, on the one hand, to the 'Nemertina and their immediate ancestors, the Platodes, and to the lowest and oldest forms of the Chordoma on the other. The Enteropneusta (Fig. 245) live in the sea sand, and are long worms of very simple shape, like the Nemertina. From the latter they have inherited : (1) The bilateral type, with incomplete segmenta- tion ; (2) the ciliary coat of the soft epi- dermis ; (3) the double rows of gastric pouches, alternating with a single or double row of gonads ; (4) separation of the sexes (the Flatode ancestors were hermaphroditic) ; (5) the ventral mouth, underneath a protruding snout ; (0) the anus terminating the simple gut-tube ; and {-) several parallel blood- canals, running the length of the body, a dorsal and a ventral principal stem. On the other hand, the En- teropneusta differ from their Nemertine ancestors in several features, some of which are important, that we may attri- bute to adaptation. The chief of these is the branchial gut (Fig. 245 k). The anterior section of the gut is converted into a respiratory organ, and pierced by two rows of gill- clefts ; between these there is a branchial (gill) skeleton, formed of rods and plates of chitine. The water that enters at the mouth makes its exit by these clefts. They lie in the dorsal half of the fore-gut, and this is completely separated from the ventral half by two longitudinal folds (Fig. 246 A*). This ventral half, the glandular walls of which are clothed with ciliary epithelium and secrete mucus, corresponds to the pharyngeal or hypo- branchial groove of the Chordoma ( '/in J, the important organ from which the later thyroid gland is developed in the Craniota (cf. p. 184). The agreement in the structure of the branchial gut of the Enteropneusts, Tunicates, and Verte- brates was first recognised bj Gegenbaur (1S7S); it is the more significant as at first we find only a couple of gill-clefts in the young animals of all three groups J the number gradual I) increases. We can infer from this the common descent of the three groups with all the more of ft n ■< )RAf-L ike . i w :bs ivfts **7 confidence when we find the BaJano- gloss us approaching the Chordonia in other respects. Thus, for instance, the chief part of the centra] nervous system is a long dorsal neural string thai runs above the gut and corresponds to the medullary tube of the Chordonia. Bateson believes he has detected a rudi- mentary chorda between the two. Of all extant invertebrate animals the Enteropneusts come nearest to the Chor- donia in virtue o\ these peculiar char- ai i< rs ; hence we may regard them ;b the survivors of the ancient gut-breathing Vermalia from which the Chordonia also have descended. Again, of all the chorda- animals the Copelata (Fig. 125) and the tailed larva of the ascidia approach nearesl to the young Balanogiossus. Both are, on the other hand, very closely related to the Atnpkioxus, the Primitive Verte- brate of which we have considered the importance (Chapters XVI. and XVII.). As we saw there, the unarticulated Tunicates and the articulated Vertebrates must be regarded as two independent stems, that have developed in divergent directions. Bui the common root v\ the two stems, the extinct group o\ the Prochordonla, must be sought in the vermalia stem ; and of all the living Vermalia those we have considered give us the safest clue to their origin. It is true that the aetual representatives of the important groups o( the Copelata, Balano- glossi, Nemertina, Icthydina, etc., have more or less departed from the primitive model owing tO adaptation to special environment. Hut we may just as con- fidently affirm that the main features of their organisation have been preserved by heredity. We must grant) however, thai in the whole stem-history of the Vertebrates the long stretch from the Gastraeads and Platodes up to the oldest Chordonia remains by far the most obscure section. We might frame another hypothesis to raise the difficulty namely, that there was a long series of very different and totally extinct forms between the Ciasii i a and the C'hordaa. Even in this modi- lied chordaa-theoi v the SIS fundamental organs of the chordula would retain their great value. The medullary tube would be originally a chemical sensory organ, a dorsal olfactory tube, taking in respira- tor)-water and food hv the neuroporus in front and conveying them by the neurenteric canal into the primitive j^ut. This olfactor) tube would afterwards become the nervous centre, while the expanding gonads (lying to right and left of the primitive mouth) would form the coeloma. The chorda may have been originally a digestive glandular groove in the dorsal middle line o\ the primitive gut. The two secondary gut-openings, mouth and anus, may have arisen in various ways by 1 hange of functions. In any case, we should ascribe the same high value to the chordula as we did before to the gastrula. In order to explain more fully the chiel Stages in the advance of our race, I add the hypothetical sketch of man's ancestry that 1 published in m\- Last Link fa trans- lation by Dr. Gadow ol the paper read at the International Zoological Congress at Cambridge in 1898J: — o — . oj r. 00 (/J • P (D ~ ~- u cj Eh m < CO W H < 4 ii • 4.JS . OS J> ijjlufillJlIiiL. hn^ o - W < e U O V oa, o h D O St u> - E •-! r\ ^J S o OS ■9 OS W > 21 a 8 k E 03 CI 2 < "? SPA lllil o| § el 3 2 C ei-S g cS S o3<: — =£•£• 2-2- !>3§i| 4) 5 c tq . T- * 8 K so 8 gi»3S|i c3^ -SI aj-H *•= 8 So 3 "^ — 5 3 5 8a 31111 — J 5 3 ."2 ,^ fs & iti5-S5slSl! .. -^ .Z •"-> * ■> ' « © <" -73 vd. — ^S 3 — — o ' OS 9 «j «E5 c5 aS c t. o J3 N Si1 f| I- OS^! ;o.S ? -03 4 ^, ,~ ^ t*»EJS 8 d SS 3 § "Jo "8 i S ° •• t 2 > *' _r >> »»1 ! C « V "- C > |c MfTi E3^ ;£s-x * £ ^ ^ 5 • > 3 (°C7i-- - £ oS t - 5 i Tj ^ a 0 0.2 to /. oi A' FISH-LIKE . I Ac /• ..s /( >/?S 229 Chapter XXI. OUR FISH-LIKE ANCESTORS Oi r task of detecting the extinct ancestoi s of our race among the vast numbers of animals known to us encounters very different difficulties in the various sections of man's stem-history. These were very great in the series of our invertebrate ancestors ; they are much slighter in the subsequent series of our vertebrate ancestors. Within the vertebrate stem there is, as we have already seen, so complete an agreement in structure and embryology that it is impossible to doubt their phylogenetic unity. In this ease the evidence is much clearer and more abundant. The characteristics that distinguish the Vertebrates as a whole from the Inverte- brates have already been discussed in our description of the hypothetical Primitive Vertebrate (Chapter XI., Figs. 98-102). The chief ot" these are : ( 1 ) The evolution of the primitive brain into a doi sal medullary tube; (2) the formation of the chorda between the medullary tube and the gut ; (3) the division of the gut into branchial (gill) and hepatic (liver) gut ; and (4) the internal articulation or meta- merism. The first three features are shared by the Vertebrates with the ascidia-larvae and the Prochordonia ; the fourth is peculiar to them. Thus the chief advantage in organisation by which the earliest Vertebrates took precedence of the unsegmented Chordoma consisted in the development of internal segmenta- tion. The whole vertebrate stem divides first into the two chief sections of Acrania and Craniota. The Amphioxus is the only surviving representative of the older and lower section, the Acrania (" skull-less"). All the other vertebrates belong to the second division, the Craniota (" skull- animals"). (The Craniota descend directly from the Acrania, and these from the primitive Chordoma. The exhaustive study that we made of the comparative anatomy and ontogeny of the Ascidia and the Amphioxus has proved these illations for us. (See Chapters XVI. and XVII.) The Amphioxus, the lowesl Vertebrate, and the Ascidia, the nearest related Invertebrate, descend from a common extinct stem-form, the Chordaa , and this must have had, substantially, the organisation of the chordula. However, the Amphioxus is important not mcrelv because it fills the deep gulf between the Invertebrates and Verte- brates, but also because it shows us to- day the typical vertebrate in all its sim- plicity. We Owe to it the most important data that we proceed on in reconstructing the gradual historical development of the whole stem. All the Craniota descend from a common Stem-form, and this was substantially identical in structure with the Amphioxus. This stem-form, the Primitive Vertebrate [Prospondylus, Figs. 98-102), had the characteristics of the vertebrate as such, but not the important features that distinguish the Craniota from the Acrania. Though the Amphi- oxus has many peculiarities of structure and has much degenerated, and though it cannot be regarded as an unchanged descendant of the Primitive Vertebrate, it must have inherited from it the specific characters we enumerated above. We may not say that "Amphioxus is the ancestor of the Vertebrates"; but we can say : " Amphioxus is the nearest relation to the ancestor of all the animals we know." Both belong to the same small family, or lowest class of the Vertebrates, that we call the A rania. In our genea- logical tree this group forms the twelfth , stage, or the first stage among the verte- brate ancestors (p. 22X). From this group of Acrania both the Amphioxus and the Craniota were evolved. The vast division of the Craniota embraces all the Vertebrates known to us, with the exception of the Amphioxus. All of them Ik ve a head clcarlv differen- tiated from the trunk, and a skull enclos- ing a brain. The head has also three pair- ot higher sense-organs (nose, eyes, and cars). The brain is very rudimentary at I first, a mere bulboui enlargement of the 230 OUR FISH-LIKE ANCESTORS q palatoquadralum, nd lower jaw. hm hyomandihular. Iiy tongue-bone. l{ gill-radii, kh gill-arches, c jaw-teeth, \; gullet-teeth, sl neckv*pine. II. Vertebral column : oi upper arches, ub lower arches, he intercentra. > ribs. III. Single li n s : s additional r.i\v, n h p V Ventral fin : £ pelvis, m I'm-axis, u single row ot fin-ray? A. additional raj rof> penis. oi 7.' FISH-LIKE . \ \( /•- .s TOfiS or double-nosed Vertebrates (Gnatho- sloma or Amphirhina ) We have to con- sider the fishes carefully .is the class which, on the evidence ot palaeontology, Fio. 249. Embryo of a shark (Scymnus lichia), -..in from the ventral tide. "• breast-fins (in front five pairs of ^'ill-clefts), h belly-fins, a anus, s tail-fin. k external gill-tuft, d \ elk-sac (removed for most part), £• eye, « none, m mouth-cleft. comparative anatomy, and ontogeny, mav be regarded with absolute certainty as the stem-class of all the higher Vertebrates or Gnathostomes. Naturally, none of the actual fishes can be considered the direct ancestor of the higher Vertebrates. But it is certain that all the Vertebrates or Gnathostomes, from the fishes to man, descend from a common, extinct, fish-like ancestor. If we had this ancient stem- form before us, we would undoubtedly class it as a true fish. Fortunately, the comparative anatomy and classification of the fishes are now so far advanced that we can get a very clear idea of these interesting and instructive features. In order to understand properly the genealogical tree of our race within the vertebrate stem, it is important to bear in mind the characteristics that separate the whole of the Gnathostomes from the Cyclostomes and Craniota. In these respects the fishes agree entirely with all the other Gnathostomes up to man, and it is oi\ this that we base our claim of relationship to the fishes. The following characteristics of the Gnathostomes are anatomic features of this kind : (1) The internal gill-arch apparatus with the jaw- arches ; (2) the pair of nostrils ; (3) the floating bladder or lungs ; and (4) the two pairs of limbs. The peculiar formation of the frame- work of the branchial (gill) arches and the connected maxillary (jaw) apparatus is of importance in the whole group of the Gnathostomes. It is inherited in a rudimentary form by all of them, from the earliest fishes to man. It is true that the primitive transformation (which we find even in the Ascidia) of the fore gut into the branchial gut can be traced in all the Vertebrates to the same simple type ; in this respect the gill-clefts, which pierce the walls of the branchial gut in all the Vertebrates and in the Ascidia, are very characteristic. But the external, super- ficial branchial skeleton that supports the gill-crate in the CyclostOma is replaced in the Gnathostomes by an internal branchial skeleton. It consists of a number of successive cartilaginous arches, which lie in the wall of the gullet between the gill-clefts, and run round the gullet from both sides. The foremost pair of gill- arches become the maxillary arches, from which we L,'et our upper and lower jaws. The olfactory organs are at first found in the same form in all the Gnathostomes, as a pair of depressions in the fore part of the skin of the head, above the mouth; hence, the) are also called the Amphirhina or A' FISH-LIKE ANCESTORS *33 ("double-nosed "). The Cyclostoma are" one-nosed" (MonorhinaJ; their nose is a single passage in the middle of the frontal surface. But as the olfactory nerve is double in both cases, it is possible that the peculiar form of the no^c in the actual Cyclo- stomes i> a secondary acquisition (bj adaptation to suctorial habits). A thirl essential character ol the Gnathostomes, that distinguishes them very conspicuously from the lower vertebrates we have dealt with, is the formation of a blind sac by invagination from the fore part of the gut, which becomes in the fishes the air-filled floating bladder. This organ acts as a hydrostatic apparatus, in- creasing it reducing the specific gravit) of the fish by compressing or alteringthe quantity of air in it. The lish can rise or sink in the water b) means of il. This is the organ from which the lungs of the higher verte- brates are developed. Finally, the fourth character of the Gnathostomes in their simple em- bryonic form is the two pairs of extremities or limbs a pair of fore legs (breast-fins in the fish, Fig. 250 z») and .t pair of hind lei,rs (ventral tins in the fish, Fig. 250 //). The com- parative anatomy of these fins is very interesting, because the\ contain the rudiments of all the skeletal parts that form the framework i^f the fore and hind legs in all the higher verte- brates right up to man. There is no trace of these pairs of limits in the Acrania and Cyclostomes. Turning, now, to a closer inspection of the fish class, we may first divide it into three groups or sub-classes, thegenealog} of which is well known to us. The first and oldest group is the sub-class of the Selpchii ov primi- tive fishes, the best-known represen- tatives of which to-day are the orders of the sharks and ravs (Figs. .'}>> 252). Next to this is the 111 ore adv ained sub-class of the plated fishes or Ganoids (Figs. 253 5). It has been long extinct fov the mosl part. and has very few living representa- tives, such as the sturgeon and the bony pike; but we Can form some idea of the earlier extent of this interesting group from the larg< numbers of fossils. From these plat< d fishes the sub-class of the bonv fishes lie. 250.- Fully- developed nian- eatinp shark (Care/mrta* me/an- otter hs), Kit \it«. n first. 1 necond clors.il tm. .1 tail-fin. a anus-fin. v in att-fin*. h helh -fins -\H ( ) ( rR FTSH- 1. IK!'. . I A 7 V-.s n ) RS or Teleostei was developed, to which t ho ?real majority of living fishes belong (especially nearly all our rive/ fishes). Comparative anatomy and ontogeny show clearly thai the Ganoids descended from the Selachii, and the Teleostei From the Ganoids. On the other hand, a collateral FtG. 251.— Fossil angel-Shark (Squatiita alifera). from the upper Jurassic at Eichstatt. (From Xittc/.) The cartilagtnou.3 skull is clearly seen in the broad head, and the gill-arches behind. The wide breast-fin and the narrower belly-fin have a number of radii ; between these and the vertebral column are a number of ribs. line, or rather the advancing chief line of the vertebrate Stem, was developed from the earlier Ganoids, and this leads ns through the group of the Dipneusta to the important division of the Amphibia. The earliest fossil remains of Verte- brates that we know were found in the Upper Silurian (p. 2Ql), and belong to two groups the Selachii and the Ganoids. The most primitive of all known repre- sentatives of the earliest fishes are pro- bably the remarkable Pfeuracattfnida, the genera Pleuracanthus, Xeneu an thus, Ortho- canthus, etc. (Fig. 24s). These ancient cartilaginous fishes agree in most points of structure with the real sharks [Figs. 241), 250); but in other respects they seem to be so much simpler in organisation that many palaeontologists separate them altogether.and regard them a.sPwseIat kit ; they are probably 1 losely related to the extinct ancestors oi the Gnathostomes. We find well-preserved remains oi them in the Permian period. Well-preserved impressions of other sharks are found in the Jurassic schist, such as oi the angel- lish {Sonatina, Fig. 251). Among the extinct earlier sharks of the Tertiary period there were some twice as large as the biggest living fishes; Carcharodon was more than 100 feet long. The sole surviving species of this genus ( ('. Rondeleti) is eleven yards long, and has teeth two inches long ; but among the fossil species we find teeth six inches long (Fig. 2S2). From the primitive fishes or Selachii, the earliest Gnathostomes, was developed the legion oi the Ganoids. There are very few genera now of this interesting and varied group — the ancient sturgeons (Accipenser), the eggs oi which are eaten as caviare, and the stratified pikes {Polypterusx Fig. 255) in African rivers, and bony pikes ( Lepidosteus ) in the rivers of North America. On the other hand, we have a great variety oi' specimens oi' this group in the fossil state, from the Upper Silurian onward. Some of these fossil Ganoids approach closely to the Selachii ; others are nearer to the DipneustS ; others again represent a transition to the Teleostei. For our genealogical purposes the most interest- ing are the intermediate forms between the Selachii and the Dipneusts. Huxley, to whom we owe particularly important works on the fossil Ganoids, classed them in the order of the Crossopterygii. Many genera and species of this order are found in the Devonian and Carboniferous strata (big. 253); a single, greatly modified survivor oi the group is still found in the large rivers oi Africa fRofyptetUS, Fig. 255, and the closely related ( 'euamichth Vi )■ In many impressions oi the Crossopterygii the floating bladder seems to be ossified, OUR FISH-LIKE ANCESTORS *35 and therefore well preserved foi instance, in the Undina (Fig. 254, immediately behind the head). Part of these Crossopterygii approach mi \ i losel) in their chief anatomic features to the Dipneusts, and thus represenl phy- logenetically the transition from the Devonian Ganoids to the earliest air- breathing vertebrates. This importanl advance was made in the Devonian period. The numerous fossils thai we have from the first two geologic al >ci tions, the Laurent ian and Cambrian periods, belong exclusive!) to aquatic plants and animals. From this palaeontology al facl , in conjunction with important geological and biological indications, we ma) infer with some confidence that there were no terrestrial animals at that time. During the whole of the vasl archeozoic period main millions o( years the living population o\ our planet consisted almost exclusivel) of aquatic organ- isms; this is a very remarkable fact, when we remember that this period embraces the larger half o\ the whole history oi life. The lower animal-stems are wholly (or with very few exceptions) aquatic. Hut the higher stems also remained in the water during the primordial epoch. It was only towards its close that some o\ them came to live on land. We find isolated fossil remains of terrestrial animals fust in the Upper Silurian, and in larger numbers in the Devonian strata, which were deposited at the begin- ning of the second chief section of geology (the paleozoic age). The number increases considerably in the Carboniferous and Permian or less modified, either m consequence of remote correlation to the preceding 01 ow hit; to new adaptations. In the vertebrate Stem it was unques- tionabl) a branch of the fishes in fact, of the Ganoids that made the first fortunate experiment during the Devonian period o\ adapting themselves to terrestrial lite and breathing the atmosphere. This led to a modification o\ the heart and the nose. Tile true fishes have merely a pair o\ blind olfactor) pits of\ the surface of the head ; hut a connection of these with the cavity oi the mouth was now formed. A canal made its appearance on each side, and led directly from the nasal depression Fig. 252.— Tooth of a gigantic shark (CarcharoAin megalodon), from the Pliocene at Malta. Half natural size. (From ZitteL) deposits. We find many species both o\ the articulate and the vertebrate into the mouth-cavity, thus conveying Stem that lived on land and breathed the atmospheric air to the lungs even when atmosphere; their aquatic ancestors of the mouth was closed. Further, in all the Silurian period only breathed water. true li shes the heart has onlv two sections This important change in respiration is the chief modification that the animal organism underwent in passing from the water* to the solid land. The first con- sequence was the formation of lungs for breathing air ; up to th.tt time the gills alone had served for respiration. I3ut there was at the same time a great change in the circulation and its organs ; these- are always very closely correlated to the respiratory organs. Moreover, the limbs and other organs were also more | —an atrium that receives the venous blood from the veins, and a ventricle that propels it through a conical artery to the gills ; the atrium was now divided into two halves, or right and left auricles, by an incomplete partition. The right auricle alone now received the venous blood from the body, while the left auricle received the venous blood that flowed from the lungs and gills to the heart. Thus the double circulation o\~ the higher vertebrates was evolved from the simple »36 Of A' FISH-HKE ANCESTORS circulation o\ the true fishes, and, in accordance with the laws ol correlation, this advance lod toothers in the structure of othei 01 gans. The vertebrate class, that thus adapted they retained the earlier gill-respiration along with the new pulmonary (lung) respiration, like the lowest amphibia. This class was represented during the paleozoic age (or the Devonian, C'ar- Fig. 255. Fig. 253. — A Devonian Crosaopterygius ( Holofttychius nobilissimus ), from the Scotch old red sandstone. (From Huxlev-) Fig. 254.— A Jurassic CrOSSOpteryglus ( Undina penicillata), from the upper Jurassic at Eichstiitt. (From y.ittil.) j jugular plates, b three ribbed scales. FlC. 255.— A living CrOSSOpteryglllS, from the Upper Nile ( Polypterus bichir ). itself to breathing the atmosphere, and was developed from a branch of the Ganoids, takes the name of the Dipneusts or Dipnoa (" double-breathers "), because coniferous, and Permian periods) by a number of different genera. There are only three genera of the class living to-day : Prutoptcrus anneciens in the rivers ()/ rR FISH-LIKE . 1 Ni 'ES TORS 237 of tropical Africa (the White Nile, the Niger, Quelliman, etc.), Lepidoswen paradoxa in tropical South America (in the tributaries of the Amazon)) and Ceratodus Forstefi in the rivers ol East Australia. This wide distribution of the three isolated survivors proves thai they represent a group thai was formerly very large, In their whole structure they though most now associate them with the fishes. Asa matter of fail, t lie characters of the two classes are so tar united in the Dipneusts thai the answer to the question depends entirely on the definition we give of "fish" and u amphibian." In habits they are line amphibia. During the tropica] winter, in the rainy Season, they swim in the water like the fishes, and Fig. 256. /> Fig. 257. Fig. 256.— Fossil DipneilSt ( Dipterus Valenciennesi). from the old red sandstone (Devon). (From Pander.) Fig. 257. The Australian DipneilSt (Ceratoditx Forsieri). B view from the right, A lower side of the skull, C lower jaw. (From Gunther.) Q11 quadrat! bone. Psph parasphenoid. /'CF pterygopaJatinum, I'o vomer, d teeth, nn nostrils, Br branchial cavity, C first rib. D lower-jaw teeth of the fossil Ceratodus Kaupi (from the Triassic). form a transition from the fishes to the amphibia. The transitional formation between the two classes is so pronounced in the whole organisation of these remarkable animals that zoologists had a lively controversy over the question whether they were really fishes or amphibia. Several distinguished zoolo- gists classed them with the amphibia, vol. n. breathe water by gills. During the dry season they bury themselves in the dry mud, and breathe the atmosphere through lungs, like the amphibia and the higher vertebrates. In this double respiration they resemble the lower amphibia, and have the same characteristic formationof the heart ; in this they are much superior to the fishes. But in most other features D OUR FISH-LIKE ANCESTORS they approach nearer to the fishes, and nre inferior to the amphibia. Externall) they are entirely fish-luce. In the Dipneusts the head is not marked off from the trunk. The skin is covered with large scales. The skeleton is- soft, cartilaginous, .md at a low stage o( development, as in the lower Selachii and the earliest Ganoids. The chorda is completely retained, and surrounded by an unsegmented sheath. The two pairs of limbs arc very simple tins of a primitive skeleton ; the cartilaginous skeleton of its two pairs of lins, for instance, has still the original form of abi-serial or feathered leaf, and was on that account described by Gegenbaur as a " primitive fin- skeleton." On the other hand, the skeleton of the pairs of lins is greatly reduced in the African dipneusl (Prot- o/>/, rus ) and the American (Lepido- siren). Further, the lungs are double in these modern dipneusts, as in all the other air-breathing vertebrates; they Fig. 258. Fig. 259. Fig. 258. — Young ceratodus, shortly after issuing- from the egg, magnified ten times, k g-ill-cover, /liver. (From Richard Semon.) Fig. 259. — Young ceratodus, six weeks after issuing- from the egg. s spiral fold of gut, b rudimentary belly-fin. (From Richard Sernon.) type, like those of the lowest Selachii. The formation of the brain, the gut, and the sexual organs is also the same as in the Selachii. Thus the Dipneusts have preserved by heredity many of the less advanced features of our primitive fish- like ancestors, and at the same time have made a great step forward in adaptation to air-breathing by means of lungs and the correlative improvement of the heart. Ceratodus is particularly interesting on account of the primitive build of its have on that account been called "double- lunged " ( Dipncumoncs ) in contrast to the Ceratodus ; the latter has only a single lung (Monopneumones). At the same time the gills also are developed as water-breathing organs in all these lung- fishes. ProtOpterus has external as well as internal gills. The paleozoic Dipneusts that are in the direct line of our ancestry, and form the connecting-bridge between the Ganoids and the Amphibia, differ in many respects OUR FIVE-TOED ANCESTORS 239 from their living descendants, bul agree with them in the above essential features. This is confirmed by a number oi interesting facts thai have lately come to our knowledge in connection with the embryonic development oi 1 lie Ceratodus and Lepidosiren ; they give us important information as to the stem-history of the lower Vertebrates, and therefore oi our early ancestors of the paleozoic age. Chapter XXII. OUR FIVE-TOED ANCESTORS With the phylogenetic study oi the four higher l lasses oi Vertebrates, which must now engage our attention, we reach much tinner ground and more light in the con- struction oi our genealogy than we have, perhaps, enjoyed up to the present. In the firsl place, we owe a number oi very valuable data to the very interesting class oi Vertebrates that come next to the DipneustS and have been developed from them- the Amphibia. To this group belong the salamander, the frog, and the toad. In earlier days all the reptiles were, on the example of Linne, classed with the Amphibia (lizards, serpents, crocodiles, and tortoises). But the reptiles are much more advanced than the Amphibia, and are nearer to the birds in the chief points oi their structure. The true Amphibia ate nearer to the Dipneusta and tlie fishes ; the) are also much older than the reptiles. There were plenty oi highly-developed (and sometimes large) Amphibia during the Carboniferous period ; but the earliest reptiles are only found in the Permian period. It is probable that the Amphibia were evolved even earlier — during t'he Devonian period — from the Dipneusta. The extinct Amphibia of which we have fossil remains from that remote period (very numerous especially in the Triassic strata) were distinguished for a graceful scaly coat or a powerful bony armour on the skin (like the crocodile), whereas (he living amphibia have usually a smooth and slippery shin. The earliest oi these armoured Amphibia ( Phimtaniphibia ) form the order oi Stegocephala (" roof-headed ") (Fig. 260). It is among these, and not among the actual Amphibia, that we must look for the forms that are directly related to the genealogy oi our race, and are the ancestors of the three, higher classes oi Vertebrates. Hut even the existing Amphibia have such important relations to us in their anatomic structure, and especially their embryonic develop- ment, that we may say : Between the Dipneusts and the Amniotes there was a series oi extinct intermediate forms which we should certainly class with the Amphibia if we had them before us. In their whole organisation even the actual Amphibia seem to be an instructive transitional group. In the important respects of respiration and circulation they approach very closely to the Dipneusta, though in other respects they are far superior to them. This is particularly true oi the develop- ment oi their limbs or extremities. In them we find these tor the first time as five-toed feet. The thorough investiga- tions of Ciegenbaur have shown that the fish's tins, oi which very erroneous opinions were formerly held, are many- toed feet. The various cartilaginous or bony radii that are found in large numbers in each fin correspond to the fingers or toes oi the higher Vertebrates. The several joints of each fin-radius correspond to the various parts oi the toe. Even in the Dipneusta the I'm is of the same construction as in the fishes ; it was afterwards gradually evolved into the five-toed form, which we first encounter in the Amphibia. This reduction of the number of the toes to six, and then to five, probable took place in the second half of the Devonian period — at the latest, in the subsequent Carboniferous period — in those Dipneusta which we regard as the ancestors of the Amphibia. We have severalfossil remains ofnve-toedAmphibia from this period. There are numbers iA~ fossil impressions oi them in the Triassic of Thuringia ( Chirotlurium J. 240 OUR Fl VE- TOED A ACES TORS The fact that the toes number five is of great importance, because they have clearly been transmitted from the Amphibia to all the higher Vertebrates, Man entirely resembles his amphibian is well known that this hereditary number o( the toes has assumed a very great practical importance from remote times ; on it our whole system of enumeration (the decimal system applied to measure- Fig. a6o.— Fossil amphibian from the Permian, found in the Plauen terrain near Dresden (Branchio- saurns amblystomus ). (From Credner.) A skeleton of a young larva. B larva, restored, with gills. C the adult form, natural size. ancestors in this respect, and indeed in the whole structure of the bony skeleton of his five-toed extremities. A careful comparison of the skeleton of the frog with our own is enough to show this. It ment of time, mass, weight, etc.) is based. There is absolutely no reason why there should be five toes in the fore and hind feet in the lowest Amphibia, th'e reptiles,' and the higher Vertebrates, unless we Of/: /■/ 1 '/•:- TOED . I N( ES /'( )RS -4' ascribe it to inheritance from a common stem-form. Heredity alone can explain it. It is true that we find less than, five tiv- in many of the Amphibia and o\ the higher Vertebrates. But in all these - we can prove that souk- o\ the toes atrophied, and were in time lost altogether. The causes o\ this evolution of the five- toed foot from the many-toed tin in the amphibian ancestor must be sought in adaptation to the entire i hange o( function that the- limbs experienced in passing from an exclusively aquatic to a partly terrestrial life. The many-toed tin had been used almost solely for motion in the water ; it had now also to support the body in creeping on the solid ground. This led. to a modification both o\ the skeleton and the muscles o\ the limbs. The number of the tin-radii was gradually reduced, and sank finally to five. Hut these five remaining radii became much stronger. The soft cartilaginous radii became bony rods. The rest o\ the skeleton was similarly Strengthened. Thus from the one-armed lever o\' the many-toed fish-fin arose the improved many-armed lever system o( the tive-toed amphibian limbs. The movements ol' the body gained in variety as well as in strength. The various parts of the skeletal system and correlated muscular l to differentiate more and more. In view of the close correlation of the muscular and nervous systems, this also made great advance in structure and function. Hence we find, as a matter of that the brain is much more developed in the higher Amphibia than in the fishes, the Dipneusta, and the lower Amphibia. The first advance in organisation that was occasioned by the adoption of life on land was naturally the construction o\ an organ for breathing air — a lung. This was formed directly from the Boating- bladder inherited from the fishes. At fust its function was insignificant beside that of the gills, the older organ for water- respiration. Hence we. find in the lowest Amphibia, the gilled Amphibia, that, like the Dipneusta, they pass the greater part o\ their life in the water, and breathe water through gills. They only come to the surface at brief intervals, or creep on to the land, and then breathe air by their lungs. But some of the tailed Amphibia — the salamanders remain entirely in the water when they are voung, and aftei wards spend most oi their time on land. In the adult state they only breathe air through lungs. The same applies to the most advanced of the Amphibia, the Batrachia (frogs and toads) ; some of them have entirely lost the gill-bearing larva form.1 This is also the case with certain small, serpentine Amphibia, the lia (which live in the ground like earth-worms). The great interest of the natural history o\ the Amphibia consists especially in their intermediate position between the lower and higher Vertebrates. The lower Amphibia approach very closely to the Fig. 261.— Larva of the Spotted Salamander (Salamandra maculata ). seen from the ventral side. In the centre a yelk-sac still hangs from the gut. The external gills are gracefully ramified. The two pairs erf legs are still very small. Dipneusta in their whole organisation, live mainly in the water, and breathe by gills ; but the higher Amphibia are just as dose to the Amniotes-, live mainly on land, and breathe by lungs. Hut in their younger state the latter resemble the former, and only reach the higher stage by a complete metamorphosis. The embryonic development o( most of the 1 The tree-frog of Martinique (/A 'lodei /tiaHitu'criisis) loses the gilk on the seventh, and the tail and yelk-sac on the eighth, da) oi tuial life. On the ninth or tenth d.i> alter fecundation the trog emerges from the egg. 242 01 rR /•'/ 1 rE- TOED A NCES TORS higher Amphibia still faithfulh reproduces the stem-history of the whole class, and the various stages of the advance that was ni.uk' by the lower Vertebrates in passing from aquatic to terrestrial life during the Devonian or the Carboniferous period are repeated in the spring by every frog that developes from an egg in our ponds. The common frog leaves the egg in the shape ofa larva, like the tailed salamander (Fig. 2b\), and this is altogether different Fig. 262.— Larva of the common grass-frog ( Kana temporaria /or "tadpole." m mouth, n a pair ol suckers for fastening on to Btones, d skin-fold from which the.gill-cover developes ; behind it the gill-clefts, from which the branching tfills f/tj protrude, s tail- muscles, /cutaneous fin-fringe ot the tail. from the mature frog (Fig. 262). The short trunk ends in a long tail, with the form and structure of a fish's tail (sj. There are no limbs at first. The respira- tion is exclusively branchial, first through external ( k ) and then internal gills. In harmony with this the heart has the same strueture as in the fish, and consists of two se< (ions- an atrium thai receives the venous blood from the body, and a ventricle that forces it through the arteries into the gills. We find the larvae of the frog (or tad- poles, Gyrint) in greal numbers in our ponds every spring in this fish-form, using their muscular tails in swimming, just like the fishes and voting Ascidia. When they have reached a certain size, the remarkable metamorphosis from the fish-form to the frog begins. A blind sac grows out of the gullet, and expands into a couple of spacious sacs : these are the lungs. The simple chamber of the heart is divided into two sections by the development of a partition, and there are at the same time considerable changes in the structure of the chief arteries. Previously all the blood went from the auricle through the aortic arches into the gills, but now only part of it goes to the gills, the other part passing to the lungs through the new-formed pulmonary artery. From this point arterial blood returns to the left auricle of the heart, while the venous blood gathers in the right auricle. As both auricles open into a single ventricle, this contains mixed blood. The dipneust form has now succeeded to the fish-form. In the further course of the metamorphosis the gills and the branchial vessels entirely disappear, and the respiration becomes exclusively pulmonary. Later, the long swimming tail is lost, and the frog now hops to the land with the legs that have grown meantime. This remarkable metamorphosis of the Amphibia is very instructive in connection with our human genealogy, and is particu- larly interesting from the fact that the various groups of actual Amphibia have remained at different stages of their stem- history, in harmony with the biogenetic law. We have first of all a very low order of Amphibia — the Sozobranchia (" gilled-amphibia "), which retain their gills throughout life, like the fishes. In a second order of the salamanders the gills are lost in the metamorphosis, and when fully grown they have only pulmonary respiration. Some of the tailed Amphibia still retain the gill-clefts in the side of the neck, though they have lost the gills themselves ( Menopoma). If we force the larvae of our salamanders (Fig. 261) and tritons to remain in the water, and prevent them from reaching the land, we can in favourable circumstances make them retain their gills. In this fish-like condition they reach sexual maturity, and remain throughout life at the lower stage of the gilled Amphibia. OUR FIVE-TOED ANCESTORS 243 We have the reverse of iliis experiment in .1 Mexican rilled salamander, the fish- like axolotl (Siredon pistiformis). It was formerly regarded as a permanent rilled amphibian persisting throughout Rfe .u the fish-stage. Hut some of the hundreds of these animals thai are kepi in the Botanical Garden at Paris got on to the land for some reason or other, losl their gills, and changed into a form closely resembling the salamander (Am- blystoma). Other species of the genus, became sexually mature for the firsl time in this condition. This lias been regarded as au astounding phenomenon, although every common frog and salamander repeats the metamorphosis in the spring. The whole change from the aquatic and gill-breathing animal to the terrestrial lung-breathing form may be followed step by step in this case. But what we Their ancestors also had long tails and gills like the gilled Amphibia, as the tail and the gill-arches of the human embryo clearly show. For comparative anatomical and onto- genetic reasons, we must not seek these amphibian ancestors of ours as one would be inclined to do, perhaps -among the tail-less Batrachia, but anion- the tailed lower Amphibia. The vertebrate form that comes next to the Amphibia in ( he series of our ancestors is .1 lizard-like animal, the earlier exis- tence of which can be confidently deduced fijom the facts of comparative anatomy and ontogeny. The living HaHena of New Zealand (Fig. 2t>4) and the ex- tinct Rkyncocephala of the Permian period (Fig. 265) are closely related to this im- portant stem-form ; we may call them the Prulamniolcs, or Primitive Amniotes. Fui. 2>>\. —Fossil mailed amphibian, from the Bohemian Carboniferous (Seeleya). (Fro n Fritsch.) The scaly 11i.1t is retained on the left. see lure in the development of the indi- vidual has happened to the whole class in the course of its stem-history. The metamorphosis goes farther in a third order of Amphibia, the Batrachia or A intra, than in the salamander. To this belong the various kinds of toads, ringed snakes, water-frogs, tree-frogs, etc. These lose, not only the gills, but also (sooner or later) the tail, during metamorphosis. The ontogenetic loss of the pills and the tail in the frog and toad can only be explained on the assumption that thev are descended from long-tailed Amphibia of the salamander type. This is also clear from the comparative anatomy of the two groups. This remarkable meta- morphoSlS is, however, also interesting because it throws a certain light on the phylogeny of the tail-less apes and man. All the Vertebrates above the Amphibia — or the three classes of reptiles, birds, and mammals — differ so much in their whole organisation from all the lower Vertebrates we have yet considered, and have so great a resemblance to each other, that we put them all together in a single group with the title of Amniotes. In these three classes alone we find the remarkable embrvonic membrane, already mentioned, which we called the amnion ; a cenogenetic adaptation that we may regard as a result of the sinking of the growing embryo into the yelk-sac. All the Amniotes known to us all reptiles, birds, and mammals (including man) agree in so many important points of internal structure and development that their descent from a common ances- tor can be affirmed with tolerable cer- tainty. If the evidence of comparative -•H ( ) ( rR /■ 7 1 A- TOED . I NCES Ti ) RS anatomy and ontogen) is ever entirely beyond suspicion, it is certainly the case here All the peculiarities that accom- pany and follow the formation of the amnion, and that we have learned in our consideration of human embryology ; all the peculiarities in the development of the organs which we will presently follow in detail ; finally, all the principal special features of the internal structure of the full-grown Amniotes prove so clearly the common origin of all the Amniotes from a single extinct stem-form that it is diili- Cult to entertain the idea of their evolu- tion from several independent stem--. This unknown common stem-form is our PKmitive Amniote (Protamnion). In outward appearance it was probably some- thing between the salamander and the lizard. It is very probable that some part of the Permian period was the age of the origin of the Protamniotes. This follows from the fact that the Amphibia are not fully developed until the Carboniferous period, and that the fust fossil reptiles ( Palcehaiteria, Ffomoeosauins, Protero- saurus ) are found towards the close of the Permian period. Among the im- portant changes of the vertebrate organi- sation that marked the rise of the first Amniotes from salamandrine Amphibia during this period the following three are especially noteworthy: the entire disap- pearance of the water-breathing gills and the conversion of the gill-arches into other organs, the formation of the allantois in- primitive urinary sac, and the develop- ment of the amnion. One of the most salient characteristics of the Amniotes is the complete loss of the gills. All Amniotes, even if living in water (such as sea-serpents and whales), breathe air through lungs, never water through gills. AH the Amphibia (with very rare exceptions) retain their gills for some, time when young, and have for a time (if not permanently) branchial respiration ; but after these there is no question of branchial respiration. The Protamniote itself must have entirely abandoned water- breathing. Nevertheless, the gill-arches are preserved by heredity, and develop into totally different (in part rudimentary) organs — various parts of the bone of the tongue, the frame of the jaws, the organ of hearing, etc. But we do not find in the embryos of the Amniotes any trace of gill-leaves, or of real respiratory organs on the gill-arches. W'idi this complete abandonment of the gills is probably connected the formation of anothei organ, to which we have already referred in embryology namely, the allantois or primitive urinary s.u- (i.-f. p. i ()(>). It is very probable that the urinary bladder of the DipneustS is the first structure of the allantois. We find in these a urinary bladder that proceeds from the lower wall of the hind end of the gut, and serves as receptacle for the renal secretions. This organ has been transmitted to the Amphibia, as wo can see in the fiotj;. The formation of the amnion and the allantois and the complete disappearance of the gills are the chief characteristics that distinguish the Amniotes from the lower Vertebrates we have hitherto con- sidered. To these we may add several subordinate features that are transmitted to all the Amniotes, and are found in these only. One striking embryonic character of the Amniotes is the great curve of the head and neck in the embryo. We also find an advance in the structure of several of the internal organs of the Amniotes which raises them above the highest of the anamnia. In particular, a partition is formed in the simple ventricle of the heart, dividing into right and left chambers. In connection with the complete metamorphosis of the gill- arches we find a further development of the auscultory organs. Also, there is a great advance in the structure of the brain, skeleton, muscular system, and other parts. Finally, one of the most important changes is the reconstruction of the kidneys. In all the earlier Verte- brates we have found the primitive kidneys as excretory organs, and these appear at an early stage in the embryos of all the higher Vertebrates up to man. But in the Amniotes these primitive kidneys cease to act at an early stage of embryonic life, and their function is taken up by the permanent or secondary kidneys, which develop from the terminal section of the prerenal ducts. Taking all these peculiarities of the Amniotes together, it is impossible to doubt that all the animals of this group — all reptiles, birds, and mammals — have a common origin, and form a single blood- related stem. Our own race belongs to this stem. Man is, in every feature of his organisation and embryonic develop- ment, a true Amniote, and has descended from the Protamniote with all the other Of 'R II I 'A- TOED . \ Nt 'As TORS ?AS Amniotes. Though the) appeared al the end (possibly even in the middle) of the Paleozoic age, the Amniotes only reached their full development during the Mesozbic age. The birds and mammals made their first appearance during this period. Even the reptiles show their greatest growth at this time, so that it is called "the reptile age." The extinct Protam- niote, the ancestor of the whole group, and onlj comes in contact with the Mammals al it-- root, is the combined group of the reptiles and birds; these two classes may, with Huxley, be conveniently grouped together as the Sauropsida. Their common stem-form is an extinct lizard-like reptile of the order o\ the Rhyncocephalia. From this have been developed in various directions the ser- pents, crocodiles, tortoises, etc. — in a Fig. 264. — The lizard ( Hatto-ia punctata=Sphenodon punctatus) of New Zealand. The solo surviving proreptile. (From lirchm.) belongs in its whole organisation to the reptile class. The genealogical tree of the amniote group is clearly indicated in its chief lines by their paleontology, comparative anatomy, and ontogeny. The group succeeding the Protamniote divided into tun branches. The branch that will claim ouv whole interest is the cla'ss of the Mammals. The other branch, which developed in u totally different direction, I word, all the members of the reptile class. But the remarkable class of the birds has also been evolved directly from a branch of the reptile group, as is now established beyond question. The embryos of the reptiles and birds are identical until a verj late stage, and have an astonishing resemblance even later. Their whole structure agrees so much that no anato- mist now questions the de-cent ol' the birds from the reptiles. On the other .V}') Of 'A' FIVE-TOED ANCESTORS hand, the mammal line has descended from the group of the Sauromammalia, a different branch of the Proreptilia. It is connected at its deepest roots with the reptile line, but it then diverges com- pletely from it and follow-, a distinctive development. Man is the highest out- come of this class, the "crown of crea- tion." The hypothesis that the three higher Vertebrate classes represent a single Amniote-stem, and that the common root of this stem is to be found in the amphibian elass, is now generally admitted. The instructive group of the Permian Tocosauria, the common root from which the divergent stems of the Sauropsids and mammals have issued, merits our particular attention as the Stem-group of all the Amniotes. Fortunately a living representative of this extinct ancestral 3(65), of which well-preserved skeletons are found in the Solenhofen schists, is perhaps still more closely related to them. Unfortunately, the numerous fossil remains of Permian and Triassic Toco- sauria that we have found in the last two decades are, for the most part, very im- perfectly preserved. Very often we can make only precarious inferences from these skeletal fragments as to the ana- tomic characters of the soft parts that went with the bony skeleton of the extinct Tocosauria. Hence it has not yet been possible to arrange these important fossils with any confidence in the ancestral series that descend from the Protamniotes to the Sauropsids on the one side and the Mammals on the other. Opinions arc particularly divided as to the place in classification and the phylogenetic signi- ficance of the remarkable Theromo) pha. Fig. 265.— Homceosaurus pulchellus, a Jurassic prorcptile from Kchlheim. (From Zittel.) group has been preserved to our day ; this is the remarkable lizard of New- Zealand, Hatteria punctata (Fig. 264). Externally it differs little from the ordinary lizard ; but in many important points of internal structure, especially in the primi- tive construction of the vertebral column, the skull, and the limbs, it occupies a much lower position, and approaches its amphibian ancestors, the Stegocephala. Hence Hatteria is the phylogenetically oldest of all living reptiles, an isolated survivor from the Permian period, closely resembling the common ancestor of the Amniotes. It must differ so little from this extinct form, our hypothetical Prot- amniote, that we put it next to the Proreptilia. The remarkable Permian Pahrhattcria, that Credner discovered in the Plauen terrain at Dresden in 1888, belongs to the same group (Fig. 266). The Jurassic genus fjomceosaums (Fig. Cope gives this name to a very interesting and extensive group of extinct terrestrial reptiles, of which we have only fossil remains from the Permian and Triassic strata. Forty years ago some of these Therosauria (fresh-water animals) were described by Owen as Anomodontia. Hut during the last twenty years the distin- guished American paleontologists, Cope and Osborn, have greatly increased our knowledge of them, and have claimed that the stem-forms of the Mammals must be sought in this order. As a matter of fact, the Theromorpha are nearer to the Mammals in the chief points of structure than any other reptiles. This is especially true of the Thereodontia, to which the Pureosauria and Pelycosauria belong (Fig. 267). The whole structure of their pelvis and hind-feet has attained the same form as in the Monotremes, the lowest Mammals. The formation of the OCR FIVE-TOED ANCESTORS 2-47 scapula and tho quadrate bone shows an appro. nil tO the Mammals such as W€ find in no oilier group of reptiles. The teeth also arc already divided into incisors, canines, and molars. Nevertheless, n is very doubtful whether the Theromorpha really arc in the ancestral lino of the Sauromammals, or lead direct from the Tocosauda to the earliest Mammals. Other experts on this group believe that it is an independent legion of the reptiles, connected, perhaps, at its lowest root, with the Sauromammals, but developed quite independently of the Mammals though parallel to them in many ways. C^ne ot the most important of the zoological facts that we rely on in our have already seen, this root-form deve- loped from the primitive Proreptile stem in a totally different direction from the birds, and soon separated from the main Stem of the reptiles. The difforci between the Mammals and I he reptiles and birds are so important and chara. istic that we can assume with complete confidence this division of the vertebrate stem at the commencement of the develop- ment of the Amniotes. The reptiles and birds, which we group together as the Savmpsids, generally agree in the characteristic structure of the skull and brain, and this is notably different from that of the Mammals. In most of (lie reptiles and birds the skull is connected investigation of the genealogy of the with the first cervical vertebra (the atlas) human race is the position of man in (he Mammal class. However different the views of zoologists may have been as to this position in detail, and as to his relations to the apes, no scientist has ever doubted that man is a true mam- mal in his whole organisation and development. Linnedrew atten- tion to this fact in the first edition of his famous Systema Natures (1735). As will be seen in any museum of anatomy or any manual of comparative anatomy, the human frame has all the characteristics that are common to the Mammals and distinguish them conspicuously from all other animals. If we examine this undoubted fact from the point of view of phytogeny, in the light of the theory of descent, it follows at once that man is of a common stem with all the other Mammals, and comes from the same root as they. But the various features in which the Mammals agree and by which they are distinguished are of such a character as to make a polyphyletic hypothesis quite inadmis- sible. It is impossible to entertain the idea that all the living and extinct Mammals come Irom a number of separate roots. If we accept the general theory of evolu- tion, we are bound to admit the mono- phyletic hypothesis of the descent of all t!ie Mammals (including man) from a single mammalian stem-form. We may call this long extinct root-form and its earliest descendants (a few genera of one family) " primitive mammals " or " stem- mammals " ( Promammalia ). As we Fir.. a66.— Skull of a Permian lizard (Paleehatteria lo»- gicaudula ). (From Crrdner.) )i nasal bone, f>j "frontal bone. / lachrymal bone. /v postorbttal bone, sq covering bone-, i cheek- bone, m vomer, im inter-maxillary. by a single, and in the Mammals (and Amphibia) by a double, condyle at the back of the head. In the former the lower jaw is composed of several pieces, and connected with the skull so that it can move by a special maxillary bone (the qvadmtum) ; in the Mammals the lower jaw consists of one pair of bony pieces, which articulate directly with the temporal bone. Further, in the Sauropsids the skin is clothed with scales or feathers; in the Mammals with hair. The red blood-cells of the former have a nucleus ; those of the latter have not. In tine, two quite characteristic features of the Mam- mals, which distinguish them not only From the birds and reptiles, but from all other animals, are the possession of a 248 OUR FIVE-TOED .lXrESTORS complete diaphragm and of mammary glands that produce the milk for the nutrition of the young. It is only in the Mammals that the diaphragm forms a transverse partition of the body-cavity, completely separating the pectoral from the abdominal cavity. It is only in the mammals that the mother suckles its young, and tliis rightly gives the name to the whole class [mamma breast). From these pregnant fads of compara- tive anatomy and ontogeny it follows absolutely that the whole of the Mammals belong- to a single natural stem, which Bb Pa* Fig. 267.— Skull of a Triassic theromorphum (Galesaums planiceps), from the Karoo formation in South Africa. (From Oven.) a from the rit;ht, b from below, c from above, d tricuspid tooth. N nostrils, .V. 7 nasal bone, Mx upper jaw, Prf pre- frontal, /-'/-frontal bone, A eye-pits, S temple-pits. Pa Parietal eve. Bo joint at back of head, /'/ pterygoid-bone, Mil lower jaw. branched oft at an early date from the reptile-root. It follows further with the same absolute certainty that the human race is also a branch of this stem. Man shares all the characteristics I have described with all the Mammals, and differs in them from all other animals. Finally, from these facts we deduce with the same confidence those advances In the vertebrate organisation by which one branch of the Sauromammals was con- verted into the stem-form of the Mammals. Of these advances the chief were : (1) The characteristic modification of the skull and the brain ; (2) the development of a hairy coat ; (3) the complete formation of the diaphragm ; and (4) the construction i^~ the mamillary glands and adaptation 10 suckling. Other important changes of structure proceeded step by step with these. The epoch at which these important advances were made, and the foundation of the Mammal class was laid, may be put with great prohability in the first section of the Meso/oic or secondarv age —the Triassic period. The oldest fossil remains of mammals that we know were found in strata that belong to the earliest Triassic period — theupper Keuper. One of the earliest forms is the genus Dromatherium, from the North American Triassicj Fig. 268). Their teeth still strikingly recall those of the Pelycosauria. Hence we may assume that this small and probably insectivorous mammal belonged to the stem- group of the Promammals. We do not find any positive trace of the third and most advanced division of the Mammals — the Placentals. These (including man) are much younger, and we do not find indisputable fossil remains of them until the Ceno- zoic age, or the Tertiary period.' This paleontological fact is very important, because it fully har- monises with the evolutionary succession of the Mammal orders that is deduced from their com- parative anatomy and ontogeny. The latter science teaches us that the whole Mammal class divides into three main groups or sub-classes, which correspond to three successive phylogenetic stages. These three stages, which also represent three important in our human genealogy, were first distinguished in 1816 by the eminent French zoologist, Blainville, and received the names of Ornilhodelphia, Didcl- pliia, and Monodclphia, according to the construction of the female organs (dciphys = uterus or womb). Huxley afterwards gave them the names of Prototheria, Metatheria, and Ejbitheria. But the three sub-classes differ so widely from each other, not only in the construction of the sexual organs, but in many other respects also, that we may confidently draw up the following stages OUR FIVE-TOED ANCESTORS 249 important phylogenetic thesis : The Mono- dclphia or Placentals descend from the Diuelphia or Marsupials ; and the latter, in turn, are descended from the Mono- tremes or Ornithodelphia. Thus we must regard as the twenty- first stage in our genealogical tree the earliest and lowest chief group of the Mammals— the sub-class of the Mono- tremes(" cloaca-animals, "Ornithodelphia, or Prototheria, Figs. 269 and -70). They take their name from the cloaca which they share with all the lower Vertebrates. This cloaca is the common outlet fov the passage of the excrements, the urine, and the sexual products. The urinary ducts and sexual canals open into the hindmost part of the gut, while in all the other Mammals they are separated from the eectum and anus. The latter have a special uro-genital outlet (poms urogeni- talis J. The bladder also opens into the cloaca in the Monotremes, and, indeed, apart from the two urinary ducts ; in all the other Mammals the latter open directly into the bladder. It was proved by Haacke and Caldwell in 1884 that the Monotremes lay large eggs like the reptiles, while all the other Mammals ire viviparous. In 1894 Richard Semon further proved that these large eggs, rich in food-yelk, have a partial segmen- tation and discoid gastrulation, as I had hypothetically assumed in 1879 ; here again they resemble their reptilian ancestors. The construction of the mammary gland is also peculiar jn the Monotremes. In them the glands have no teats for the young animal to suck, but there is a special part of the breast pierced with holes like a sieve, from which the milk issues, and the young Monotreme must lick it off. Further, the brain of the Monotremes is very little advanced. It is feebler than that of any of the other Mammals. The fore-brain or cerebrum, in particular, is so small that it does not coyer the cerebellum. In the skeleton (Fig. 270) the formation of the scapula among other parts iscurious; it is quite different from that of the other Mammals, and rather agrees with that of the reptiles and Amphibia. Like these, the Monotremes have a strongly deve- loped caracoidcum. From these and other less prominent characteristics it follows absolutely that the Monotremes occupy the lowest place among the Mammals, and represent a transitional group between the Tocosauria and the rest of the Mammals. All these remark- able reptilian characters must have been posstssitl by the stem-form of the whole mammal class, the IVomammal of the Triassjc period, and have been inherited from the Proreptiles. During the Triassic and Jurassic periods the sub-class of the Monotremes was represented bv a number of different stem-mammals. Numerous fossil remains of them have lately been discovered in the Mesozoic strata of Europe, Africa, and America. To-day there are only two surviving specimens of the group, which we place together in the family of the duck-bills, Omithostoma. They are con- fined to Australia and the neighbouring island of Van Diemen's Land (or Tasmania) ; they become scarcer every year, and will soon, like their blood- relatives, be counted among the extinct animals. One form lives in the rivers, Fig. 268.— Lower jaw of a Primitive Mammal OP Promammal ( Dromaiherinnr silvestre ) from the North American Triassic. i incisors, c canine, p pre- molars, »i molars. (From Duderlein.) and builds subterraneous dwellings on the' banks; this is the Ornithorhyncus paradoxus, with webbed feet, a thick soft fur, and broad flat jaws, which look very much like the bill of a duck (Figs. 269, .270). The other form, the land duck- bill, or spiny ant-eater (Echidna hrs/iixj, is very much like the ant- eaters in its habits and the peculiar construction of its thin snout and very long tongue ; it is covered with needles, and can roll itself up like a hedgehog. A cognate form ( Parechidna Bruyni) has lately been found in New Guinea. These modern Omithostoma are the scattered survivors of the vast Mesozoic group of Monotremes ; hence they have the same interest in connection with the stem history of the Mammals as the living stem-reptiles ( Hattcria) for that of the reptiles, and the isolated Acrania ( Am- phioxus) for the phytogeny of the Verte- brate stem. The Australian duck-bills are distin- guished externally by a toothless bird- 250 Ol'K II \ '/•:- 7X )ED . \ \< As V( >A\s like beak o<- snout. This absence of real bony teeth is a late result o\ adaptation, as in the toothless Placentals {Edentata, armadillos and ant-raters). The extinct Monotremes, to which the Promammalia belonged, inusl have had developed teeth, inherited from the reptiles. Lately small rudiments of real molars have been discovered in the young of the Ormtho- Fig. 269. — The Ornithorhyncus or Duck-mole. fOrn ithorhynens pa radoxiisj. rhynrus, which has horny pJafes in the jaws instead of real teeth. The living Ornithostoma and the stem- forms of the Marsupials (or Didclphia) must be regarded as two widely diverge inir line.-, from the Promammals. This second sub-class of the Mammals is very interesting as a perfect intermediate stage between the other two. While the Marsupials retain a great part of the characteristics of the Monotremes, they Fin. 270.— Skeleton of the preceding. have also acquired some of the chief feat ures of the Placentals. Some features o ( rR /■/ \ '/•;- to/-: n . i Ni ES tors 25> arc also peculiar t^ 'he Marsupials, such a- ihe construction of the male -11111 female sexual organs and the form of the lower jaw. The Marsupials are distin- guished by a peculiar hook-like bony process thai bends from tin- corner of the lower jaw and points inw.ui.ls. As most of the Placentals have not this process, we can, with some probability, recognise the Marsupial from this feature alone. Most of the mammal remains that we have from the Jurassic and Cretaceous deposits are merely lower jaws, and mQSl of the jaws found in the Jurassic deposits at Stonesfield and Purbeck have the peculiar hook-like process that characterises the lower jaw of the Marsupial. On the strength of this paleontological fact, we may suppose that they belonged to Marsupials. Placentals do not seem to have existed at the middle of the Mesozoic age not until towards its close (in the Cretaceous period). At all events, we have no fossil remains of indubitable Placentals from that period. The existing Marsupials, of which the plant-eating kangaroo and the carnivo- rous opossum (Fig. 272) are the best known, differ a good deal in structure, shape, and si/e, and correspond in many respects to the various orders of Placentals. Most of them live in Australia, and a small part of the Australian and East Malayan islands. There is now not a single living Marsupial on the mainland of Europe, Asia, or Africa. It was very different during the Mesozoic and even during the Cenozoic age. The sedimen- tary deposits of these periods contain a -great number and variety of marsupial remains, sometimes of a colossal size, in various parts of the earth, and even in Europe. We may infer from this that the existing Marsupials are the remnant of an extensive earlier group that was distributed all over the earth. It had to give way in the struggle for life to the more powerful Placentals during the Tertiary period. The survivors of the group were able to keep alive in Australia and South America because the one was completely separated from the other parts of the earth during the whole of the Tertiary period, and the other during the greater part of it. From the comparative anatomy and ontogeny of the existing Marsupials we may draw very interesting conclusions 3s to their intermediate position between i he earlier Monotremes and the later Placentals. The detective development of the brain (especially the cerebrum), the possession of marsupial bones, and the simple construction of the allantois (without any placenta as yet) wire inherited by the Marsupials, with many other features, from the Monotremes, and preserved. On the other hand, they have lost the independent bone ( ninuoi- deumj at the shoulder-blade. But we have 8 more important advance in the disappearance of the cloaca ; the rectum and anus are separated by a partition from the uro-genital opening (sinus urogeiiitalis). Moreover, all the Marsu- pials have teats on the mammary glands, at which the new-born animal sucks. The teats pass into the cavity of a pouch or pocket on the ventral side of the mother, and this is supported by a couplfl of marsupial bones. The young are born in a very imperfect condition, and carried Fie. 27:.— Lower jaw of a Promammal (Dryo- testes firhcus), from the Jurassic of the Felsen strata. (From Marsh.) by the mother for some time longer in her pouch, until they are fully developed (Fig. 272). In the giant kangaroo, which is as tall as a man, the embryo only developes for a month in the uterus, is then born in a very imperfect stale, and finishes its growth in the mother's pouch ( marsupium) ; it remains in this about nine months, and at first hangs con- tinually on to the teat of the mammary gland. From these and other characteristics (especially the peculiar construction of the internal and external sexual organs in male and female) it is clear that we must conceive the whole sub-class of the Marsupials as one stem group, which has been developed from the Promam- malia. From one branch of these Marsupials (possibly from more than one) the stem-forms of the higher Mammals, the Placentals, were afterwards evolved. Of the existing forms of the Marsupials, 25-' OUR FIVE-TOED ANCESTORS which have undergone various modifica- tions through adaptation to different environments, the family of the opossums (Didelpkida or Pedimttna) seems to be the oldest and nearest to the common stem-form o\ the whole class. To this Family belong the crab-eating opossum ol' Brazil (Fig. 272) and the opossum of Lemurs, were evolved directly from the opossum. We must not forget, however, that the conversion of the five-toed foot into a prehensile hand is polyphyletic. By the same adaptation to climbing trees the habit of grasping their branches with the feet has in many different cases brought about that opposition of the Fig. 272.— The crab-eating Opossum (Philander cancrivorus ). The temate has three young in the (Fr« •pouch. (From Brehm. Virginia, on the .embryology of which Selenka has given us a valuable work (cf. Figs. 63-7 and ^'"S)- These' Didelphida .climb trees like the apes, grasping the branches with their hand- shaped hind feetf We may conclude from this that the stem-forms of the Primates, which we must regard as the earliest thumb or great toe to the other toes which makes the hand prehensile. We see this in the climbing lizards (chame- leon), the birds, and the tree-dwelling mammals of various orders. Some zoologists have lately advanced the opposite opinion, that the Marsupials represent a completely independent sub- or A' APE ANCESTORS »53 class of the Mammals, with no direct relation to the Placentals, and developing independently of them from the Mono- tremes. But t li i ^ opinion is untenable if wt examine carefully the whole organisa- tion of the three sub-classes, and do not lay the chief stress ow incidental features and secondar) adaptations (such as the formation of the marsupium). It is then clear that the Marsupials viviparous Mammals, without placenta are a neces- sary transition from the oviparous Mono- tremes to the higher Placentals with chorion-villi. In this sense the Marsupial class certainly contains some of man's ancestors. Chapter XXIII. OUR APE ANCESTORS Tin: long scries o\ animal forms which we must regard as the ancestors of our race has been confined within narrower and narrower circles as our phylogenetic inquiry has progressed. The great majority of known animals do not fall in the line Of our ancestry, and even within the vertebrate stem only a small number are found to do so. In the most advanced class oi the stem, the mammals, there are Only a few families that belong directly to our genealogical tree. The most important o\ these are the apes and their predecessors, the half-apes, and the earliest Placentals f Prochoriata). The Placentals (also called Choriata, Monodelphia, Eutheria or Epitheria) are distinguished from the lower mammals we have just considered, the Monotrcmes and Marsupials, by a number of striking peculiarities. Man has all these distinc- tive features ; that is a very significant fact. We may, on the ground of the most careful comparative - anatomical and ontogenetic research, formulate the thesis : " Man is in ever] respect a true Placental." He has all the characteristics of structure and development that distin- guish the Placentals from the two lower divisionsof the mammals, and, in fact, from all other animals. Among these charac- teristics we must especially notice the more advanced development of the brain. The fore-brain or cerebrum especially is much more developed in them than in the lower animals. The COTpUS callosttm, which forms a sort of wide bridge con- necting the two hemispheres o\ the cerebrum, is only fully formed in the Placentals ; it is very rudimentary in the Marsupials and Monogenics. It is true that the lowest Placentals are not far removed from the Marsupials in cerebral development ; but within the placental group we can trace an unbroken grada- tion of progressive development of the brain, rising gradually from this lowest stage up to the elaborate psychic organ of the apes and man. The human soul — a physiological function of the brain— is in reality only a more advanced ape-soul. The mammary glands of the Placentals are provided with teats like those of the Marsupials ; but we never find in the Placentals the pouch in which the latter carry and suckle their young. Nor have they the marsupial bones in the ventral wall at the anterior border of the pelvis, which the Marsupials have in common with the Monotremes, and which are formed by a partial ossification ol the sinews of the inner oblique abdominal muscle. There are merely a few insignifi- cant remnants of them in some of the Carnivora. The Placentals are also generally without the hook-shaped pro* es> at. the angle of the lower jaw which is found in the Marsupials. However, the feature that characterises the Placentals above all others, and that has given its name to the whole sub- class, is the formation of the placenta. We have already considered the formation and significance of this remarkable embryonic organ when we traced the development of the chorion and the allantois in the human embryo (pp. 165 o). The urinary sac or the allantois, the 254 OUR APE ANCESTORS curious vesicle thai grows out oi the hind p. nt oi the gut, lias essentially the same structure and function in the human embryo as in thai oi all the other Am- niotes (cf. Figs. 194 t>). There is a quite secondary difference, on which great stress lias wrongly been laid, in the fact that in man and the higher apes the original cavity oi the allantois quickly degenerates, and the rudiment of it sticks out as a solid projection from the primitive gut. The thin wall of the allantois consists of the same two layers or membranes as the wall oi the gut — the gut-gland Jayer within and the gut-fibre Fig. 273.— Foetal membranes of the human embryo [diagrammatic), m the thick muscular wall of the womb. plu placenta [the inner layer (phi' J or which penetrates into the chorion-villi ( chzj with its processes], cAftufaqi, chl smooth chorion, a amnion, ah amniotic cavity, as amniotic sheath of the umbilical cord (which passes urjder into the navel of the embryo —not given here), dg vitelline duct, ds yelk sac, dv, dr decidua (vera and reflexa). The uterine cavity ( uh ) opens below into the vagina and above on the right Jhto an oviduct (t). (From Kolliker.) layer without. In the gut-fibre layer of the allantois there are large blood-vessels, which serve for the nutrition, and especially the respiration, of the embryo — the umbilical vessels (p. 170). In the reptiles and birds the allantois enlarges into a spacious sac, which encloses the embryo with the amnion, and does not combine with the outer fcetal membrane (the chorion). This is the case also with the lowest mammals, the oviparous Monotremes and most of the Marsupials. It is only in some of the later Marsupials ( Peratneluia ) and all the Placentals that the allantois developes into the distinctive and remarkable structure that we call the placenta. The placenta is formed by the branches of the blood-vessels in the wall oi the allantois growing into the hollow BCtodermic tufts (villi) of the chorion, which run into corresponding depressions in the mucous membrane of the womb. The latter also is richly permeated with blood-vessels which bring the mother's blood to the embryo. As the partition in the villi between the maternal blood- vessels and those of the fcetus is extremely thin, there is a direct exchange of fluid between the two, and this is of the greatest importance in the nutrition of the young mammal. It is true that the maternal vessels do not entirely pass into the fcetal vessels, so that the two kinds of blood are simply mixed. But the partition between them is so thin that the nutritive fluid easily transudes through it. By means of this transudation or diosmosis the exchange of fluids takes place without difficulty. The larger the embryo is in the placentals, and the longer it remains in the womb, the more necessary it is to have special structures to meet its great - consumption of food. In this respect there is a very con- spicuous difference between the lower and higher mammals. In the Marsupials, in which the embryo is only a comparatively short time in the womb and is born in a very immature condition, the vascular arrangements in the yelk-sac and the allantois suffice for its nutrition, as we find them in the Monotremes, birds, and reptiles. But in the Placentals, where gestation lasts a long time, and t he embryo reaches its full development under the protection of its enveloping mem- branes, there has to be a new mechanism for the direct supply of a large quantity of food, and this is admirably met by the formation of the placenta. Branches of the blood-vessels penetrate into the chorion-villi from within, starting from the gut-fibre layer of-the allantois, and bringing the blood of the fcetus through the umbilical vessels ("Fig. 273 chs). On the other hand, a thick network of blood-vessels developes in the mucous membrane that clothes the inner surface of the womb, especially in the region of the depressions into which the chorion- villi penetrate (phi). " This network of arteries contains maternal blood, brought by the uterine vessels. As the connective tissue between the enlarged capillaries of OCR ARE ANCESTORS 255 the uterus disappears, wide cavities filled with maternal blood appear, and into these the chorion-villi of the embryo penetrate. The sum of these vessels oi both kinds, that are so intimately corre- lated at this point, together with the connective ;md enveloping tissue, is the placenta. The placenta consists, there- fore, properly speaking, of two different though intimately connected parts the foetal placenta (Fig. .7.; cAm) within and the maternal or uterine placenta ( tin J without. The latter is made up oi the mucous coal of the uterus and its blood- vessels, the former of the tufted chorion and the umbilical vessels of the embryo (cf. Fig. 196). The manner in which these tWO kinds of vessels combine in the placenta, and the structure, form, and size of it, differ ;i good deal in the various Placentals ; to some extent they give us valuable data at birth the foetal placenta alone comes a\va\ ; the Uterine placenta i- fid torn ;tu ,iy with it. The formation iA' the placenta : different in the second and highei section of the Placentals, the Deciduata. Here again the whole surface of the chorion is thickly covered with the \illi in the beginning. But they afterwards dis- appear from one part of the BUrface, and grow proportionate!) thi< ker on the other part, we thus get a differentiation between the smooth chorion {chorion laeve, Fig. 27^ chl) and the thickly- tufted chorion (chorion frvndasum, I 273 ch/J. The former lias only a few small villi or none at all; the latter is thickly covered with large and well- developed villi ; this alone now constitutes the placenta. In the great majorit) of the Deciduata the placenta has the same shape as in man (Figs. 197, 200) — namely I'k:. --74. -Skull of a fossil lemur (Adapt* f>an'sie>ish), from the Miocene at Quercy. A lateral view from the ri^ht, halt natural size. B lower jaw, C lower molar, i incisors, r canines, / premolars, /;; molars. for the natural classification, and there- fore the plnlogcny, of the whole of this sub-class. On the ground of these differences we divide it into two principal sections ; the lower Placentals or Indecidua, and the higher Placentals or Deciduata, To the Indecidua belong three im- portant groups of mammals : the Lemurs ( I'rosimiiF y.the Ungulates (tapirs, horses, pi^s, ruminants, etc.), and the Celacea (dolphins and whales). In these Indecidua the villi are distributed over the whole surface of the chorion (or its greater part ), either singly or in groups. They arc- only loosely connei ted with the mucous coat of the uterus, so that the whole futal membrane with its \illi can be easily withdrawn from the uterine depressions like a hand from a glove. There is no real coalescence of the two placentas at any part of the Surface of contact. Hence a thick, circular disk like a cake ; so we find in the Insectivora, Chiroptera Rodents, and Apes. This disCOplacenta lies on one side of the chorion. Hut in the Sarcotheria (both the Carnivora and the seals, Pinnipcdin) and in the elephant and several other Dcciduates we find a eonoplacenta ; in these the rich mass oi villi runs like a girdle round the middle of the ellipsoid chorion, the two poles ol it being free from them. Still more characteristic of the Deciduates is the peculiar and very intimate connection between the chorion Jrondosuni and the corresponding part oi the mucous COat of the womb, which we must regard as a real coalescence of the two. The villi of the chorion push their branches into the blood-lillcd tissues ol the COat of the uterus, and the vessels oi each loop together .so intimately that it is no longei possible to separate the foetal •S6 OUR APE ANCESTORS from the maternal placenta ; they form henceforth a compact and apparently simple. placenta. In consequence of this coalescence^ a whole piece of the lining of the womb comes away at birth with the fnt.il membrane thai is interlaced with it. Thi> piece is called the " falling-awa\ membrane (decidua). It is also railed the serous (spongy) membrane, because Frc. 275.— The tail-less lemur. Slender Lori (Stcnof** gracilis) of Ceylon it is pierced like a sieve or spouse. All the higher Placentals that have this decidua are classed together as the Deciduates." The tearing away of the decidua at birth naturally causes the mother to lose a quantity of blood, which does not happen in the Indecidua. The last part of the uterine coat has to be repaired by a new growth after birth in the Deciduates. (Cf. Figs. 'c)9, 200, pp. 168 70.) In the various orders of the Deciduates the placenta differs considerably both in outer form and internal structure. The extensive investigations of the last ten years have shown that there is more variation in these respects among the higher mammals than was formerly supposed. The physiological work of this important em- bryonic organ, the nutrition of the foetus during its long sojourn in the womb, ;s accomplished in the various groups of the Placentals by very different and sometimes verv elaborate structures. They have lately been fully described by Hans Strahl. The phytogeny of the placenta has become more intelligible from the fact that we have found a number of transitional forms of it. Some ofthe Marsupials ( Perameles) have the beginning of a placenta. In some of the Lemurs (Tarsias ) a discoid placenta with decidua is developed. While these important results of comparative em- bryology have been throwing further light on the close blood-relationship of man and the anthropoid apes in the last few years (p. 172), the great advance of paleontology has at the same time been affording us a deeper insight into the stem-history of the Placental group. In the seventh chapter of my Syste- matic Phytogeny of the Verte- brates I advanced the hypo- thesis that the Placentals form a single stem with many branches, which has been evolved from an older group of the Marsupials < Pnnii- delphia). The four great legions ofthe Placentals Rodents, Ungulates, C'ar- nassia, and Primates are sharply separated to-day by important features of organisation. Hut if we consider their extinct ancestors of the Tertiary period, the differences gradually disappear, the deeper we go in the Cenozoic deposits ; in theend we find that they vanish altogether. OUR APE ANCESTORS 257 The primitive sU'in-forms of t ho RodeatS ( EsthonychidaJ, the Ungulates (Ckondy- larthraj, the Carnassia flctofistdaj, and the Primates /" Lemurnvida j are so ^ lose!) related .it the beginning of the Tertiary period thai we might group them together ,i> different Families of one order, the Proplacentals ( Afallotheria or ho ria la J. Heme the great majority of the Placentals have no direct and close relationship to man, but only the legion of the Primates. This is now general 1) divided into three orders' the halt-apes (Prosimia*), apes (Simiat), and man (Anthropi). The lemurs or half-apes are the stem-group, descending from the older Mallotherux of the Cretaceous period. From them the apes were evolved in the IVrtiaiv peiiod, and man was formed from these towards its I lose. The Lemurs ( ProsimiaJ have few Kving repre tentatives. But thej areverj interest- ing, and are the last survivors of a oiue extensive group. We find many fossil remains of them in the older Tertiary deposits of Europe and North America, in the Eocene and Miocene. We distin- guish two sub-orders, the fossil Lemura- vida and the modern Lemurogona. The earliest and most primitive forms of the Lemuravidn are the Pachylemursf- Hybofi- sodina); they come next to the earliest Placentals (ProchoriataJ, and have the typical full dentition, with fort \ -four teeth ). The Necrolemurs [Adapida, Fig. J74I have only forty teeth, and have lost an incisor in each jaw (" \ The dentition is still further reduced in the Lemurogona (Autolemures), which usually have Onlj thirty-six teeth I ' j These living survivors are scattered fat- over tile southern part of the Old World. Most of the species live in Madagascar, some in the Sunda Islands, others on the mainland of Asia and Africa. They are gloomy and melancholic animals ; they live a quiet life, climbing trees, and eating fruit and insects. They are of different kinds. Some are closely related to the Marsupials (especially the opossum). Dthers fMacrotarsi) are nearer to the Insectivora, others again (Chiromys) to the Rodents. Some Of the lemurs ( Brachy tarsi ) approach closely to the true apes. The numerous fossil remains of half-apes and apes that have been recently found in the Tertiary deposits justify us in thinking that man's ancestors were represented by several different species during this long period. Some of these were almost as big as men, sin h as the diluvial lemurogonon Megaladapis of Madagascar. Next to the lemurs come the true apes f Simia j, the twenty-sixth stage in our ancestry. It has been hcvond question for some time now that the apes approach nearest to man in every respect of all the animals. J list as the lowest apes come close to the lemurs, >o the highest come next to man. When we carefully study the comparative anatomy of the apes and man, we can trace a gradual and uninter- rupted advance in the organisation of the ape up to the purely human frame, and, after impartial examination of the"ape- Fig. 276.— The white-nosed ape (CemfithecuM frtanrista ).• problem " that has been discussed" of late years with such passionate interest, we come infallibly to the important conclu- sion, first formulated by Huxley in 1863 : " Whatever systems of organs we take, the comparison of their modifications in the seriesofapes leads to the same result : that the anatomic differences that separate man from the gorilla and chimpanzee are not as great as those that separate the gorilla from the lower apes." Translated into phylogenetic language, this " pithe- cometra-law, "formulated in such masterly fashion by Huxley, is quite equivalent to the popular saving: "Man is descended from the apes.'' In the very first exposition of his pro- found natural classification (1735) Linne »5* OCR ARE ANCESTORS placed the anthropoid mammals at the head of the animal kingdom, with three genera : man, the ape, and the sloth. 1 [e afterwards called them the " Primates" — the "lords" o\ the animal world ; ho then also separated the lemur from the true ape, and rejected the sloth. Later zooloi;iM> divided the order of Primates. and Quadrumana was retained by Cuvier and most of the suhsdquent zoologists. It seems to be extremely important, hut, as a matter of fact, it is totally wrong. This was first shown in 1863 by Huxley, in his famous Man's Place in Nature. On the strength o( careful comparative- anatomical research he proved that the Fig. 277.— The drill-baboon ( Cynocephalus leucofihcrtis) (From Rrehm.) First the Gottingen anatomist, Blumen- bach, founded a special order for man, which he tailed Bimana (" two-handed ") ; in a second order he united the apes and lemurs under the name of Quadnonaiia ("four-handed"); and a third order was formed of the distantly-related ChirOptera (bats, etc.). The separation of the Bimana apes .are just as truly " two-handed " as man ; or, if we prefer to reverse it, that man is as truly four-handed as the ape. He showed convincingly that the ideas of hand and foot had been wrongly defined, and had been improperly based on physio- logical instead of morphological grounds. The circumstance that we oppose the OCR APE ANCESTORS 259 thumb to the other four fingers in Kin- hand, and so can grasp things, seemed to be a spe< i.il distinction of the hand in contrast to the foot, in which the corre- sponding great toe cannoi be opposed in this u.i\ to the others. Bui the apes can grasp with the hind-fool as well a> the fore, and so were regarded as quadru- manous However, the inability to grasp that we find in the fool of civilised man is a consequence of the habit of clothing it with tight coverings for thousands of years. Many of the bare-footed lower races of men, especially among the negroes, use the foot very freely in the same way ;i^ the hand. As a result of earl] habit and continued practice, they can grasp with ihe fool (in climbing trees, lor instance) just as well as with the hand. Even new-horn infants of our own race can grasp very strongly with the great toe, and hold a spoon with it as firmly as with the hand. Hence the physiological distinction between hand and foot can neither be pressed very tar, nor lias it a scientific- basis. We must look to morphological characters. As a matter of fact, it is possible to draw such a sharp morphological distinc- tion a distinction based on anatomic structure - between the fore and hind extremity. In the formation both of the bony skeleton and of the muscles that are connected with the hand and foot before and behind there are material and con- stant differences ; and these are found both in man and the ape. For instance, the number and arrangement of the smaller bones of the hand and foot are quite different. There are similar con- stant differences in the muscles. The hind extremity always has three muscles (a short flexor muscle, a short extensor muscle, and a long calf-muscle) that are not found in the fore extremity. The arrangement of the muscles also is different before and behind. '1 hese charac- teristic differences between the fore and hind extremities are found in man as well as the ape. There can be no doubt, there- fore, that the ape's foot deserves that name just as much as the human foot does, and that all true apes are just as " bimanous " as man. The common distinction of the apes as " quadrumanous " is altogether wrong morphologically. Hut it ma\ be asked whether, quite apart from this, we can find any Other features that distinguish man more sharply from the ap< than the various spot ies ot apis are distinguished from' each other. Huxlej gave so complete and demonstrative a reply to this question thai the opposition still raised on many s'aks is absolutely Without foundation. On the ground of careful comparative* anatomical research, Huxley prosed that in all morphological respects the differ* ences between the highest and lowest .n\' greater than the corresponding differences between the highest apes and man. He thus restored Lmne's order of the Primates (excluding the bats), and divided it into three sub-oi tiers, t ho first composed of the half-apes (Lemurida)^ the second of the true apes ( Simiadc y, the third of men f AtUhfOpidCB) lint, as we wish to proceed quite con- sistently and impartially on the laws of systematic logic, we may, on the strength o\ Huxley's own law, go a good deal farther in this division. We are justified in going at least one important step farther, and assigning man his natural place within one of the sections of the order of apes. All the features that characterise this group of apes are found in man, and not found in the other apes. We do not seem to be justified, therefore, in founding for man a special order distinct from the apes. The order of the true apes ( Simice or Pxtheca) — excluding the lemurs — has long been divided into two principal groups, which also differ in their geographical distribution. One group (I/tiptropit/iica, or western apes) live in America. The other group, to which man belongs, are the Eopitheca ox eastern ,1 pis ; they are found in Asia and Africa, and were formerly in Europe. All the eastern apes agree with man in the features that are chiefly used in zoological classifica- tion to distinguish between the two simian groups, especially in the dentition. The objection might be raised that the teeth are too subordinate an organ physio- logically for us to lay stress m\ them in so important a question. Hut there is a good reason for it ; it is with perfect justice that zoologists have for more than a century paid particular attention to the teeth in the systematic division and arrangement of the orders of mammals. The number, form, and arrangement ol the teeth are much more faithfully inherited in the various orders than most other characters. Hence the form of dentition in man is very important. In the fully developed 260 OUR APE ANCESTORS condition we have thirty-two teeth , of these eighl are incisors, tour canine, and twenty molars. The eight incisors, in Uic middle of the jaws, have certain Next to these, at each side of hoth jaws, is a canine (or "eye tooth "), which is larger than tlie incisors. Sometimes it is very prominent in man, as it is in most 6~< — S 5 ~ ;? ocf <2* S* T «->u. Vw ■ajss go eS ■S-^ * «l g?j c 2 rigf £0% rt^, *- = o-c 00 ^ V 00 u -C-O org! «<* G<5$ rf.l aJ3 fa<-> characteristic differences ahove and below. In the upper jaw the inner incisors are larger than the outer ; in the lower jaw the inner are the smaller. apes and many of the other mammals, and forms a sort of tusk. Next to this there are 6ve molars above and below on each side, the first two of which (the OUR APE ANCESTORS 261 * pre-molars ") are small, have only one root, and are included in the change o( teeth ; the three bai k ones are much larger, have two roots, and only come with the second teeth. The apis of the Old World, or all the living Or fOSSil apes o\ Asia, Africa, and Europe, have the same dentition as man. On the other hand, all the American apes have an additional pre-molar in each half oi the jaw. They have six molars above and helow 011 each side, or thirty-six teeth altogether. This charac- teristic difference between the eastern and western apes has been so faithfully inherited that it is very instructive for us. It is true that there seems to he an excep- tion in the ease of a small family of South American apes. The small silky apes ( Arctopitheca or ffapaJidigJ, which in- clude the tamarln (Midas) and the brush-monkey fjaccivs), have only Gve molars in each half of the jaw (instead of six), and si) seem to he nearer to the eastern apes. Hut it is found, on closer examination, that they have three pre- molars, like all the western apes, and that only the last molar has been lost. Hence the apparent exception really confirms the above distinction. Of the other features in which the two groups of apes differ, the structure of the nose is particularly instructive and con- spicuous. All the eastern apes have the same type of nose as man — a compara- tively narrow partition between the two halves, so that the nostrils run down- wards. In some of them the nose protrudes as far as in man, and has the same characteristic structure. We have already alluded to the curious long-nosed apes, which have a Long, finely-curved nose. Most of the eastern apes have, it is true, rather Hat noses, like, for instance, the white-nosed monkey (Fig. 276) ; but the nasal partition is thin and narrow in them all. The American apes have a different type of nose. The parti- tion is very broad and thick at the bottom, and the wings of the nostrils are not developed, so that they point outwards instead of downwards. This difference- in the form of the nose is so constantly inherited in both groups that the apes of the New World are called "flat-nosed" ( ' I'latyirhituv J, and those of the Old World " narrow-nosed " ( Catarrhince ) . The bony passage of the ear (at the bottom of which is the tympanum) is short and wide in all the Platyrrhines, but long and narrow in all the Catar- rhines ; and in man this difference also is significant. This division of the apes into Platyr- rhines and (.'atari bines, on the ground of the above hereditary features, is now generall) admitted in zoology, and receives strong support from the geo- graphical distribution of the two groups in the east and west. It follows at once, as regards the phylogeny of the apes, that tWO divergent lines proceeded from the common stem-form of the ape-o'dcr in the early Tertiary period., one of which spread over the OJd, the other over the New, World. It is certain that all the Platyrrhines* come oi' one stock, and also all the Catarrh ines ; but the former are phylogenetically older, and must be regarded as the stem-group of the latter. What can we deduce from this with regard to our own genealogy? Man has just the same characters, the same form of dentition, auditory passage, and nose, as all the Catarrh ines ; in this he radi- cally differs from the Platyrrhines. We are thus forced to assign him a position among the eastern apes in the order of Primates, or at least place him alongside of them. But it follows that man is a direct blood relative of the apes of the Old World, and can be traced to a common stem-form together with all the Catarrhines. In his whole organisation and in his origin man is a true Catar- rhine ; he originated in the Old World from an unknown, extinct group of the' eastern apes. The apes of the New World, or the Platyrrhines, form a divergent branch of our genealogical tree, and this is only distantly related at its root to the human race. We must assume, of course, that the earliest Eocene apes had the full dentition of the Platyrrhines ; hence we may regard this stem-group as a special stage (the twenty- sixth) in our ancestry, and deduce from it (as the twenty-seventh stage) the earliest Catarrhines. We have now reduced the circle of our nearest relatives to the small and com- paratively scanty group that is repre- sented by the sub-order of the Catar- rhines, and we are in a position to answer the question of man's place in this sub-order, and say whether we can deduce anything further from this posi- tion as to our immediate ancestors. In answering this question the comprehen- sive and able studies that Huxley gives of ?r>j or A' .!/*/: ANCESTORS the comparative anatomy of man and the various Catarrhines in his Man's /'/an- i/i Nature are of great assistance to us. It is quite clear from these that the differences between man and the highest' Catarrhines (gorilla, chimpanzee, and orang) are in every respect slighter than the corresponding differences between the highest and the lowest Catarrhines (white-nosed monkey, macaco, baboon, etc.). In fact, within the small group o\ the tail-less anthropoid apes the differences between the various genera ate not less than the differences between them and man. This is seen by a glance at the skeletons that Huxlev has put together (FigS. 278-282). Whether we take the skull or the vertebra! column or the ribs or the fore or hind limbs, or whether we extend the comparison to the muscles, blood-vessels, brain, placenta, etc., we always reach the same result on impartial examination— that man is not more different from the other Catarrhines than the extreme forms o\ them (for instance, the gorilla and baboon) differ from each other. We may now, therefore, complete the Iluxleian law we have already quoted with the following thesis: "Whatever system ol' organs we take, a comparison of their modifications in the series oi Catarrhines always leads to the same conclusion ; the anatomic differences that separate man from the most advanced Catarrhines (brang, gorilla, chimpanzee) are not as great as those that separate (lie latter from the lowest Catarrhines (white-nosed monkey, macaco, liaboon)." We must, therefore, consider the desi em of man from other Catarrhines to be fully pinned. Whatever further information on the comparative anatomy and onto- geny of the living Catarrhines we may obtain in the future, it cannot possibly disturb this conclusion. Naturally, our Catarrhine ancestors must have passed through a long series of different forms before' the human type was produced. The chief advances that effected this "creation of man," or his differentiation from the nearest related Catarrhines, wire : the adoption of the erect posture and the consequent greater differentiation ol the fore and hind limbs, the evolution of articulate speech and its organ, the larynx, and the further development i>i' the brain and its function, the soul ; sexual selection had a great influence in this, as Darwin showed in his famous work. With an eye to these advances we can distinguish at least four important sta^es- in our simian ancestry, which represent prominent points in the historical procesf oi the making of man. We may take, after the Lemurs, the earliest and lowest Platyrrhines of South America, with thirty-six teeth, as the twenty-sixth stage oi our genealogy ; they were developed from the Lemurs by a peculiar modifica- tion of the brain, teeth, nose, and fingers. From these Eocene stem-apes were formed the earliest Catarrhines or eastern apes, with the human dentition (thirty -two teeth), by modification of the nose, lengthening of the bony channel oi the ear, and the loss of four premolars. These oldest stem-forms oi the whole Catarrhine group were still thickly coated with hair, and had long tails- baboons ( Cynopitheca) or tailed apes (Jtfenocerca, Fig. 276). They lived during the Tertiary period, and are found fossilised ill the Miocene. Of the actual tailed apes perhaps the nearest to them are the Semnopitheci. If we take these Semnopitheci as the twenty-seventh stage in our ancestry,, we may put next to them, as the twenty- eighth, the tail-less anthropoid apes. This name is given to the most advanced and man-like of the existing Catarrhines. They were developed from the other Catar- rhines by losing the tail and part oi the hair, and by a higher development oi the brain, which found expression in the enor- mous growth ol' the skull. Oi this re- markable family there are only a few genera to-dav, and we have ahead) dealt with them (Chapter XV.) — the gibbon (//)7ol>a/cs, Fig. 203) and orang (Salvias, Figs. 204, 205) in South-Eastern Asia and the Archipelago ; and the chimpanzee [Anthropithecus, Figs. 206, 207) and gorilla [Gorilla, Fig. 208) in Equatorial Africa. The great interest that every thought- ful man takes in these nearest relatives of ours has found expression recently in a fairly large literature. The most distin- guished of these works for impartial treat- ment o\' the question of affinity is Robert rlartmann's little work- on The Anthro- poid A pes. I [art man n divides the primate order into two families: (1) Primarii (man and the anthropoid apes); and (2) SiaiiaiuP (true apes, Catarrhines and Plal vrrhiius). Professor Klaatsch, of Heidelberg, has advanced a different view in his interest- ing and richly illustrated work on The Origin ami lievelopment of ike Human OCR APE ANCESTORS *3 Ran*. This is a substantial supplemeiK to my Antkrvpogeny, in so tar as it gives the chief results of modern research on t ho early history of man and civilisation. But when Klaatsch declares the descent of man from the apes to be "irrational, narrow-minded, and false," in the belief that wo are thinking of some firing species of ape, we must remind him that no competent scientisl has ever held sd narrow a view. All of us look merely - in the sense of Lamarck and Darwin to the original unity (admitted by Klaatsch) of the primate stem. This common descenl of all the Primates (men, apes, and lemurs) from one primitive stem- form, from which the most far-reaching conclusions follow tor the whole of anthro- pology and philosophy, is admitted by Klaatsch as well as by mysell and all other competent zoolo- gists who accept the heory of evolution in general. He says explicitly (p. 17-'): "The three anthropoid apes - - gorilla," chimpanzee, and orang seem to he branches from a com- mon root, and this was not far from that of the gibbon and man." That is in the main the opinion that I have maintained (especially against Virchow) in a number of works ever since 1866. The hypo- thetical common ancestor of all the Primates, which must have lived in the earliest Tertian- period (more prob- ably in the Cretaceous), was called by me Archiprimus ; Klaatsch now calls it Prima- luiii Dubois has proposed the appro- priate name of Protkylobates for the common and much younger stem-form of the anthropomorpha (man and the anthro- poid apes). The actual HylobaUs is nearer to it than the other three existing anthro- poids. None of these <-o\ be said to he absolutely the most man-like. The gorilla conies next to man in the structure of the hand and loot, the chimpanzee in the chief features of the skull, the orang in hrain development, and the gibbon in the formation of the chest. None of these existing anthropoid apes is among the direct ancestors of our race ; they are scattered survivors of an ancient branch of the Catarrhines, from which the human race developed in a particular direction. Although manisdirectlv connected with (his anthropoid family and originates from it, we may assign an important inter- mediate* form between the Protkylobates and him (the l w entv-ninth stage in our ancestrj , the ape-nun (Pithecanthropi-). I gave this name in the History <>> ('ra- tion to the " Speechless primitive men" ( Aliili), which were nun in the ordinary sense aS t.ii as the general structure is concerned (especially in the differentia- tion of the limbs), hut lacked one of the chief human characteristics, articulate speech and the higher intelligence that goes with it, and so had a less developed hrain. The phylogenet.c hypothesis of the organisation of this "ape-man " which I then advanced vva.s brilliantly confirmed twenty-four years afterwards hy the famous discovery of the fossil Pithec- Fio. 28}.— Skull of the fossil ape-man of Java ( Pithec- anthropus erect 'us j. restored bj Eugen Dubois. anthropus erectus by Eugen Dubois (then military surgeon in Java, afterwards professor at Amsterdam). In 1892 he found at Trinil, in the residency of Madiun in Java, in Pliocene deposits, certain remains of a large and very man- like ape (roof of the skull, femur, and teeth), which he deserihed as "an erect ape-man" and a survivor of a "stem-form of man" (Fig. 283). Naturally, the Pithecanthropus e\(ited the liveliest interest, as the long-SOUght transitional form between man and the ape: we seemed to have found "the missing link." There were very interesting scientific dis- cussions of it at the last three International Congresses of Zoolog) (Leyden, 1895, Cambridge, [898, and Berlin, 1901). 1 took an active part in the discussion at 2(H OCR APE ANCESTORS Cambridge, and may refer the reader to the paper I road there on "The Presenl Position of elm Knowledge of the Origin of Man " (translated by Dr. Gadow with the title of The Last Link). An extensive and valuable literature has grown up in the last ten years on the Pithecanthropus and the pithecoid theory connected with it. A number of distin- guished anthropologists, anatomists, paleontologists, and phylogenists have taken pan in the controversy, and made use of the important data furnished by the new science of pre-historic research. Hermann Klaatsch has given a good ! summary of them, with many fine illus- trations, in the above-mentioned work. I refer the reader to it as a valuable supplement to the present work, especially as I cannot go any further here into these anthropological and pre-hist6ric questions. I will only repeat that I think he is wrong in the attitude of hostility that he affects to take up with regard to my own views on the descent of man from the apes. The most powerful opponent of the pithecoid theory — and the theory of evolu- tion in general— during the last thirty years (until his death in September, 1902) was the famous Berlin anatomist, Rudolf Virchow. In the speeches which he delivered every year at various con- gresses and meeting-, on this question, he was never tired of attacking the hated "ape theory." His constant categorical position was : " It is quite certain that man does not descend from the ape or any other animal." This has been repeated incessantly by opponents of the theory, especially theologians and philo- sophers. In the inaugural speech that he delivered in 1894 al lne Anthropological Congress at Vienna, lie said that " man might just as well hare descended from a sheep or an elephant as from an ape." Absurd expressions like this only show that the famous pathological anatomist, who did so much for medicine in the establishment of cellular pathology, had not the requisite attainments in compara- tive anatomy and ontogeny, systematic /oology and paleontology, for sound judg- ment in the province of anthropology. The Strassburg anatomist, Gustav Schwalbe, deserved great praise for having the moral courage to oppose this dogmatic and ungrounded teaching of Virchow, and showing its untenability. The recent admirable works of Schwa I be on the Pithecanthropus, the earliest races of men, and the Neanderthal skull (1897- 1901 ) will supply any candid and judicious reader with the empirical material with which he can convince himself of the baselessness of the erroneous dogmas of Virchow and his clerical friends (J. Ranke, J. Bumuller, etc.). As the Pithecanthropus walked erect, and his brain (judging from the capacity of his skull, Fig. 283) was midway between the lowest nun and the anthro- poid apes, we must assume that the next great step in the advance from the Pithec- anthropus to man was the further develop- ment of human speech and reason. Comparative philology has recently shown that human speech is polv- phyletic in origin ; that we must dis- tinguish several (probably many) different primitive tongues that were developed independently. The evolution of lan- guage also leaches us (both from its ontogeny in the child and its phytogeny in the race) that human speech proper was only gradually developed after the rest of the body had attained its charac- teristic form. It is probable that language was not evolved until after the dispersal of the various species and races of men, and this probably took place at the com- mencement of the Quaternary or Diluvial period. The speechless ape-men or Alali certainly existed towards the end of the Tertiary period, during the Pliocene, possibly even the Miocene, period. The third, and last, stage of our animal ancestry is the true or speaking man (Homo), who was gradually evolved from the preceding stage by the advance of animal language into articulate human speech. As to the time and place of this real "creation of man" we can only express tentative opinions. It was prob- ably during the Diluvial period in the hotter y.on<: of the Old World, either on the mainland in tropical Africa or Asia, or on an earlier continent (Lemuria — now sunk below the waves of the Indian Ocean), which stretched from East Africa (Madagascar, Abyssinia) to East Asia (Sunda Islands, Further India). I have given fully in my History of Creation (chap, xxv hi.) the weighty reasons for claiming this descent of man from the anthropoid eastern apes, and shown how we may conceive the spread ot the various races from this " Paradise " over the whole earth. I have also dealt fully with the relations of the various races and species of men to each other. SYNOPSIS OF THE CHIEF SECTIONS OF OUR STEM-HISTORY First stage : The Protists. Man's ana stora are unicellular protozoa, originally unnudeated Monera like tin- Chromacea, structureless green particles of I plasm ; afterwards real nucleated cells (firs! plasmodomous Protophyta, like the Palmella ; then plasmophagous ProtoMoa, 1 iki- the AllHl-l'.l I. Second stage: The Blastaeads. Man's ancestors are round ccenobia or colonies of Protozoa ; theyconsisl of a close association of many homogeneous cells, and thus are individuals of the second order. Tlu-v resemble the round cell-communities of the Magosphasras and Volvocina, equi- valent to the ontogenetic blastalai hollow globules, the wall of which consists of a single layer of ciliated cells (blastoderm). Third stage : The Gastraeads. M.m's ancestors are Gastraeads, like the simplest of the actual Metazoa ( Prophysema, Olynthus, Hydra, Pemmatodiscus). Their bodj consists merely of a primitive gut, the wall of which is made up of the two primary germinal layers. Fourth stage : The Platodes. Man's ancestors have substantially the organisation of simple Platodes (at first like the cryptoccelic Platodana, later like the | rhabdoccelic Turbellaria). The leaf-shaped bilateral-symmetrical body has only one gut- opening, and developes the first trace of a nervous centre from the ectoderm in the middle line of the back (i'igs. 239, 240). Fifth stage: The Vermalia. Man's ancestors have substantially the organisation of unarticulated Vermalia, at fust Gastrotricha (Icluhydina), afterwards Frontonia | Nemertina, Enteropneusta). Four secondary germinal layers develop, two middle layers arising between the limiting layers tVeeloma). The dorsal ectoderm forms the vertical plate, acroganglion Sixth stage : The Prochordonia. Man's ancestors have substantially the organisation of a simple unarticulated t'hor- donium (Copelata and Ascidia-larvas). The unsegmented chorda developes between the dorsal medullary tube and the ventral gut- tube. The simple « ivlom-pouches divide by a frontal septum into two on each side: the dorsal pouch (episornite) forms a muscle- plate ; the ventral pouch (hyposomite) tonus a gonad. Head-gul with gill-clefts. Seventh stage: The Acrania. Man's ancestors are skull-less Vertebrates, like the AmphioxuS. The body is a series >-i metamera, as several of the primitive segments are developed. The head contains in I la- ventral half the branchial gut, the trunk the hepatic- gut. The medullary tube is still simple. No skull, jaws, or limbs. Eighth stage : The Cyclostoma. Man's ancestors are jaw-less Craniotes (like th. Myxinoida and l'etromy/onta). The number of metamera increases. The fore-end of the medullary tube expands into a vesicle and forms the brain, which .soon divides into live cerebral vesicles. In the sides of it appear the three higher sense- organs: nose, eyes, and auditory vesicles. No jaws, limbs, or floating bladder. Ninth stage : The Iehthyoda. Man's ancestors arc fish-1ike Craniotes : (1) Primitive fishes (Selachii) ; (2) plated fishes (Ganoida); (3) amphibian fishes (Dipneusta); (4) mailed amphibia (Stego- ccphala). The ancestors of this series develop two pairs of limbs : a pair of fore (breast-fins) and of hind (belly-fins) legs. The gill-arches are formed between the gill- clefts : the first pair form the maxillary arches (upper and lower jaws). The floating bladder (lung) and pancreas grow out of the gut. Tenth stage : The Amniotes. Man's ancestors are Amniotes ov gill-less Vertebrates: (1) Primitive Amniotes (Pro- rcptilia) ; (2) Sauromammals ; (3) Primitive Mammals (Monotremes) ; (4 1 Marsupials; (ej Lemurs (Prosimise) ; (6) Western apes (Platyrrhin.e) ; (7) Eastern apes (Valar- rhinae): at first tailed Cynopitheca, then tail- less anthropoids ; later speechless ape-men (Mali) ; finally speaking man. The ances- tors of these Amniotes develop an amnion and allantois, ami gradually assume the mammal, and finally the specifically human, form. 265 -•(X) EVOLUTION OF THE NERVOUS system Chaptkr XXIV. EVOLUTION OF THE NERVOUS SYSTEM Tlir. previous chapters have taught us how the human body as a whole deve- lopes from the first simple rudiment, a single layer ol cells. The whole human race owes its origin, like the individual man, to i simple cell The unicellular stem-form ol" the race is reproduced daily in the unicellular embryonic stage of the individual. We have now to consider in detail the evolution of the various parts that make up the human frame I must, naturally, confine myself to the most general and principal outlines ; to make a special study of the evolution of each organ and tissue is both bevond the scope of this work, and probably beyond the anatomic capacity of most of my readers to appreciate In tracing the evolution of the various organs we shall follow the method that has hitherto guided us, except that we shall now have to consider the ontogeny and phytogeny of the organs together. We have seen, in studying the I evolution of the body as a whole, that ' phylogeny casts a light over the darker paths of ontogeny, and that we should be almost unable to find our way in it without the aid of the former We shall have the same experience in the- study of the organs in detail, and 1 shall be | compelled to give simultaneously their ontogenetic and phvlogenetic origin. | The more we go into the details of organic ' development, and the morel closely we follow the rise of the various parts, the J more wv see the inseparable connection | of embryology and stem-history The ontogeny of the organs can only be understood in the light of their phylogeny, ' just as we found of the embryology of ; the whole body Each embryonic form is determined by a corresponding stem- form. This is true of details is well as of the whole We will consider first the animal and then the vegetal systems ol organs of the body. The first group consists of the psychic and the motor apparatus To | the former belong the skip, the nervous system, and the sense-organs. The motor apparatus is composed of the passive and the active organs ol move- ment (the skeleton and the muscles) The second or vegetal group consists of the nutritive and the reproductive appa- ratus To the nutritive apparatus belong the alimentary canal with all its appen- dages, the vascular system, and the renal (kidney) system The reproductive appa- ratus comprises the different organs of sex (embryonic glands, sexual ducts, and copulative organs) As we know from previous chapters (XI. -XIII.), the animal systems oi organs (the organs ot sensation and presentation) develop for the most part out of the outer primary germ-layer, or the cutaneous (skin) layer On the other hand, the vegetal systems of organs arise for the most part from the inner primary germ- layer, the visceral layer li is true that this antithesis of the animal and vegetal spheres of the body in man and all the higher animals is by no means rigid ; several parts of the animal apparatus (for instance, the greater part of the muscles) are formed from cells that come originally from the entoderm , and a great part of the vegetative apparatus (for instance, the mouth-cavity and the gonoducts) are composed of cells that come from the ectoderm In the more advanced animal body there is so much interlacing and displace- ment of the various parts that it is often very difficult to indicate the sources of them. But, broadly speaking, we may take it as a positive and important fact that in man and the higher animals the chief part of the animal organs comes from the ectoderm, and the greater part of the vegetative organs from the entoderm It was for this reason that Carl Ernst von Iiaer called the one the animal and the other the vegetative~layer (see p. 16). The solid foundation of this important thesis is the gastrula, the most instructive embryonic form in the animal world, which we still find in the same shape in the most diverse classes of animals. This form points demonstrably to a EVOLUTION OF THE NERVOUS SYSTEM 267 common stem-form «>! all Ihe Metazoa, the Gustreta . in this long-extinct stem- form the whole body consisted throughout life si the two primary germinal layers, as is now the case temporarily in the gastrula, in the Gastraea the simple cutaneous (skin) layei actually repre- sented all the animal organs and func- tions, and the simple visceral (gut) layei all the vegetal organs and functions. Tins is the rasL- with the modern Gastraeads (Fig. 233)1 and it is also the cast potentially with the gastrula We shall easily sec thai the gastraea theory is thus able to throw a good deal of light, both morphologically and physio- logically, on some of the chief features of embryonic development, it we take up first the consideration ol the chut' element in the animal sphere, the psychic apparatus or sensormm and its evolution. This apparatus consists oi two very different parts, which seem at first to have very little connection with each Other the outer skin, with all its hairs, nails, sweat-glands, etc., and the nervous system. The latter comprises the central nervous system (brain and spinal cord), the peripheral, cerebral, and spinal nerves, and the sense-organs. In the fully- formed vertebrate body these two chief elements oi the sensonum lie far apart, the skm being external to, and the central nervous system in the very centre of, the body. The one is onlv connected with the other by a section oi the peripheral nervous system and the sense-organs. Nevertheless, as we know from human embryology, the medullar) tube is formed from the cutaneous layer The organs that discharge the most advanced func- tions oi the animal body the organs oi the SOUl, OT Of psychic life develop from the external skin. This is a perfectly natural and necessary process If we reflect on the historical evolution of the psychic and sensory functions, we are forced to conclude that the cells which accomplish them must originally have been located on the outer surface of the bod) Only elementary organs ill this superficial position could directly receive the influences of the environment. AJfter- wards, under ihe influence of natural selection, the cellular group in the skin which was specifically " sensitive " with- drew into the inner and more protected part of ihe body, and formed there the foundation oi a leiiiral nervous organ.. As a result oi increased differentiation, the skin and the central nervous system became further and further separated, and in the end the two were onlj perma- nent!) connected by the afferent peripheral sensoi \ nei \ es The observations oi the comparative anatomist are in complete accord with this \ ieu I le tells us that large nuinlu i s of the lower animals have no nervous system, though thej exercise the functions of sensation and will like the higher animals. In the unicellular IVoto/oa, which do not form germinal layers, there is, oi course, neithei nervous system nor Fig. 2K4 -The human skin in vertical section (from Ecker), highly magnified. « horny layer ol the epidermis, b mucous layer of the epidermis, c papillie of the corrurn, ). In the same way, all the parts and appendages of the epidermis develop by differentiation from the homogeneous cells of this horny plate ( Fig, 285). At an early Stage the simple cellular layer of this horny plate divides into two. The inner and softer stratum (Fig. 284 b) is known as the mucous stratum, the outer and harder (a) as ihe horny (corneous) stratum. This horny layer is being constantly used up and rubbed away at the surface ; new layers of cells grow up in their place out of the underlying mucous stratum. At first the epidermis is a simple covering of the surface of the body. Afterwards various appendages develop from it, some internally, others externally. The internal appendages are the cutaneous glands— -sweat, fat, etc. EVOLUTION (>/■ THE NERVOUS SYSTEM 269 The external appendages are the hairs and nails. The cutaneous elands are originally merely solid cone-shaped growths of the epidermis, which sink into the underlying cerium (Fig. 286 /). Afterwards 1 canal (2, ; ) is formed inside them, either by the softening and dissolution of t In- central cells or by the secretion of fluid internally. Some o\ the glands, such as the sudoriferous, do not ramify (Fig- 2X4 tfjgft. These glands, which secrete the perspiration, are very long, and have a spiral coil at the end, but they never ramify , so also the wax-glands ol the eats. Most of the other cutaneous glands give out buds ;\\\d ramify; thus, tor instance, the lachrymal glands of the upper eye-lid thai secrete tears (Fig. 286), md the sebaceous glands which secrete the fat m the skin and generally open into the hair-follicles. Sudoriferous and sebaceous glands are found only ill mammals. Bui we find lachrymal glands in all the three classes o\' Amniotes — reptiles, birds, and mammals. They are wanting in the lower, aquatic vertebrates. The mammary glands (Figs. 287 and 288) are very remarkable ; they are found in all mammals, and in these alone. They secrete the milk for the feeding of the new-born mammal. In spite of their unusual size, these structures are nothing more than large sebaceous glands in the skin. The milk is formed by the lique- faction o\ the fatty milk-cells inside the branching mammary-gland tubes (Fig. 287 c), in the same way as the skin- grease or hair-fat, by the solution of fatty cells inside the sebaceous glands. The outlets ot the mammary glands enlarge and form sac-like mammary ducts ( b ) ; tlw se narrow again (a), and open in the teats or nipples of the breast by sixteen to twenty-four fine apertures. The first structure of this large and elaborate gland very simple cone in the -epidermis, which penetrates into the corium and ramifies. In the new-born infant it consists of twelve to eighteen radiating lobes (Fig. 2X.S). These gradually ramify, their ducts become hollow and larger, and rich masses of fat accumulate between the lobes. Thus is formed the prominent female breast (mamma J, on the top of which rises the teat or nipple (mammilla ). The latter is only developed later on, when the mammary gland is fully formed ; and this ontogenetic phenomenon is extremely interesting, because the earlier VOL. II. mammals (the stem-forms of the whole class) have no teats, in them the milk comes out through a flat portion of the ventral skin 1h.1i is pierced like a sie\e, as we still find in the lowest living mammals, the OViparOOS Monott vines of Australia. The young animal licks the milk from the mother instead ol sucking it. In many of the lower mammals ue find a number o\ milk-glands at different parts of the vcntial surface. In the human female there is usually only one pair of glands, .it the breast ; and it is the same with the apes, bats, elephants, and several other mammals. Sometimes, however, we find tWO Successive pairs of Fig. 286.— Rudimentary lachrymal glands from a human embryo of four months. (From Kblliker.) 1 earliest structure, in the shape of a simple solid cone, 2 and j more advanced structures, ramifying and hol- lowing'out. a solid buds. <■ cellular coat of the hollow buds, /structure of the fibrous envelope, which after- wards forms the corium about the glands. glands (or even more) in the human female. Some women have four or five pairs of breasts, like pigs and hedgehogs (Fig. 103). This polymastism points back to an older stem-form. We often find these accessory breasts in the male also (Fig. 103 Z>). Sometimes, moreover, the normal mammary glands are fully developed and can suckle in the male ; but as a rule they are merely rudimentary organs without functions in the male. We have already (Chapter XI.) dealt with this remarkable and interesting instance of atavism. While the cutaneous glands arc inner growths of the epidermis, the appendages E 270 evolution of the nervous system which we call hairs and nails are external local growths in it The nails ( Ungues) which form important protective structures on the back of the most sensitive pans o\' our limbs, the tips of the fingers and toes, are horny growths oi the epidermis, which we share with the apes. The lower mammals usually have claws instead of them , the ungulates, hoofs. The stem-form of the mammals certainly had claws ; we find them in a rudimentary form even in the salamander The horny claws are highly developed in most of the reptiles (Pig. 2(>.i, p. 245), and the mam- mals have inherited them from the earliest representatives o( this class, the stem- reptiles ( Tocosauna ) Like the hoofs FlC. 287— The female breast (mamma) in verti- cal section, c racemose glandular Jobcs. b enlarged milk-ducts, n narrower outlets, which open into the nipple. (From // .lAjr'J (ungnhv.) of the I 'ngulates, the nails of apes and men have been evolved from the claws of the older mammals. In the human embryo the first rudiment of the nails is found (between the horny and the mucous stratum of the epidermis) in the fourth month But their edges do not penetrate through until the end of the sixth month. The most interesting and important appendages of the epidermis are the hairs . on accoum of their peculiar com- position and origin we must regard them as highly characteristic of the whole mammalian class. It is true that we also find hairs in'many of the lower animals. such as insects and worms. But these bans, like the hairs of plants, are thread- like appendages o\' the surface, and differ entirely from the hairs of the mammals in the details of their structure and develop- ment. The embryology of the hairs' is known in all its details, but there are two different views as to their phylogeny On the older view the hairs of the mammals are equivalent or homologous to the feathers of the bird or the horny scales of the reptile. As we deduce all three classes of Ammotes from a common stem-group, we must assume that these Permian stem- reptiles had a complete scaly coat, inherited from their Carboniferous ances- tors, the mailed amphibia ( Stegocephala) ; the bony scales of their corium were covered with horny scales. In passing from aquatic to terrestrial life the horny scales were further developed, and the bony scales degenerated in most of the reptiles. As regards the bird's feathers, it is certain that they are modifications of the horny scales of their reptilian ancestors. But it is otherwise with the hairs of the mammals In their case the hypothesis has lately been advanced on the strength of very extensive research, especially by Friedrich Maurer, that they have been evolved from the cutaneous sense-organs of amphibian ancestors by modification of functions , the epidermic structure is very similar in both in its embryonic rudiments This modern view, which had the support of the greatest expert on the vertebrates, Carl Gegenbaur, can be harmonised with the older theory to an extent, in the sense that both formations, scales and hairs, were very closely connected originally Probably the conical budding of the skin-sense layer grew up under the protection of the homy scale, and became an organ of touch subsequently by the cornification of the hairs . many hairs are still sensory organs (tactile hairs on the muzzje and cheeks of manv mammals • pubic hairs). This middle position of the genetic con- nection of scales and hairs was advanced in my Systematic Phylogeny of the Verte- brates (p. 433) It is confirmed bv the similar arrangement of the two cuta- neous formations As Maurer pointed out. the hairs, as well as the cutaneous sense-organs and the scales, are at first arranged in regular longitudinal series, and they afterwards break into alternate groups. In the embryo of a bear two EVOIA'TIOX OF THE XERVOVS SYSTEM 271 inches long, which I owe to the kind- 1 ness of Herr von Schmertziog (oi Arva Vrarallia, Hungary), the back is covereJ with sixteen to twenty alternating longi- tudinal row-, of scaly protuberances (Fig -••Si)). They are it tlie same time arranged in regular transverse rows, which con- verge at an acute angle from both sides towards the middle of the back. The tip of the scale-like wart is turned in- wards. Between these larger hard scales (or groups of hairs) we find numbers of rudimentary smaller hairs The human embryo is, as a rule, en- tirely clothed with a thick coat of fine wool during the last three or four weeks of gestation. This embryonic woollen coat (Lanugo) generally disappears in part during the last weeks of foetal life ; bur- in any case, as a rule, it is lost imme- diately after birth, and is replaced by the thinner coat of the permanent hair. These permanent hairs grow out of hair- follicles, which are formed from the root- sheaths of the disappearing wool-fibres. The embryonic wool-coat usually, in the case of the human embryo, covers the whole body, with the exception of the palms of the hands and soles of the feet These parts are always bare, as in the case of apes and of most other mammals. Sometimes the wool-coat of the embryo has n striking effect, by its colour, on the later permanent hair-coat. Hence it happens occasionally, for instance, among our Indo-Germanic races, that children of blond parents seem — to the dismay of the latter — to be covered at birth with a dark brown or ejven a black woolly coat Not until this has disappeared do we see the permanent blond hair which the child has inherited Sometimes the darker coat remain? for weeks, and even months, after birth This remarkable woolly coat of the human embryo is a legacy from the apes, our ancient long-haired ancestors. It is not les^ noteworthy that many of the higher apes approach man in the thinness of the hair on various parts of the body With most of the apes, espe- cially the higher Catarrhines (or narrow- nosed apes), the face is mostly, or entirely, bare, or at least it has hair no longer or thicker than that of man. In their case, too, the back of the head is usually pro- vided with a thicker growth of hair ; this is lacking, however, in the case of the bald-headed chimpanzee {Anthropithecus calvus). The males of many species of apes have a considerable beard on the cheeks and chin , this sign of the mascu- line sex has been acquired by sexual Selection Many species of apes have a very thin covering oi hair on the breast and the upper side oi tin' limbs — nuich thinner than on the hack or the under side oi' the limbs. On the other hand, we are often astonished to find tufts of hair on the shoulders, back, and extremities oi' members of our Indo-Germanic and of the Semitic races. Exceptional hair on the face, as on the whole body, is hereditary in certain families of hairy men. The quantity and the quality of the hair on head and chin are also con- spicuously transmitted in families. These extraordinary variations in the total and partial hairy coat of the body, which are so noticeable, not only in comparing Fig. 288— Mammary gland of a new-born Infant, a original central gland, b small and c large buds of same. (From Longer.) different races of men, but also in com- paring different families of the same race, can only be explained on the as- sumption that in man the hairy coat is, on the whole, a rudimentary organ, a useless inheritance from the more thickly- coated apes. In this man resembles the elephant, rhinoceros, hippopotamus, whale, and other mammals of various orders, which have also, almost entirely or for the most part, lost their hairy coats by adaptation. The particular process of adaptation by which man lost the growth of hair on most parts of his body, and retained or augmented it at some points, was most probably sexual selection. As Darwin luminously showed in his Descent of Man, sexual selection has been very active 272 EVOLUTION OF THE NERVOUS SYSTEM in thi- respect As the male anthropoid apes chose the females with the least hair, and the females favoured the males with the finest growth9 on chin and head, the (genera] coating of the body gradually degenerated, and the hair o( the beard and head was more Btrongly developed. The growth of hair at other parts of the body (arm-pit, pubic region) was also probably due to sexual selection. More- over, changes of climate, or habits, and other adaptations unknown to us, may A poid apes — gorilla, chimpanzee, orang, and several species of gibbons — besides man (Figs. 203, 207). In other species of gibbon the hairs are pointed towards the hand both in the upper and lower arm, as in the rest of the mammals. We can easily explain this remarkable pecu- liarity of the anthropoids and man on the theory that our common ancestors were accustomed (as the anthropoid apes are to-day) to place their hands over their heads, or across a branch above their Fig. 289. — Embryo Of a bear ( Ursus arrtos), twice natural size. A seen from ventral side, B from the left have assisted the disappearance of the hairy coat The fact that our coat of hair is in- herited directly from the anthropoid apes is proved in an interesting way, according to Darwin, by the direction of the rudi- mentary hairs on our arms, which cannot be explained in any other way Both on the upper and the lower part of the arm they point towards the elbow. Here they meet at an obtuse angle. This curious arrangement is found only in the anthro- heads, during rain. In this position, the fact that the hairs point downwards helps the rain to run off. Thus the direction of the hair on the lower part of our arm reminds us to-day of that useful custom of our anthropoid ancestors. The nervous system in man and all the other Vertebrates is, when fully formed, an extremely complex apparatus, that we may compare, in anatomic structure and physiological function, with an extensive telegraphic system. The chief station of wt El'Of.CTIO.V OF THE NERVOUS SYSTEM 273 the System is the ecu' ral marrow or central nervous system, the innumerable ganglionic cells or nenrona (Fig <») of which are connected by branching pro- cesses with each other and with numbers ofverj fine conducting wires I"he latter art.- the peripheral and ubiquitous nerve- fibres ; with their terminal ipparatus, the sense-organs, etc they constitute the Conducting marrow >r peripheral nervous system. Some of them the sensor) nerve-fibres conduct the impressions from the skin and other sense-organs to the central marrow , others— the motor nerve-fibres conve) the commands of the will to the mu^ les. The central nereous system or central marrow f medulla centralis) is the real organ of psychic action in the narrower sense I lowever we conceive the intimate connection of this organ and its functions, it i- certain that its characteristic actions, which we call sensation, will, and thought, are inseparably dependent on the normal development t^f the material organ in man and all the higher animals We must, therefore, pay particular attention to the evolution of the latter As it can give us most important information re- garding the nature of the " soul," it should he full of interest If the central marrow developes in just the sanie way in the human embryo as in the embryo of the other mammals, the evolution of the human psychic organ from the central organ of the other mammals, and through them from the lower vertebrates, must be beyond question. Xo one can doubt the momentous bearing of these embryonic phenomena In order to understand them full) we must first sa\ a word or two of the general form and the anatomic composition of the mature human central marrow Like the central nervous system of all the other Craniotes, it consists of two pirts, the head-marrow or brain (medulla capitis or encephalon ) and the spinal-marrow (medulla spinalis or nolomye/on J The one is enclosed in the bony skull, the other in the bony vertebral column. Twelve pairs of ceiebral nerves proceed from the brain, and thirty-one pairs of spinal turves from the spinal cord, to the rest of the body (Fig 171) On general anatomic investigation the spinal marrow is found to be a cylindrical cord, with a spindle-shaped bulb both in the region of the neck above (at the last cervical vertebra) and the region of the loins (at the first lumbar vertebra) below (Fig. -■mi \t 'he cervical bulfa the strong nerVes of the upper limbs, and at the lumbar bulb those of the lower limbs, proceed from 1 he spinal cord Above, 'he latter passes into 'he brain through the medulla oblongata (Fig 2Qt mo). The spinal cord seems to be a thick mass of nervous matter, but it has a narrow canal a' its i\is, which passes into the further Fig. 290. Fig. 291 Fig. 290.— Human embryo, three months old. natural size, from the dorsal side brain and spinal cord exposed. (From Kolliker.) h cerebral hemi- spheres (fore brain), m corpora quadrigemina (middle brain), r cerebellum (hind brain) : under the latter is the triangular medulla oblongata (after brain). Fig 291.— Central marrow of a human embryo, four months old. natural size, from the back. (From Kolliker.) h large hemispheres, t> quadrigemina., c cerebellum, mo medulla oblongata • underneath it the spinal cord. cerebral ventricles above, and is filled, like these, with a clear fluid. The brain is a large nerve-mass, oc- cupying the greater part of the skull, of most elaborate structure. On general examination it divides into two parts, the cerebrum and cerebellum. The cerebrum lies in front and above, and has the familiar characteristic convolutions and furrows on its surface (Figs. 292, 293). On the upper side it is divided by a deep longitudinal fissure into two halves, the ?74 / I O/.C 77 OX OF THE S'ERVOl'S SYSTEM cerebral hemispheres these are connected by the corpus cal/osu»i The large cerebrum is separated from the small Cerebellum by a deep transverse furrow Tlie latter lies behind and below and has also numbers of furrows but much finer and more regular with convolutions between, at its surface The cerebellum ;il glance for a moment at the lower animals, which have no brain. Even in the skull-less vertebrate, the Amphioxus, we find wo independent brain, as uc have seen. The whole central marrow is merel) a simple cylindrical cord which runs the length OI the body, and ends equally simply at both extremities a plain medullary tube. All thai we can discover is a small vesicular bulb at the foremost part oi the tube, a degenerate rudiment o\ a primitive brain, we meet the same simple medullary tube in the lii si structure o\ the ascidia larva, in the same characteristic position, above the chorda. On closer examination we find here also a small vesicular swelling at the fore end of the tube, the first trace of a differentiation of it into brain and spinal cord. It is probable that this differentiation was more advanced in the extinct Pro- vertebrates, and the brain- bulb mote pronounced ll'l^. 98-102). The brain is phylo- genetically older than the spinal cord, as the trunk was not developed until after the head. It' we consider the undeniable affinity of the Ascidia? to the Vermalia, and remember thai we can trace all the Chordoma to lower Vermalia, it seems probable that the simple central marrow of the former is equivalent to the simple nervous ganglion, which lies above the gullet in the lower worms, and has long been known as the "upper pharyngeal ganglion " (ganglion pharyngeum si//>r- ftus); it would be better to call it the primitive or vertical brain (acroganglion). Probably this upper pharyngeal gang- lion of the lower worms is the structure from which the complex central marrow of the higher animals has been evolved. The medullary tube of the Chordoma has been formed by the lengthening of the vertical brain on the dorsal side. In all tli.- other animals the central nervous system has been developed in a totally different way from the upper pharyngeal ganglion ; in the Articulates, especially, a pharyngeal ring, with ventral mat row, has been added. The Molluscs also have a pharyngeal ring, but it is not found in the Vertebrates. In these the central marrow Mas been prolonged down the dorsal side; in the Articulates down the ventral side. This fact proves of it-elf that there is no direct relationship between the Vertebrates and the Articu- lates. The unfortunate attempts to derive the dorsal marrow of the former from tlie ventral marrow of the laitor have totally failed (cf. p. 2K|). When we examine the embryology of the human nervous system, we must start from the important fact, which we have alread) seen, that the first structure of it in man and all the higher Vertebrates is the simple medullary tube, and (hat this separates from the outer germinal layer in the middle line of the so'e-shajx;d PO Fie;. 293.— The human brain, seen from the left. (From H. Afiyrr.) The furrows of the cerebrum are indicated by thick, and those o( the cerebellum by finer lines. Under the latter we Can see the medulla oblongata. f\-f-i frontal convolutions, C centra] con- volutions, S fissure of Sylvius, T temporal furrow, Pa parietal 1 ■:>. . .hi angular gyrus, Po parieto-occipital fissure. embryonic shield. As the reader wilt remember, the straight medullary furrow first appears in the middle of the sandal- shaped embryonic shield. At each side of it the parallel borders curve over in the form of dorsal or medullary swellings. These bend together with their free borders, and thus form the closed medul- lary tube (Figs. 133-137). At first this tube lies directly underneath the horny plate ; but it afterwards travels inwards, the upper edges of the prevertebral plates growing together between the horny plate and the tube, joining above the latter, and forming a completely closed canal. As Gegenbaur very properly observes, " this gradual imbedding in the 276 E\ OLUTION OF THE NERVOUS SYSTEM Inner p.ut of the body is a process acquired with the progressive differentia- tion and the higher potentiality that this secures ; by iliis process the organ of greater value to the organism is buried within the frame." (Cf. Figs. 143-141)). In the Cyclostoma a stage above the 'Acrania the tore ^.nd of the cylindrical medullary tube begins early to expand into a pear-shaped vesicle ; this is the first outline of an independent brain. In this way the central marrow of the Vertebrates divides clearly into its two chief sections, brain and spinal cord. The simple vesicular form of the brain, which persists for some time in the Cyclostoma, is found also at first in all the higher Vertebrates (Fig. 153 hb). But in these it soon passes away, the one vesicle being divided into .several succes- sive parts by transverse constrictions. There are first two of these constrictions, 3 1. 1. Fig. 294. Fig. 295. Fig. 296. Figs. 294-296.— Central marrow of the human «mbryo from the seventh week, } inch long. (From KoUiker.) Fig. 296 back view of the whole embryo : brain and spinal cord exposed. Fig. 295 the brain with th&jjppermost part of the cord, from the left. Fig. 294 the brain from above. i< fore brain, 2 intermediate brain, m middle brain, h hind-brain, n after brain. dividing- the brain into three consecutive vesicles (fore brain, middle brain, and hind brain, Fig. 154 v, m, h). Then the first and third are sub-divided by fresh constrictions, and thus we get five suc- cessive sections (Fig. 155). In all the Craniotcs, from the Cyclo- stoma up to man, the same parts develop from these five original cerebral vesicles, though in very different ways. The first vesicle, the fore brain (Fig. i55?')> forms by far the largest part of the cerebrum — namely, the large hemispheres, the olfac- tory lobes, the corpora striata, the callo- sum, and the fornix. From the second vesi- cle, the intermediate brain ftjt originate especially the optic thalami, the other parts that surround the third cerebral ventricle, and the infundibulum and pineal gland. The third vesicle, the middle brain (m ), produces the corpora quadrigemina and the aqueduct of Sylvius. From the fourth vesicle, the hind brain ( ' h J, developes the greater part of the cerebellum — namely, the vermis and the two small hemispheres. Finally, the fifth vesicle, the after brain fnj, forms the medulla oblongata, with the quadrangular pit (the floor of the fourth ven bodies, etc. ar pi ), th fourth ventricle), the pyramids, olivary We must certainly regard it as a com- parative-anatomical and ontogenetic fact of the greatest significance that in all the Craniotes, from the lowest Cyclostomes and fishes up to the apes and man, the brain developes in just the same way in the embryo. The first rudiment of it is always a simple vesicular enlargement of the fore end of the medullary tube. In every case, first three, then five, vesicles develop from this bulb, and the perma- nent brain with all its complex anatomic structures, of so great a variety in the various classes of Vertebrates, is formed from the five primitive vesicles. When we compare the mature brain of a fish, an amphibian, a reptile, a bird, and a mammal, it seems incredible that we can trace the various parts of these organs, that differ so much internally and exter- nally, to common types. Yet all these different Craniote brains have started with the same rudimentary structure. To convince ourselves of this we have only to compare the corresponding stages of development of the embryos of these' different animals. This comparison is extremely instruc- tive. If we extend it through the whole series of the Craniotes, we soon discover this interesting fact : In the Cyclostomes (the Myxinoidaand Petromyzonta), which we have recognised as the lowest and earliest Craniotes, the whole brain remains throughout life at a very low stage, which is very brief and passing in the embryos of the higher Craniotes ; they retain the five original sections of the brain un- changed. In the fishes we find an essential and considerable modification of the five vesicles ; it is clearly the brain of the Selachii in the first place, and sub- sequently the brain of the Ganoids, from which the brain of the rest of the fishes on the one hand and of the Dipneusts and Amphibia, and through these of the higher, Vertebrates, on the other hand, must be derived. In the fishes and Amphibia (Fig. 300) there is a preponderant de-, velopment of the middle brain, and also the after brain, the^ first, second, arid EVOLUTION OF THE NERVOUS SYSTEM 277 fourth sections remaining very primitive. It is just the reverse in the higher Verte- brates, in which the first and third sections, the Cerebrum and cerebellum, are exceptionally developed ; while the F10. 297.— Head of a chick embryo (hatched fifty- eight hours), from the back, magnified forty times. (From MOialkovics.) vw anterior wall of the fore brain, 9k its ventricle, ait optic vesicles, mh middle brain, i/t hinJ brain, uh after brain, hz heart (seen from below), vm vitelline veins, us primitive segment, rm spinal cord. middle brain and after brain remain small. The corpora quadrigemina are mostly covered by the cerebrum, and the oblongata by the cerebellum. But we find a number o\ stages of development within the higher Vertebrates themselves. From the Amphibia upwards the brain (and with it the psychic life) developes in two different directions; one of these is followed by the reptiles and birds, and the other by the mammals. The development of the first section, the fore brain, is particularly characteristic of the mammals. It is only in them that the cerebrum becomes so large as to cover all the other parts of the brain (Figs. 293, 301-304). There are also notable variations in the relative position of the cerebral vesicles. In the lower Craniotes they lie originally almost in the same plane. When we examine the brain laterally, we can cut through all five vesicles with a straight line. But in the Amniotes there is .1 con- siderable curve in the brain along with the bending of the head and neck ; the 'whole of the upper dorsal surface of the brain developes much more than the under ventral surface. This causes a curve, BO that the parts Come to lie as follows: The fore brain is right in front and below, the intermediate brain a little higher, and the middle brain highest of all ; the hind brain lies a little lower, and the after brain lower still. We find this only in the Amniotes the reptiles, birds, and mammals. Thus, while the brain of the mammals agrees a good deal in general growth with that of the birds and reptiles, there are some striking differences between the two. In the Sauropsids (birds and reptiles) the middle brain and the middle pari of the hind brain are well developed. In the mammals these parts do not grow, and the fore-brain developes so much that it overlies the other vesicles. As it con- tinues to grow towards the rear, it at last COVerS the whole of the rest of the brain, and also encloses the middle parts from Fig. 298. Fig. 299. Fig. 298.— Brain of three eranioie embryos in vertical section. A of a shark ( Hefitarchits). />* o(a serpent (Coluber), C of a goat ( Capra ). u fore brain, b intermediate brain, c middle brain, d hind brain, c alter brain, 5 primitive cleft. (From Gtgtnbattr.) Fig. 299.— Brain of a shark fSeylh'um), back view. £• fore-brain, h olfactory lobes, which send the l&rge olfactory ner\es to the nasal capsule (ok d inter- mediate brain, b middle brain ; behind this the insigni- ficant structure of the hind brain, a alter brain. (Front Gtgtnbaur.) the sides (FigS. 301-303). This process is of great importance, because the lore brain is the organ of the higher psychic life, and in it those functions of the nerve- cells are discharged which we sum up in *?8 A" VOLUTION OF THE NER \ 'O CS S J 'ST/C. 1/ the word "soul." The highest achieve- ments oi the animal body die wonderful manifestations oi' COOSClOUSIieSS and tlie complex molecular processes of thought — have their seat in the forehrain. We can remove the large hemispheres" piece by Fig. 300.— Brain and spinal cord of the frog. A from the dorsal, B from the ventral side, a olfac- tory lobes before the (b) fore brain, t infundibulum at thebase of the intermediate brain, c middle brain, d hind brain, s quadrangular pit in the after brain, m spinal cord (very short in the frog), rri roots of the spinal nerves, t terminal fibres of the spinal cord. (From Gegenbaur.) Fig. 301.— Brain of an ox-embryo, two inches in length. (From Mihalkovics, magnified^ three times.) Left view ; the lateral wall of_ the left hemisphere has been removed, st corpora striata, ml Monro-foramen, ap- arterial plexus, ah Amnion's horn, mfi middle brain, bh cerebellum, d e (equilibrium). Comparative anatomy and physiology teach us that there are no differentiated sense-organs in the lower animals ; all their sensations are received by the sur- face of the skin. The undifferentiated skin-layer or entoderm of the Gastnea is the simple stratum of cells from which Hie differentiated sense-organs of all the Mcta/oa (including the Vertebrates) have beene\ oh ed. Starting from the assump- tion that necessarily only the superficial parts of the body, which are in direct touch with the outer world, could he con- cerned in the origin of sensations, we can see at once that the sense-organs also must have arisen there. This is really the case. The chief part of all the sense- organs originates from the skin-sense layer, partly directly from the horny plate, partly from the brain, the foremost part of the medullary tube', after it has separated from the horny plate. If we compare the embryonic development of the various sense-organs, we see that they all make their appearance in the simplest conceivable form ; the wonderful contrivances that make the higher sense- organs among the most remarkable and elaborate structures in the body develop only gradually. Jn the phylogenetic ex- planation of them comparative anatomy and ontogeny achieve their greatest triumphs. Hut at first all the sense- organs are merely parts of the skin in which sensory nerves expand. These nerves themselves were originally of a homogeneous character. The different functions or specific energies of the differentiated sense-nerves were only gradually developed by division of labour. At the same time, their simple terminal expansions in the skin were converted into extremely complex organs. The great instructiveness of these historic. il fuis in connection with the life of the soul is not diffii ult to see. The whole philosophy of the future will be transformed as soon as psychology takes cognisance of these genetic phenomena and makes them the basis of its specula- te its. W'lun we examine impartially the manuals of psychology that have been published by the most distinguished specu- lative philosophers and are still widely distributed, we are astonished at the naivete with which the authors raise their air) metaphysical speculations, regardless of the momentous embryological facts that completely refute them. Vet the science of evolution, in conjunction with the great advance of the comparative anatomy and physiology of the sense- Organs, provides the one sound empirical basis of a natural psychology. Fir.. }c>v -Head of a shark (SeyUutm), from the ventral side, m mouth, o olfactory pits, r nasal groove. M nasal told in natural position, ft nasal fold drawn up. (The dots arc openings of the mucous canals.) (From Gegenbaur.) In respect of the terminal expansions of the sensory nerves, we can distribute the human sense-organs in three groups, which correspond to three stages of de- velopment. The first group comprises those organs the nerves of which spread out quite simply in the free surface of the skin itself (organs of the sense of pres- sure, warmth, and sex). In the second groupthe nerves spread out in the mucous coat of cavities which are at first depres- sions in or invaginations of the skin (organs of the sense of smell and taste. The third group is formed of the very elaborate organs, the nerves of which spread out in an internal vesicle, rated from the skin (organs of the sense of sight, hearing, and space). There is little to be said of the develop- ment of the lower sense-organs. We jS> EVOLUTION OF THE SENSE-ORGANS have already considered (p. j<>8) the organ of touch and temperature in the skin. I need only add that in the corium of man and all the higher Vertebrates countless microscopic sense-organs de- velop, but the precise relation o( these to the sensations of pressure or resistance, of w. ninth and cold, has not yet been explained. Organs of this kind, in or on which sensory cutaneous nerves termi- nate, are the " tactile corpuscles" (or the z. canal to which these parts belong (Chap- ter XXVII.). I will only point out for the present that the mucous coat of the tongue and palate, in which the gustatory nerve ends, originates from a part of the outer skin. As we have seen, the whole of the mouth-cavity is formed, not as a part of the gut-lube proper, but as a pit- i«ke fold in the outer skin (p. 139). Its mucous lining is thsrefore formed, not from the visceral, but from the cutaneous <,? *?> Fig. .506. Fig. 307. Fig. 310. Fig. 309. Figs. 306 and 307.— Head of a chick embryo, three days old : 306 front view, 307 from the right., n rudi- mentary nose (olfactory pits), I rudimentary eyes (optic pits), g rudimentary ear (auscultory bit), v fore brain, gl eye-cleft, 0 process ol upper jaw, u process of lower jaw of the first gill-arch. PlG. 308. — Head Of a Chick embryo, four days old, from below, n nasal pit. o upper-jaw process of the first gill-arch, 11 lower-jaw process of same, k" second gill-arch, sf> choroid fissure of eye, s gullet Figs. 309 and 310. — Heads of Chick embryos : 309 from the end of the fourth, 310 from the beginning of the fifth week. Letters as in Fig. 308, except: in inner, an outer, nasal process, nf nasal furrow, st frontal process, in mouth. (From Kolliker.) Figs. 306-310 are magnified to the same extent. Pacinian corpuscles) and end-bulbs. We find similar corpuscles in the organs of the sexual sense, the male penis and the female clitoris ; they are processes of the skin, the development of which we will consider later (together with the rest of the sexual parts, Chapter XXIX.)- The evolution of the organ of taste, the tongue and palate, will also be treated later, together with that of the alimentary layer, and the taste-cells at the surface of the tongue and palate are not products of the gut-fibre layer, but of the skin-sense layer. This applies also to the mucous lining of the olfactory organ, the nose. How- ever, the development of this organ is much more interesting. Although the nose seems superficially to be simple and single, it really consists, in man and all EVOLUTION OF THE SENSE-ORGANS 283 other Gnathostomes, of two completely separated halves, the right ahd leu cavi- ties. They -in.' divided by ■ vertical partition, so lli.it the right nostril leads into the li^ln cavity alone and the left nostril into the left cavity. They open internal!) (and separately) bj the postei ioi nasal apertures into the pharynx, so that are can i^et direct into the gullet through the na-.il passages without touching the mouth. This is the way the air usually passes in respiration ; the mouth being elosed, it goes through the nose Into the gullet, and through the larynx and bronchial tubes into the lungs. The nasal cavities an separated from the mouth In the horizontal bony palate, to which is attached behind (as a dependent process) the soft palate with the uvula. In the upper and hinder parts o\ the nasal cavities the olfactory nerve, the first pair of cerebral nerves, expands in the mucous coat which clothes them. The terminal branches o\ it spread partly over the septum (partition), partly on the side- walls of the internal cavities, to which are attached the turbinated bones. These hones are much more developed in many of the higher mammals than in man, but there are three of them in all mammals. The sensation of smell arises by the passage o( a current o\~ air containing odorous matter over the mucous lining of the cavities, and stimulating the olfactoi v cells of the nerve-endings. Man has all the features which distin- guish the olfactory organ of the mammals from that oi the lower Vertebrates. In all essential points the human nose entirely resembles that of the Catarrhme apes, some o\ which have quite a human external nose (compare the face of the long-nosed apes). However, the first structure of the olfactory organ in the human embryo gives no indication of the future ample proportions of our catar- rhine nose. It has the form in which we find it permanently in the fishes — a couple of simple depressions in the skin at the outer surface of the head. We find these blind olfactory pits in all the fishes ; sometimes they 1 i *-- near the eyes, some- times more forward at the point of the muzzle, sometimes lower down, near the mouth (Fig. 24. . .. - . thkk walls, and thus we get the solid Crystalline lens. fhis is, therefore, a purely epidermic structure. Together with the lens the small underlying piece of corium-plate also separates from the skin. As the lens separates from the corneous plate and grows inwards, it necessarily hollows out the Contiguous primary optic vesicle (Fig. 318, / j). This is done in just the same way as the invagina- *- t ion of the blastula, which gives rise to the gastmla in the amphi- oxus 1 Fig. 38 ( * /•'). In both cases tlie hollowing of the closed vesicle on one side goes so far that at last the inner, folded part touches the outer, not folded part, and the cavity disappears. As in the gas- tmla the first part is converted into the entoderm and the latter into the ectoderm, so in the in- vagination of the primary optic vesicle the retina ( r) is formed from the first (inner) part, and the black pigment membrane ( u ) from the latter (outer, non- invaginated) part. The hollow stem of the primary optic vesicle is converted into the optic nerve. The lens ftj, which has so important a part in this process, lies at first directly on the in- vaginated part, or the retina (r). But they soon separate, a new structure, the corpus vitreum ( gl ' ), growing between them. While the lenticular sac is being detached and is causing the invagination of the primary- optic vesicle, another invagination is taking place from below ; this proceeds from the superficial part of the skin-fibre layer— the corium of the head. Behind and under the lens a last-shaped process rises from the cutis-plate (Fig. 319 g), hollows out the cup-shaped optic vesicle from below, and presses between the lens ( I ) and the retina f i ). In this way the optic vesicle acquires the form of a hood. Finally, a complete fibrous envelope, the fibrous capsule of the eye-ball, is its outer side. The round wall of the capsule soon divides into two different membranes by surface-cleavage. The inner membrane becomes the choroid or vascular coat, and in front the ciliary corona and iris. The outer membrane is Fig. 317.— The human eye in section. a sclerotic coat," J cornea, c conjunctiva, d circular veins of the iris, e choroid coat, f Ciliary muscle, £■ corona ciliaris, h iris, / optic nerve, /• anterior border of the retina, / crystalline lens, m inner covering of the cornea (aqueous membrane), n pigment membrane, o ictina, £ Petit's canal, q yellow spot of the retina. (From Helmholtz.) converted into the white protective or sclerotic coat— in front, the transparent- cornea. The eye is now formed in all its essential parts. The further development — the complicated differentiation and com- position of the various parts — is a matter of detail. The chief point in this remarkable evo- lution of the eye is the circumstance that the< optic nerve, the retina, and the pig-. ment membrane originate really from a part of the brain — an outgrowth of the intermediate brain — while the lens, the chief refractive body, developes from the outer skin. From the skin — the horny EVOLUTION OF THE SENSE-ORGANS plate -also arises the delicate conjunctiva, which afterwards covers the outer surface of the eyeball. The lachrymal glands arc ramified growths from the conjunctiva (Fig. 2Xh). All these important parts Of Fu; -,i*. -Eye of the chick embryo m longitudinal section (/. from an embryo sixtv-fivc noun old ; 2. from a somen hat older embrj o ; \. from an embryo tour days old). A horny plate. 0 lens-pit. /lens (m / still part of tin- epidermis, in -'. andj separated from it), v thicken- HiLr of the horn) plate at the point where the lens has severed itself, gl corpus vitrcum, >■ retina, u pigment membrane. (From Rcmak.) the eye are products of the outer germinal layer. The remaining parts — the corpus vitreum (with the vascular capsule of the lens), the choroid (with the iris), and the sclerotic (with the cornea)— are formed from the middle germinal layer. The outer protection of the eye, the eye-lids, are merely folds of the skin, which are formed in the third month of human embryonic life. In the fourth month the upper eye-lid reaches the lower, and the eye remains covered with them until birth. As a rule, they open wide shortly before birth (sometimes only after birth). Our craniote ancestors had a third eye-lid, the nictitating membrane, which was drawn over the eye from its inner angle. It is still found in many of the Selachii and Amniotes. In the apes and man it has degenerated, and there is now only a small relic of it at the inner corner of the eye, the semilunar fold, a useless rudimentary organ (cf. p. 32). The apes and man have also lost the Harderian "gland that opened under the nictitating membrane ; we find this in the rest of the mammals, and the birds, rep- tile-, and amphibia. The peculiar embryonic development of •the vertebrate eye docs not enable us to draw any definite conclusions as to its 'obscure phylogeny ; it is clearly ccno- genctic to a great extent, or obscured by the reduction and curtailment of its original features. It is probable that many of the earlier stages Of its ph> logenv have disappeared without leaving a trace. It can only be said positively that the peculiar ontogeny of the complicated optic apparatus in man follows just the same laws as in all t he other Vertebrates. Their eye is a part of the fore brain, which has grown forward towards the skin, not an original cutaneous sense-organ, as in the Invertebrates. The vertebrate ear resembles the eye and nose in many important respects, but is different in others, in its development. The auscultory organ in the fully- developed man is like that of the other mammals, and especially the apes, in the' main features. As in them, it consists of two chief parts —an apparatus for conduct- ing sound (external and middle ear) and an .apparatus for the sensation of sound (internal ear). The external ear opens in the shell at the side of the head (Fig. 320 a). From this point the external passage fb), about an inch in length, leads into the head. The inner end of it is closed by the tympanum, a vertical, but not quite upright, thin membrane of an oval shape ( c ). This tympanum separates the external passage from the tympanic cavity (d ). This is a small cavity, tilled with air, in the temporal hone ; it is con- nected with the mouth by a special tube. Fie. 310— Horizontal transverse section of the eye of a human embryo, four weeks old (magnified one hundred times), (from Kolliker.) t lens (the dark wall of which is as thick as the diameter of the central cavity), c~ corpus vitrcum (connected by a stem. %, with the corium). v vascular loop (pressing behind the lens inside the corpus vitrcum by means i-t this stem g), i retina (inner thicker, invaginated layer oi the primary optic Vesicle), a pigment membrane (outer, thm, non- in vagina ted layer o( same), h space between retina and pigment membrane (remainder ol the cavity of the primary optic vesicle). This tube is rather longer, but much narrower, than the outer passage, leads inwards obliquely from the anterior wall of the tympanic cavity, and opens in the throat below, behind the nasal EVOLUTION OF THE SENSE-ORGANS 289 openings. It is called the Eustachian tube (e); it serves to equalise the pres- sure of the air within the tympanic cavity and the outer atmosphere tli.it enters by the external passage. Both Fig. 320.— The human ear (left ear. seen from the front, natural size). a shell of ear. A external passage, c tympanum, d tympanic cavity, e Eustachian tube. f, g. h the three bones of the ear (/"hammer, g anvil, h siirrup). 1 utricle, k the three setni-circular canals, / the lacculua, «/ cochlea, n auscultory nerve. the Eustachian tube and the tympanic cavity are lined with a thin mucous coat, which is a direct continuation of the mucous lining of the throat. Inside the tympanic cavity there are three small bones which are known (from their shape) as the hammer, anvil, and stirrup (Fig. 320, /, g, h). The hammer (f) is the outermost, next to the tympanum. The anvil ( gj fits between the other two, above and inside the hammer. The stirrup (h) lies inside the anvil, and touches with its base the outer wall of the internal ear, or auscultory vesicle. All these parts of the external and middle ear belong to the apparatus for conduct- ing sound. Their thief task is to convey the waves of sound through the thick wall of the head to the inner-lying auscul- tory vesicle. They are not found at all in the fishes. In these the waves of sound are conveyed directly by the wall of the head to the auscultory vesicle. The internal apparatus for the sensation of sound, which receives the waves of sound from the conducting apparatus, consists in man and all other mammals o\~ a dosed auscultory vesicle filled with fluid and an auditory nCTVC, the ends of which expand over the wall of this vesicle. The vibrations of the sound-waves are Conveyed by these media tO the nerve- endings. I'n the labvrinthic water that fills the auscultory vesicle there are small stones at the points of entry of the acoustic nerves, which are composed o\ groups of microscopic calcareous crystals (otoliths). The auscultory organ of most of the Invertebrates has substantially the same CO/nposition. It usually consists of a closed vesicle, filled with fluid, and con- taining otoliths, with the acoustic nerve expanding on its wall. But, while the auditory vesicle is usually of a simple round or oval shape in the Invertebrates, it has in the Vertebrates a special and curious structure, the labyrinth. This thin-membraned labyrinth is enclosed in a bonv capsule of the same shape, the osseous labyrinth (Fig. 321), and this lies in the middle of the petrous bone of the skull. The labyrinth is divided into two vesicles in all the Gnathostomes. The larger one is called the u/riculus, and has three arched appendages, called the "semi-circular canals" fc, d, e). The smaller vesicle is called the saccu/us, and is connected with a peculiar appendage, with (in man and the higher mammals) a spiral form something like- a snail's shell, and therefore called the cochlea (= snail, b). On the thin wall of this delicate labyrinth the acoustic nerve, which comes from the after-brain, spreads out in most elaborate fashion. It divides, into two main branches — a coch- lear nerve (for the cochlea) and a ves- tibular nerve (for the rest of the labyrinth). The former seems to have more to dowith thequality, the latter with the quantity, of the acoustic sensa- tions. Through the cochlear nerves we learn the height and timbre, through the vestibular nerves the intensity, of tones. The hrst structure of this highly elabo- rate organ is very simple in the embryo of man and all the other Craniotes ; it is a » J* Fig. 321.— The bony labyrinth of the human ear (left side). a vcstibulum. b cochlea, nipper canal, (/posterior canal, e outer canal, f oval fenestra, g round fenestra. (From Meyer.) 2QO EVOIA'TIOX OF THE SENSE-ORGANS C it-like depression in the skin. At the ack part of the head at both sides, near the after brain, a small thickening of the horny plate is t'ormed at the upper end of the second gill-cleft (Fig. 322 A //). This sinks into a sort of pit, and severs from the epidermis, just as the lens of the eye does. In this way is formed at each side, directly under the horny plate of the back part of the head, a small vesicle filled with fluid, the primitive auscultory vesicle, or the primary laby rinth. As it separates from its source, the horny plate, and presses inwards and backwards into the skull, it changes from round to pear-shaped (Figs. 322 R h\ 323 0). The outer part of it is length- ened into a thin stem, which at first still opens outwards by a narrow canal. This is the labyrinthic appendage (Fig. 2,22 //-). In the lower Vertebrates it developes into a special cavity filled with Fig. 322.— Development of the auscultory labyrinth of the chick, in five successive stages (A-E). (Vertical transverse sections of the skull.) fl auscultory pits, Iv auscultory vesicles, Ir labyrinthic appendage, c rudi- mentary cochlea, csp posterior canal, cse external canal, jv jugular vein. (From tteissner.) calcareous crystals, which remains open permanently In some of the primitive fishes, and opens outwards in the upper part of the skull. But in the mammals the labyrinthic appendage degenerates. In these it has only a phylogenetic interest as a rudimentary organ, with no actual physiological significance. The useless relic of it passes through the wall of the petrous bone in the shape of a narrow canal, and iscalled thevestibularaqucduct. It is only the inner and lower bulbous part of the separated auscultory vesicle that developes into the highly complex and differentiated structure that is after- wards known as the secondary labyrinth. This vesicle divides at an early stage into an upper and larger and a lower and smaller section. From the one we get the utriculus with the semi-circular canals ; from the other the sacculus and the cochlea (Fig. 320 c). The canals are formed in the shape of simple pouch-like involutions of the utricle ( cse and csp). The edges join together in the middle part of each fold, and separate from the utricle, the two ends remaining in open connection with its cavity. All the Gnathostomes have these three canals like man, whereas among the Cyclo- stomes the lampreys have only two and the hag-lishes only one. The very com- plex structure of the cochlea, one of the most elaborate and wonderful outcomes of adaptation in the mammal body, developes originally in very simple fashion as a flask-like projection from thesacculus. As Hasse and Retzius have pointed out, we find the successive ontogenetic stages of its growth represented permanently in the series of the higher Vertebrates. The cochlea is wanting even in the Mono- tremes, and is restricted to the rest of the mammals and man. The auditory nerve, or eighth cerebral nerve, expands with one branch in the cochlea, and with the other in the remaining parts of the labyrinth. This nerve is, as Gegenhaur has shown, the sensory dorsal branch of a cerebro - spinal nerve, the motor ventral branch of which acts for the muscles of the face (nervus facialis). It has therefore origi- nated phylogenetically from an ordinary cutaneous nerve, and so is of quite different origin from the optic and olfactory nerves, which both represent direct outgrowths of the brain. In this respect the auscul- tory organ is essentially different from the organs of sight and smell. The acoustic nerve is formed from ectoderm ic cells of the hind brain, and developes from the nervous structure that appears at its dorsal limit. On the other hand, all the membranous, cartilaginous, and osseous coverings of the labyrinth are formed from the mesoderm ic head- plates. The apparatus for conducting sound which we find in the external and middle car of mammals developes quite sepa- rately from the apparatus for the sensa- tion of sound. It is both phylogeneti- cally and ontogenetically an independent secondary formation, a later accession to EVOLUTION OF THE SENSE-ORGANS •Ml the primary internal ear Nevertheless, its development is not less interesting, .nul is explained with the same ease i\\ comparative anatomy. In all the fishes and in the lowest Vertebrates there is no Fu;. -,_••,. Primitive skull of the human embryo, four irceks old, \ i-rtu.il set tion, left half seen internally . v. s, mt, h. >i ilii' five pits of the cranial cavity, in which tho five cerebral vesicles lie (fore, intermediate, -middle, bind, and after brains), o pear-shaped primary auscul- tor) veside (appearing through), « eye (appearing through), no optic nerve, f canal o( the hypophysis,./ central prominence of the skull. (Prom Kullik-rr.) special apparatus for conducting sound, no external or middle oar ; they have only a labyrinth, an internal oar, which lies within the skull. They are without the tympanum and tympanic- cavity, and all it> appendages. From many observa- tions made in the last few decades it seems that many of the fishes if not all) cannot distinguish tones; their labyrinth seems to be chiefly (if nol exclusively) an organ for tlu' sense of space (or equi- librium). If it i- destroyed, the fishes lose their balance and tall. In the opinion of recent physiologists this applies aUo to many of the Invertebrates (including the nearer ancestors of the Vertebrates). The round vesicles which are onsklcred to he their auscultory vesicles, and which contain an otolith, are supposed to be merely organs of the sense ot space (" static vesicles or stato- The middle ear makes its first appear- ance in the amphibian class, where we find a tympanum, tympanic cavity, and Eustachian tube; these animals, and all terrestrial Vertebrates, certainly have the faculty of hearing. All these essential parts of the middle ear originate from the first gillrclefl and its. surrounding part ; in the Seluchii this remains throughout life an open squirting-hole, and lies between the first and second gill-arch. In the embryo of the higher Vertebrates it i loses up in the centre, and thus forms thi' lympanu membrane. The outlying remainder o( the tirst gill-cleft is the rudiment of the external meatus. From its inner pan ue gel the tympanic cavity, and, furthei inward still, the Eustachian tube. Connected with this is the develop- ment oi' the three bones of the mammal eai from the first two gill-arches; the hammer and anvil are formed from the first, the Stirrup flOm the upper end of (lie second, ^ill-arch. Finally, the shell (pinna or concha) and external meatus (passage to the tym- panum') of the outer ear are developed in a very simple fashion from the skin that benders the external aperture oi the first gill-cleft. The shell rises in the shape of a circular fold of the skin, in which cartilage and muscles are afterwards formed (Figs. 313 and 315). This organ is oid\ ,1'ound in the mammalian class. It is viiy rudimentary in the lowest section, the Monotremes. In the others it is found at very different stages of development, and sometimes of degenera- tion. It is degenerate in most of the aquatic mammals. The majorit v of them have lost it altogether for instance, the walruses and whales and most of the seals. On the other hand, the pinna is Fig. 324— The rudimentary muscles of the ear in the human skull a raising muscle ( .'/. aftolietuj, b drawing muscle (M. attrahens), cwfthdrawing muscle ( .1/ rrlrahF THE ORG. 1 NS OF MO l 'EMENT injects, we find .1 similar feature, with tlu- difference that in tins case the skin forms a solid armour .,1 rigid cutaneous skeleton madeof chitinejfand often alsoof carbonate of lime). This external chitine coat undergoes a ; very elaborate ar- ticulation both on tho trunk and the limbs of the Articu- lates, and in conse- quence the muscular system also, the con- tractile fibres of which are attached inside the chitine tubes, is highly arti- culated. The Verte- brates form a direct contrast to this. In these alone a solid internal skeleton is de\ i loped, of cartil- age of bone, to which the muscles are at- tached. This bony skeleton is a complex lever apparatus, or Passive apparatus of movement. Its rigid parts, thearmsof the levers, or the bones, are brought together by the actively mo- bile muscles, as if by drawing-ropes. This admirable locomo- toriurri, especially its solid central axis, the vertebral column, is a special feature of the Vertebrates, and has given the name to the group. In order to get a clear idea of the chief features of the devel- opment of the human skeleton, we must first examine its composition in the adult frame (Fig. 325, the human skeleton seen from the right ; Fig. 326, front view of the whole skeleton). As in other mam- mals, we distinguish first between the axial or dorsal skeleton and the skeleton of the limbs. The axial skeleton con- sists, of the vertebral column (the skeleton of the trunk) and the skull (skeleton of the head); the latter is a peculiarly modified part of the former. As appen- dages of the- vertebral column we have Fig. 327.— The human vertebral column (standing upright. from the ri^ht side). (From H. Meyer.) the ribs, and of the skull we have the hyoid bone, the lower jaw, and the other products of the gill-arches. The skeleton of the limbs or extremities is composed of two groups of parts (he skeleton of the extremities proper and the /one-skeleton, which connects these with the vertebra] column. The /one-skeleton of the arms (or fore legs) is the shoulder- zone , the /one-skeleton of the legs (or hind legs) is the pelvic zone. The vertebral column (Fig. 327) in man is composed of thirty-three to thirty- five ring-shaped bones in a continuous series (above each other, in man's upright position). These vertebra are separated from each other by elastic ligaments, and at the same time connected by |OtntS, so that the whole column forms a firm and solid, but flexible and elastic, axial skeleton, moving freely in all directions. The vertebra- differ in shape and connec- tion at the various parts ol the trunk, and we distinguish the following groups in the series, beginning at the top . Seven cervical vertebra, twelve dorsal vertebra;, five lumbar vertebrae, five sacral vertebra?, and four to six caudal vertebras. The uppermost, or those next to the skull, are the cervical vertebra' (Fig. 327) ; they have a hole in each of the lateral pro- cesses. There are seven of these vertebra; in man and almost all the other mam- J mals, eveniftheneck is as long as that iA' the camel or giraffe, or as short as that of the mole or hedgehog. This constant number, which has few ex- ceptions (due to adaptation), is a strong proof of the common descent of the mammals ; it can only beexplained by faithful heredity from a common stem-form, a primi- tive mammal with seVen cervical verte- bra-. If each species had been created separately, it would to have given the long-necked mam- mals more, and the short-necked animals less, cervical vertebra-. Next to these come the dorsal (or pectoral). Fig. 328.— A piece of the axial rod (chorda dor salts), from a sheep embryo. a cuticular sheath, b cells. (From KbUiker.) have been better EVOLUTION OF THE ORGANS OF MOVEMENT -^5 vertebra, which number twelve to thirteen (usual!) twelve) in man and most of the other mammals. (Fig. 165) has at V\c. v.). Three dorsal vertebrae, from .1 human embryo, Bight wiiks old. in lateral longitudinal section, f cartilagin- ous vertebra] body, It intir-vtrtilir.il disks. th chorda. (From KdllU . Each dorsal vertebra the ^i Jv'. connected by joints', a'couple of ribs, long bony arches that lie in and pTOtecl the wall of the chest. The twelve pairs of ribs, together with the connecting intercostal muscles' and the ster- num, which joins the ends of the right and left rills in front, form the chest (thorax). In this Strong and elastic frame are the I unrjs, and between them the heart. \'e\l to the dorsal vertebra' comes a short but stronger section of the column, formed of five large vertebrae. These are the lumbar vertebras (Fig. 166); they have no ribs and no holes in the trans- verse processes. To these succeeds the sacral bone, which is fitted between the two halves of the pelvic zone. The sacrum is formed of five verte- bras, completely blended together. Finally, we have at the end a small rudimentary caudal column, the coccyx. This consists of" a varying number (usually four, more rarely three, or five W six) of small degenerated vertebrae, and is a useless rudimentary organ with no actual physio- logical significance. Morphologically, however, it is of great interest as an irrefragable proof of the descent of man and the anthropoids from long-tailed apes. On no other theory can we explain the existence ol this rudimentary tail. In the earlier stages of development the tail of the human embryo protrudes con- siderably. It afterwards atrophies; but the relic of the atrophied caudal vertebra' and of the rudimentary muscles that once moved it remains permanently. Some- times, in fact the external tail is pre- served. The older anatomists sav that the tail is usually onv vertebra longer in the human female than in the male (or four against five); Steinbach says il is the reverse. In the human vertebral column there are usually thirty-three vertebrae. It is interesting to find, however, that the number often changes, one or two vertebrae dropping out or an additional o\w appealing. Often, also, a mobile rib is formed at the last cervical or the firsl lumbar vertebra, so that there are then thirteen dorsal vertebra', besides six cervical and four lumbar. In this way the contiguous vertebrae of the various sec- tions of the column may take each other's places. In order to understand the embryology of the human vertebral column we must fust carefully consider the shape and con- nection of the vertebrae. Each vertebra has, in general, the shape of a seal-ring (Figs. 164 16(1). The thicker portion, which is turned towards (he ventral side,, is called the body of the vertebra, and' forms a short OSSeOUS disk ; the thinner part forms a semi-circular arch, the. vertebral arch, and is turned towards the back. The arches of the successive vertebrae are connected by thin intercrural ligaments in such a way that the cavity they collectively enclose represents a long canal. In this vertebral canal we find the trunk part of the central nervous System, the spinal cord. Its head part, the brain, is enclosed by the skull, and the skull itself is merely the. uppermost part of the vertebral column, distinctively modified. The base or ventral side of the vesicular cranial capsule corresponds originally to a number of developed vertebral bodies ; its vault or dorsal side to their combined upper vertebral arches. While the solid, massive bodies of the vertebrae represent the real central axis of to. — A dorsal vertebra of tin- same embryo, in Literal transverse section. CV cartilaginous vertebra] body, rli chorda. />>■ transverse process, n vertebral arch (upper arch), c upper end of the rib (lower aiilil (Front Kolliker.) the skeleton, the dorsal arches serve to protect the central marrow they enclose. Hut similar arc lies develop o\\ the ventral side for the protection of the viscera in the breast and belly. These, lower or *q6 EVOLUTION OF THE ORGANS OF MOV EM EXT ventral vertebral arches, proceeding from the ventral side o\ the vertebral bodies, form, in many of the lower Vertebrates, a canal in Which the large blood-vessels are enclosed on the lower surface o\ the Fig. 331.— Intervertebral disk of a new-bom in- fant, transverse section, a rest of the chorda. (From Kiilliker.) vertebral column (aorta and caudal vein). In the higher Vertebrates the majority of these vertebral arches are lost or become rudimentary. But at the thoracic section of the column they develop into inde- pendent strong osseous arches, the ribs (costce). In reality the ribs are merely large and independent lower vertebral arches, which have lost their original connection with the vertebral bodies. If we turn from this anatomic survey of the composition of the column to the question of its development, I may refer the reader to earlier 'pages with regard to the first and most important points (pp. 145-148). It will be remembered that in the human embryo and that of the other vertebrates we find at first, instead of the segmented column, only a simple unarticulated cartilaginous rod. This solid but flexible and elastic rod is the axial rod (or the chorda dorsal is). In the lowest Vertebrate, the Amphioxus, it retains this simple form throughout life, and permanently represents the whole internal skeleton (Fig. 210 i). In the Tunicates, also, the nearest Invertebrate relatives of the Vertebrates, we jneet the same chorda— transitorily in the passing larva tail of the Ascidia, permanently in the Copelata (Fig. 225 c). Undoubtedly both the Tunicates and Acrania have inherited the chorda from a common unsegmented stem-form ; and these ancient, long-extinct ancestors of all the [ chordoma are our hypothetical Prochor- donia. Long before there is any trace of the ! skull, limbs, etc., in the embryo of man or any of the higher Vertebrates— at the early stage in which the whole body is merely a sole-shaped embryonic shield — there appears in the middle line o( the shield, directly under the medullary furrow, the simple chorda. (Cf. Figs. W-^S**)' ,l follows the long axis of the body in the shape of a cylindrical axial rod of elastic but firm composition, equally pointed at both ends. In every case the chorda originates from the dorsal wall of the primitive gut ; the cells that compose it (Fig. 328 li) belong to the entoderm (Figs. 216-221). At an early stage the chorda developes a transparent structureless sheath, which is secreted from its cells (Fig. 328 a). This chorda- lemma is often called the " inner chorda- sheath," and must not be confused with the real external sheath, the mesoblastic perichorda. But this unsegmented primary axial skeleton is soon replaced by the segmented secondary axial skeleton, which we know as the vertebral column. The prever- tebral plates (Fig. 124 s) differentiate from the innermost, median part of the visceral layer of the coelom-pouches at each side of the chorda. As they grow round the chorda and enclose it they form the skeleton plate or skeletogenetic layer — that is to say, the skeleton-forming stratum of cells, which provides the mobile foundation of the permanent vertebral column and skull (scleroblast). In the head-half of the embryo the Fig. 332.— Human skull. skeletal plate remains a continuous, simple, undivided layer of tissue, and presently enlarges into a thin-walled capsule enclosing the brain, the primordial skull. In the trunk-half the provertebral E l (>/. I '7 /OX OF THE ORG A NS OF Mi > 1 7. Ml. Nl 297 plate divides into a number of homo- geneous, cubical, successive pieces ; these are the several primitive vertebrae. They are not numerous at first, but soon increase as the embryo grows longer (Figs. 153-155). Vu- 333.— Skull Of a new-born Child. (From Kallmann.) Above, in the three hones of the root' of the skull, we see the lines that radiate from the central points ot ossification ; in front, the frontal bone; behind, the occipital hone; between the two the large parietal hone, /. .•; the scurf bone, H mastoid fontanelle, f petrous bone, t tympanic bone. /-lateral part, b bulla, j cheek-bone, a large wing ^ cuneiform bone, k fontanelle of cuneiform bone. In all the Craniotes the soft, indifferent cells of the mesoderm, which originally compose the skeletal plate, are afterwards converted for the most part into carti- laginous cells, and these secrete a firm and elastic intercellular substance between them, and form cartilaginous tissue. Like most of the other parts of the skeleton, the membranous rudiments of the vertebrae soon pass into a carti- laginous state, and in the higher Verte- brates this is afterwards replaced by the hard osseous tissue with its character- istic stellate cells (Fig. 6) The primary axial skeleton rem, tins a simple chorda throughout life in the Acrania, the Cyclo- Stomes. and the lowest fishes. In most of the other Vertebrates the chorda is more or less replaced by the cartilaginous tissue of the secondary perichorda that grows round it. In the lower Craniotes (especially the fishes) a more or less considerable part of the chorda is pre- served in the bodies of the vertebrae. In the mammals it disappears for the most part. Bj the end of the second month in the human embryo the chorda is merely a slender thread, running through the axis of the thick, cartilaginous vertebral column (Figs. l8a ch, 32Q ch). In the cartilaginous vertebral bodies themselves, which afterwards ossify, the slender remnant ot the chorda presently disappears (Fig. \J,o ch). Hut in the elastic inter- vertebral disks, which develop from the skeletal plate between each pair of vertebral bodies (Fig. 320, //), a relic of the chorda remains permanently. In the new-born child there is a large pear-shaped cavity in each intervertebral disk, filled with a gelatinous mass of cells (Fig. 331 a). Though less sharply defined, this gela- tinous nucleus of the elastic cartilaginous disks persists throughout life in the mam- mals, but in the birds and most reptiles the last trace of the chorda disappears. In the subsequent ossification of the cartilaginous vertebra the first deposit of bony matter ("first osseous nucleus ") takes place in the vertebral body immedi- ately round the remainder of the chorda, and soon displaces it altogether. Then there is a special osseous nucleus formed in each Fig. 334.— Head-skeleton of a primitive fish. ■ nasal pit. rth cribriform bone region, orb orbit ot eye. In wall of auscultory labyrinth, ore occipital region of primitive skull. n. EVOLUTION OF THE ORGANS OP MOVEMENT 200 thai they connect with the osseous vertebral bodies. The bony skull (cranium), the head- part of tlu- secondary axial skeleton, developes in just the same way BS the vertebral column. The skull forms a bony envelope for the brain, just as the vertebral canal dees for the spinal cord ; and as the brain is only a peculiarly differentiated part of the head, while the spinal cord represents the longer trunk- section oi~ the originally homogeneous medullary tube, we shall expect to find vault above. The other thirteen boney Form the facial skull, which is especialls the bony envelope oi' the higher sense- organs, and at the same lime em loses the entrance of the alimentary canal. The lower jaw is articulated at the base of the skull (usually regarded as the XXI. cranial bone). Hehind the. I uver jaw we find the hyoid bone at the root of the tongue, also formed from the gill-arc lies, ana a part. of the lower arches that have developed as " head-ribs" from the ventral side of the base of the cranium. Fig. 336. ftO, 3.T7- Fto. 338. Fig. 336. — Skeleton Of the breast-fin of Ceratodus (biserial feathered skeleton). A, B, cartilaginous sarics of the tin-stem, rr cartilaginous tin-radii. (From Giinther. ) Fig. 337. —Skeleton of the breast-fin of an early Selachius ( Acanthias). The radii of the median fin-border (/? J haye disappeared for the most part ; a few only (R) are left. R, R, radii of the lateral fin-border, ml meUiplerygium, ms mesopterygium, f> propterygium. (From Gegenbaur.) Fig. 338.— Skeleton Of the breast-fiin of a young Selachius. The radii of the median fin-border have wholly disappeared. The shaded part on the right is the section that persists in the five-fingered hand of the higher Vertebrates, (b the three basal pieces of the fin : mi metapterygium, rudiment of the humerus, ms meso- pterygium. p propterygium.^ (From Gegenbaur.) that the osseous coat of the one is a I special modification of the osseous envelope of the other. When we examine the adult human skull in itself (Fig- 333)i 'l is difficult to conceive how it can Ik- merely the modified fore part of the vertebral column. It is an elaborate and extensive bony structure, composed of no less than twenty bones of different shapes and sizes. Seven -of them form the spacious shell that surrounds the brain, in which we distinguish the solid ventral base below and the curved dorsal Although the fully-developed skull of the higher Vertebrates, with its peculiar shape, its enormous size, and its complex composition, seems to have nothing in common with the ordinary vertebras, nevertheless even the older comparative anatomists came to recognise at the end of the eighteenth century that it is really nothing else originally than a series ol modified vertebra.-. When Goethe in 1790 " picked up the skull of a slain victim from the sand of the Jewish cemetery at Venice, he noticed at once 3oo EVOLUTION OF THE ORGANS OF MOVEMENT th.it the bones of the f.uo also could be j traced to vertebrae (like the three hind- most cranial vertebrae)," And when Oken (without knowing anything of Goethe's discovery) found at Ilenstein "a fine bleached skull of a hind, the thought Hashed across him like light- ning : ' It is a vertebral column.'" This famous vertebral theory of the skull has interested the most distin- guished zoologists for more than a century : the chief representatives of comparative anatomy have devoted their highest powers to the solution of the problem, and the interest has spread far mammal skull, and had compared the several hones that compose it with the several parts of the vertebra (Fig. \\\) ; they- thought they could prove in this way that the fully-formed mammal skull was made of from three to six vertebrae. The older theory was refuted by simple rand obvious facts, which were first pointed out by Huxley. Neverthe- less, the fundamental idea of it — the belief that the skull is formed from the head-part of the perichordal axial skeleton, just as the brain is fiom the simple medullary tube, by differentiation F'c- 339 Fig. 340. Fig. 341. Fig. 339.— Skeleton of the fore leg of an amphibian, h uppcr-ann (humerus), ru lower arm (r radius, u ulna), rcicu' wrist-bones of first series (r radiale, I intermedium, c centrale, u' ulnare). /, 2, J, 4, S wrist-bones of the second scries. (From Getfcnbaur.) Fig. 340.— Skeleton of gorilla's hand. (From Huxley.) Fig. 341— Skeleton of human hand, back. (From Meyer.) beyond their circle. But it was not until 1872 that it was happily solved, after seven years' labour, by the comparative anatomist who surpassed all other experts of this science in the second half of the nineteenth century by the richness of his empirical knowledge and the acuteness and depth of his philosophic speculations. Carl Gegcnbaur lias shown, in his classic Studies of the Comparative Anatomy of the Vertebrates (third section), that we find the most solid foundation for the vertebral theory of the skull in the head-skeleton of the Selachii. Earlier anatomists had wronglv started from the and modification— remained. The work now was to discover the proper way of supplying this philosophic theory with an empirical foundation, and it was reserved for Gegenbaur to achieve this. He first opened out the phylogenetic path which here, as in all .morphological questions, leads most confidently to the goal. He showed that the primitive fishes^ (Figs. 249-251), the ancestors of all the Gnatho- stomes, still preserve permanently in the form of their skull the structure out of which the transformed skull of the higher Vertebrates, including man, has been evolved. He further showed that the EVOLUTION OF THE ORGANS OF MOVEMENT 3°' branchial arches of the Selachii prove tli.it their skull originally consisted of a 1 large number of (at least nine or ten) provertebne, and thai the cerebral nerves that proceed from the base of (he brain entirely confirm this. These cerebral nerves are (with the exception of the first and second pair, the ol factor) Ai\d optic nerves) merely modifications of spinal nerves, and are essentially similar to them in their peripheral expansion. The comparative anatomy of these cerehral nerves, their origin and their expansion, furnishes one of the strongest arguments for the new vertebral theory of the skull. We have not space here to go into the details of (Jegenbaur's theory of the each side the primitive upper jaw (us palafo-quadrtttum, o) and the primitive lower jaw (u); IV, the hyaloid bone ( 11 ) . finally, I A', six branchial arches in the narrower sense ( III I'lll j. From the anatomic features of these nine to ten cranial ribs or " lowei verte- bral arches " and (he cranial nerves that spread over them, it is clear dial the apparently simple cartilaginous primitive skull of (he Selachii was originally formed from so many (a! least nine) somites or prevertebral. The blending of these primitive segments into a single capsule is, however, so ancient that, in virtue of the law of curtailed heredity, the original division seems to have dis- * 3 Fig. 342.— Skeleton Of the hand or fore foot of six mammjls. /man, //dog, /// pig. //'ox, V tapir, I'/ horsi-. r radius, u ulna, a icaphoideum, b lunare. c triquetrum, d trapezium, r trapezoid, / capitatum, g hamatum. /> pisiforme. / thumb, J index finger. J middle finger, j ring linger. 5 little finger. (From (ugrnbaur.) skull. I must be content to refer the reader to the great work I have men- tioned, in which it is thoroughly estab- lished from the empirico-philosophical point of view. He has also given a com- prehensive and up-to-date treatment of the subject in his Comparative Anatomy of the X'crtebratcs (1898). Gegenbaur indicates as original "cranial ribs," or "lower arches of the cranial vertebi.i," II h side of the head of the Selachii •1, the following pairs of arches : / ' //, two lip-cartilages, the anterior ( u ) ot which is composed of an upper piece only, the posterior (be) from an upper and lower piece ; ///, the maxillary arches, also consisting of two pieces on VOL. II. appeared ; in the embryonic development it is very difficult to detect it in isolated traces, and in some respects quite impos- sible. It is claimed that several (three to six) traces of provertcbra- have been dis- COVed in the anterior (pre-chordal) part of the Selachii-skull ; this would bring up the number of cranial somites to twelve or sixteen, or even more. In the primitive skull of man (Tig. 323) and the higher Vertebrates, which has been evolved from that of the SUachii, live consecutive sections are discoverable at a certain early period of development, and one might be induced to trace these to fire primitive vertebra' ; hut these sections are due entirely to adaptation to Fie. 343. Fig. 344. Fig- 34S. Fics. 343-45.— Arm and hand Of three anthropoids. Fig-. 343 Chimpanzee ( Anthropithecus niger.) Fig;. 344 \ eddah of Ceylon (Homo veddatis). rig. 345. European (Homo meditetraneus J. (From Paul and EVOLUTION OF THE ORGANS OF MOVEMENT 303 the five primitive cerebral vesicles, and correspond, like these, to a large number of metamera. That wo have in the primitive skull of the mammals a greatly modified and transformed organ, and not at all a primitive formation, is clear from tlio circumstance that its original soft membranous form only assumes the cartilaginous character for the most part | at the base and the sides, and remains membranous at the root". At this part the bones o\ the subsequent osseous skull develop as external coverings over the membranous Structure, without an inter- mediate cartilaginous st.i^e. as there is at the base o\ the skull. Thus a large part o\ the cranial bones develop originally as covering hones from the corium, and only secondarily come into close touch with the primitive skull (Fig. 333). We have previously seen how this very rudimentary beginning of the skull in man is formed ontOgenetically from the "head-plates," and thus the fore end of the chorda is enclosed in the base o\ the skull. (Cf. Fig. 145 and pp. 138, 144, and 149.) The phytogeny of the skull has made threat progress during the last thre'e decades through the joint attainments o( comparative anatomy, ontogeny, and paleontology. By the judicious and comprehensive application o\ the phylo- geneticmethod in the sense of Gegenbaur) we have found the key to the great and important problems that arise from the thorough comparative studv of the skull. Another school of research, the school of what is called " exact craniology " (in the sense of Virchow), lias, meantime, made fruitless efforts to obtain this result. We may gratefully acknowledge all that this descriptive school has done in the way of accurately describing the various forms and measurements of the human skull, as compared with those of other mammals. But the vast empirical material that it has accumulated in its extensive literature is mere dead and sterile erudition until it is vivified and illumined by phylogenetic speculation. Virchow confined himself to the most careful analysis of large numbers of human skulls and those of anthropoid mammaU. He saw only the differences between them, and sought to express these in figures. Without adducing a single solid reason, or offering an\ alternative explanation, he rejected evolution as an un proved hypo- thesis. He played a most unfortunate part in the controversy as to the signifi- cance of the fossil human skulls 01 Spy and Neanderthal, and the Comparison of them with the skull of the Pithecanthropus (Fig. 283). AH the interesting features of these skulls that clearly indicated the transition from the anthropoid to the man were declared by Virchow to be chance pathological variations. He said that the roof of the skull of Pithecanthropus (Fig. 335i 0) must have belonged to an ape, because so pronounced an orbital stricture (the horizontal constriction between the outer edge of the eve-orbit and the temples) is not found in any human being. Immediately afterwards Nehring showed in the skull of a Brazilian Indian (Fig. 335, 2), found in the Sambaquis of Santos, that this stricture can be even deeper in man than in many of the apes. It is very Fig. 346— Transverse section of a fish's tall (from toe tunny). (From Johannes Miiller.) a upper (dorsal) lateral muscles, a', l> lower (ventral) lateral muscles, il vertebral bodies, b sections of incomplete conical mantle. />' attachment lines, of the intcr-rnuscular ligaments (from the side). instructive in this connection to compare the roofs of the skulls (seen from above) of different primates. I have, therefore, arranged nine such skulls in Fig. 335, and reduced them to a common size. We turn now to the branchial arches, which were regarded even by the earlier natural philosophers as " head-ribs." (Cf. Figs. 167-170). Of the four original gill- arches of the mammals the first lies between the primitive mouth and the first gill-cleft. From the base of this arch is formed the upper-jaw process, which joins with the inner and outer nasal processes on each side, in the manner we have previously explained, and forms the chief parts of the skeleton of the upper jaw (palate bone, pterygoid bone, etc.) (Cf. K. 284.) The remainder of the first ranchial arch, which is now called, by 3°4 EVOLUTION OF THE ORGANS OF MOVEMENT way of contrast, the " upper-jaw process," forms from its base i wo of ihe ear-ossicles (hammer and anvil), and as to the rest is converted into a long strip of cartilage that is known, after ifs discoverer, as " Meckel's cartilage, "or the promandibula. At the outer surface of the latter is formed from the cellular matter of the corium, as covering or accessory bone, the permanent bony lower jaw. From the first part or base of the second branchial arch we get, (in the mammals, the third ossicle of the ear, the stirrup ; and from the succeeding parts we get (in this order) the muscle of the stirrup, the styloid process of the temporal bone, thestyloid-hyoid ligament, and the little horn of the hyoid bone. The third branchial arch is only cartilaginous at the foremost part, and here the body of the hyoid bone and its larger horn are formed at each side by the junction of its two halves. The fourth branchial arch is only found transitorily in the mammal embryo as a rudimentary organ, and does not develop special parts;. and there is no trace in the embryo of the higher Vertebrates of the posterior branchial arches (fifth and sixth pair), which are permanent in the Selachii. They have been lost long ago. Moreover, the four gill-clefts of the human embryo are only- interesting as rudimentary organs, and they soon close up and disappear. The first alone (between the first and second branchial arches) has any permanent significance ; from it are developed the tympanic cavity and the Eustachian tube, (Cf. Figs. 169, 320.) It was Carl Gegenbaur again who solved the difficult problem of tracing the skeleton of the limbs of the Vertebrates, to a common type. Few parts of the vertebrate body have undergone such infinitely varied modifications in regard to size, shape, and adaptation of structure as the limbs or extremities ; yet we are in a position to reduce them all to the same hereditary standard. We may generally distinguish three groups among the Vertebrates in relation to the forma- tion of their limbs. The lowest and earliest Vertebrates, the Acrania and Cyclostomes, had, like their invertebrate ancestors, no pairs of limbs, as we see in the Amphioxus and the Cyclostomes to-day (Figs. 210, 247). The second group is formed of the two classes of the true fishes and the Dipneusts ; here there are always two pairs of limbs at first, in the shape of many-toed fins— one pair of breast-fins or fore legs, and one pair of belly-fins or hind legs (Figs. 248 259). The third group comprises the four higher classes of Vertebrates the amphibia, reptiles, birds, and mammals ; in these quadrupeds there are at first the same two pairs of limbs, but in the shape of five-toed feet. Frequently we find less than five toes, and sometimes the feet are wholly atrophied ("as in the serpents). Hut the original stem-form of the group had five toes or fingers before and behind (Figs. 263-265). The true primitive form of the pairs of limbs, such as they were found in the primitive fishes of the Silurian period, is preserved for us in the Australian dipneust, the remarkable Ccratodus (Fig. 257). Both the breast-fin and the belly-fin are flat oval paddles, in which we find a biserial cartilaginous skeleton (Fig. 336). This consists, firstly, of a much segmented fin-rod or "stem" (A, BJ, which runs through the fin from base to tip ; and secondly of a double row of thin articu- lated fin-radii (r> r)t which are attached to both sides of the fin-rod, like the feathers of a feathered leaf. This primi- tive fin, which Gegenbaur first recognised, is attached to the vertebral column by a simple zone in the shape of a cartilaginous arch. It has probably originated from the branchial arches.1 We find the same biserial primitive fin more or less preserved in the fossilised remains of the earliest Selachii (Fig. 248), Ganoids (Fig. 253), and Dipneusts (Fig. 256). It is also found in modified form in some of the actual sharks and pikes. But in the majority of the Selachii it has already degenerated to the extent that the radii on one side of the fin-rod have been partly or entirely lost, and are retained only on the other (Fig. 337). We thus get the uniserial fin, which has been transmitted from the Selachii to the rest of the fishes (Fig. 338). Gegenbaur has shown how the five- toed leg of the Amphibia, that has been inherited by the three classes of Amniotes, was evolved from the uniserial fish-fin. " * While Gegenbaur derives the fins from two pairs of posterior separated branchial arches. Balfour holds that they have been developed from segments of a pair of originally continuous lateral fins or folds of the skin. 3 The limb of the four higher classes of Vertebrates is now explained in the sense that the original fin-rod passes along its outer (ulnar or fibular) side, and ends in the fifth toe. It was formerly believed to go along the inner (radial or tibial) side, and end in the first toe, as Fig. 3j9 shows. Fio. 347. P io- 348- Fio. 347— Human skeleton. (Cf. Fig. 316.) Fig. 348— Skeleton of the giant gorilla. (Cf. Fig. 209 306 EVOLUTION OF THE ORGAXS OF MOVEMENT In the dipneusl ancestors of the Amphibia the radii gradually atrophy, and are lost, for the most pan, on tlu olher side of the iln-rod as well (the lighter cartila Fig. 338). Only the four lowest radii (shaded in the illustration) are preserved ; and these are the four inner toes oi the fool (lust to fourth). The little or fifth toe is developed from the lower end oi the fm-rod. From the middle and upper part of the fin-rod was developed the long stem of the limb the important radius and ulna (Fig. 330 rand u) and humerus ( h ) oi the higher Vertebrates. In this way the five-toed foot oi the Amphibia, which we fust meet in the Carboniferous Steeocephala (Fig. 260), and which was inherited from them by the reptiles on one side and the mammals on the other, was formed by gradual degeneration and differentiation from the many-toed fish-fin (Fig. 341). The reduc- tion of the radii to four was accompanied bv a further differentiation of the fin-rod, its transverse segmentation into upper and lower halves, and the formation of the zone oi the limb, which is composed originally oi three limbs before and behind in the higher Vertebrates. The simple arch of the original shoulder-zone divides on each side into an upper (dorsal) piece, the shoulder-blade (scapula), and a lower (ventral) piece ; the anterior part of the latter forms the primitive clavicle ( procoracoideum ), and the posterior part the caracoideum. In the same way the simple arch of the pelvic v.o\w breaks up into an upper (dorsal) piece, the iliac-bone ( os ilium), and a lower (ventral) piece ; the anterior pari oi the latter forms the pubic bone (os pubis), and the posterior the ischial bone (os ischii). There is also a complete agreement between the fore and hind limb in the stem or shaft. The first section oi the stem is supported by a single strong bone — the humerus in the fore, the femur in the hind limb. The second section con- tains two bones : in front the radius (r ) and ulna ( u ), behind I he tibia and fibula. (Cf. the skeletons in Fjgs! 260, 265, 270, 278-282, and 348.) The succeeding numerous small bones oi the wrist (carpus) and ankie ( tarsus ) are also similarly arranged in the fore and hind extremities) and so are the five bones of the middle-hand (metacarpus ) and middle-foot (metatarsus J. Finally, it is the same with the toes themselves, which have a similar characteristic composition from a .series oi bony pieces before and behind. We find a complete parallel in all the parts of the fore leg and the hind leg. When we thus learn from comparative anatomy that the skeleton of the human limbs is composed oi jusl the same bones, put together in the same way, as the skele- ton in the four higher classes oi Verte- brates, we may at once infer a common descent oi them from a single stem-form. This stem-form was the earliest am- phibian that had five toes oct each foot. It is particularly the outer parts of the limbs that have been modified bv adapta- tion to different conditions. We need only recall the immense variations they offer within the mammal class. We have the slender legs of the deer and the strong springing legs oi the kangaroo, the climbing feet oi the sloth and the digging feet of the mole, the tins of the whale and the wings of the bat. It willl readily be granted that these organs oi locomotion differ as much in regard to size, shape, and special function as can be conceived. Nevertheless, the bony skeleton is substantially the same in every case. In the different limbs we always find the same characteristic bones in essentially the same rigidly hereditary connection ; this is as splendid a proof of the theory of evolution as comparative anatomy can discover in any organ of the body. It is true that the skeleton of the limbs of the various mammals undergoes many distortions and degenerations be- sides the special adaptations (Fig. 342). Thus we find the first finger or the thumb atrophied in the fore-fool (or hand) of the dog (II). It has entirely disappeared in the pig (III) and tapir (V). In the rumi- nants (such as the ox, IV) the second and fifth toes are also atrophied, and only the third and fourth are well developed (VI, 3). Nevertheless, all these differed* fore-feet, as well as the hand oi thq ape (Fig. 340) and, of man (Fig. 341), were pngi rial ly developed from a common pentadactyle stem-form. This is proved by the rudiments of the degenerated toes, and bv the similarity oi the arrangement oi the wrist-hones in all the pentanomes (Fig. 3-1- " PY II we candidly compare the bony skele- ton of the human arm and hand with that of the nearest anthropoid apes, we find an almost perfect identity. This is especially true of the chimpanzee. In regard to the proportions of the various EVOLUTION OF THE ORGANS OF MOVEMENT 3°7 parts, the lowest living races of men (the Vedd.ihs of Ceylon, Fig. (44) are midway between the chimpanzee (Fig. 543) and the European (Fig. J45) More consider- able are the differences in structure and the proportions of the various parts be- tween the different genera of anthropoid apes (Figs. 278 282 ; and still greater is the morphological distance between these and the lowest apes [the Cynopitheca). Here, again, impartial and thorough ana- tomic comparison confirms the accuracj of Huxley's pithecometra principle (p. 171 1 The complete unity of structure which is thus revealed by the comparative anatomy of the limits is fully confirmed by their embryology. However different the extremities of the four-footed Craniotes may be in their adult state, they all de- velop from the same rudimcnlatv struc- ture. In every ease the first trace of the limb in the embryo is a very simple pro- tuberance that grows OUt of the side of the hyposoma. These simple structures develop directly into Tins in the fishes and Dipneusts by differentiation of their cells. In the higher classes of Vertebrates ea< h of the four takes the shape in its further growth of a leaf with a stalk, the inner half becoming narrower and thicker and the outer half broader and thinner. The inner half (the stalk of the leaf) then divides into two sections — the upper and lower parts of the limb. Afterwards four shallow indentations are formed at the five edge of the leaf, and gradually deepen ; these are the intervals between the five toes (Fig. 174). The toes soon make their appearance. But at first all five toes, both of fore and hind feet, are connected by a thin membrane like a swimming-web ; they remind us of the original shaping of the loot as a paddling tin. The further development of the limbs from this rudimentary structure takes place in the same way in all the Vertebrates according to the laws of heredity. The embryonic development of the muscles, or active organs of locomotion, is* not less interesting than that of the skeleton, or passive orgafis. Hut the comparative anatomy and ontogeny of the muscular system are much more diffi- cult and inaccessible, and consequently have hitherto been les^ studied. We can therefore only draw some general phylo- genetic conclusion-, therefrom. It is incontestable that the musculature of the Vertebrates has been evolved from tint of lower Invertebrates ; and among these we have to consider especially the unarticulated Vermalia. The) hav< simple cutaneous muscular layer, develop- ing from (he mesoderm. This was after- wards replaced In a pairof internal lateral muscle-., that developed from the middle wall of the ccelom-pouches ; vve still find the first rudiments of the must les arising from the muscle-plate of these in the embryos of all the Vertebrates (cf. Figs. 124, 158 (m), 222 4 nip). In the unar! un- filed stem-forms of the Chordoma, which we have called the Pi oehoi donia, the two COelom-pOUCheS, and therefore also the muscle-plates of their walls, were not vet segmented. A gn it advance was made in the articulation of them, as vv e have followed it step by step in the Amphioxus (Figs. 1 -*4, 158). This segmentation of the muscles was the momentous historical process with which vertebratiun, and the development of the vertebrate Stem, began. The articulation of the skeleton came alter this segmentation of the muscular system, and the two entered into very 1 lose corre- lation. The episomites ordorsal ccelom-pouches of the Acrania, Cyclostomes, and Selachii (Fig. 161 h) first develop from their inner or median wall (from the cell-layer that lies directly on the skeletal plate [sk\ and the medullary tube \">'\) a strong muscle- plate ( nip J. By dorsal growth f'.rj it also reaches the external wall of the ccelom-pouches, and proceeds from the dorsal to the ventral wall. From these segmental muscle-plates, which are chiefly concerned in the segmentation of the Vertebrates, proceed the lateral muscles of the stem, as we find in the simplest form in the Amphioxus (Fig. 210). By the formation of a horizontal frontal septum they divide on each side into an upper and lower series of myotomes, dorsal and ventral lateral muscles. This is seen with typical regularity in the transverse section of the tail of a fish (Fig. 346). From these earlier lateral muscles of the trunk develop the greater part of the subsequent muscles of the trunk, and also the much later " muscular buds " of the limbs.' The ontogeny of the muscles is mostly cenoprcnctic. The greater part of the muscles of the head (or the viscera] muscles) helon^ originally to tin hyposoma of the vertebrate organism, ana develop from the wall of the hyposomitea or vcntr.il coelom-pouches. This also applies originally to the primary muscles of the linils. • is these 100 belong phylogcneticallv to the h\posoma. at Chapter XIV.) 308 THE EVOLUTION OF THE ALIMENTARY SYSTEM Chapter XXVII. THE EVOLUTION OF THE ALIMENTARY SYSTEM The chief of the vegetal organs of the human frame, to the evolution of which we now turn our attention, is the alimen- tary canal. The gut is the oldest of all the organs o\ the metazoic body, and it leads us hack to the earliest age of the formation of organs — to the first section of the Laurentian period. As we have already seen, the result of the first division of labour among the homogeneous cells of the earliest multicellular animal body was the formation of an alimentary cavity. The first duty and first need of every organism is self-preservation. This is met by the functions of the nutrition and the covering of the body. When, there- fore, in the primitive globular Blastcea the homogeneous cells began to effect a division of labour, they had first to meet this twofold need. One half were con- verted into alimentary cells and enclosed a digestive cavity, the gut. The other half became covering cells, and formed an envelope round the alimentary tube and the whole body. Thus arose the primary germinal layers — the inner, alimentary, or vegetal layer, and the outer, covering, or animal layer (Cf. pp. 214-17.) When we try to construct an animal frame of the simplest conceivable type, that has some such primitive alimentary canal and the two primary layers consti- tuting its wall, we inevitably come to the very remarkable embryonic form of the gastrula, which we have found with extraordinary persistence throughout the whole range of animals, with the excep- tion of the unicellulars— in the Sponges, Cnidaria Platodes, Vermalia, Molluscs, Articulates, Echinoderms, Tunicates, and Vertebrates. In all these stems the gastrula recurs in the same very simple form. It is certainly a remarkable fact that the gastrula is found in various animals as a larva-stage in their indi- vidual development, and that this gastrula, though much disguised by cenogenetic modifications, has everywhere essentially the same palingenetic structure (Figs. 30-35). The elaborate alimentary canal of the higher animals developes onto- genetically from the same simple primi- tive gut of \X\o, gastrula. This gastraea theory is now accepted by nearly all zoologists. It was first supported and partly modified by Professor Ray-Lankester ; he proposed three years afterwards (in his essay on the develop- ment of the Molluscs, 1875) to give the name of archenteron to the primitive gut and blastoporus to the primitive mouth. Before we follow the development of the human alimentary canal in detail, it is necessary to say a word about the general features of its composition in the fully-developed man. The mature alimen- tary canal in man is constructed in all its main features like that of all the higher mammals, and particularly resembles that of the Catarrhines, the narrow-nosed apes of the Old World. The entrance into it, the mouth, is armed with thirty- two teeth, fixed in rows in the upper and lower jaws. As we have seen, our denti- tion is exactly the same as that of the Catarrhines, and differs from that of all other animals (p. 257). Above the mouth- cavity is the double nasal cavity ; they are separated by the palate-wall. But we saw that this separation is not there from the first, and that originally there is a common mouth-nasal cavity in the embryo ; and this is only divided after- wards by the hard palate into two — the nasal cavity above and that of the mouth below (Fig. 311). At the back the cavity of the mouth is half closed by the vertical curtain that we call the soft palate, in the middle of which is the uvula. A glance into a mirror with the mouth wide open will show its shape. The uvula is interesting because, besides man, it is only found in the ape. At each side of the soft palate are the. tonsils. Through the curved opening that we find THE EVOLL'TIOX OF THE ALIMENTARY SYSTEM 309 underneath the soft palate we penetrate into the gullet or pharynx behind the mouth-cavity. Injto this opens on either side a narrow canal (tin- Eustachian tube), through which there is direct communica- tion with the tympanic cavity of the ear 1 320 f). The pharynx is continued in a long, narrow tube, the OBSOphagUS (sr ). By this the food passes into the Stomach when masticated and swallowed. IntO the gullet also opens, right ahove, the trachea ( lr), that leads to the lungs. The entrance to it is covered by the epiglottis, over which the food slides. The cartilaginous epiglottis is found only in the mammals, and has developed from the fourth branchial arch of the fishes and amphibia. The lungs are found, in man and all the mammals, to the right and left in the pectoral cavity, with the heart between them. At the upper end of the trachea there is, under the epiglottis, a specially differentiated part, strengthened by a cartilaginous skeleton, the larynx. This important organ of human speech also developes from a part oi the alimentary canal. In front of the larynx is the thyroid gland, which sometimes enlarges and forms goitre. The oesophagus descends into the pectoral cavity along the vertebral column, behind the lungs and the heart, pierces the diaphragm, and enters the visceraJ cavity. The diaphragm is a membrano- muscular partition that completely separates the thoracic from the abdominal cavity in all the mam- mals (and these alone). This separa- tion is not found in the beginning ; there is at first a common breast-belly cavity, the cceloma or pleuro-peritoneal cavity. The diaphragm is formed later on as a muscular horizontal partition between the thoracic and abdominal cavities. It then completely separates the two cavities, and is only pierced by several organs that pass from the one to the olhec. One of the chief of these organs is the oesophagus. After this has passed through the diaphragm, it expands into the gastric sac in which digestion chiefly takes place. The stomach of the adult man (Fig. 349) is a long, somewhat oblique sac, expanding on the left into a blind sac, the fundus of the stomach ( b' J, but narrowing on the right, and passing at the pylorus (e) into the small intestine. At this point there is a valve, the pyloric valve (d), between the two sections of the canal ; it opens only when the pulpy food passes from the stomach into the intestine. In man and the higher Veite- brates the stomach itself is the chief organ o\ digestion, and is especially occupied with the solution o\ the food ; this is not the case in many of the lower Vertebrates, which have 1^0 stomach, and discharge its function by a pari of the gut farther on. The muscular wall o! the stomach is Comparatively thick ; it has externally strong muscles that accomplish the diges- tive movements, and internal!} a large quantity of small glands, the peptic glands, which secrete the gastric juice. Next to the stomach comes the longest section of the alimentary canal, the middle gut or small intestine. Its chief function is to absorb the peptonised fluid Fig. 349.— Human stomach and duodenum, longi- tudinal section, a cardiac (end of oesophagus), b Hindus (blind sac of the left side), c pylorus-fold, d pylorus-valves. e pvlorus-cavity, fgh duodenum, i entrance ot the gall-duct and the pancreatic duct. (From Meyer.) mass of food, o\ the chyle, and it is sub- divided into several sections, of which the first (next to the stomach) is called the duodenum (Fig. 349./^). It is a short, horseshoe-shaped loop of the gut. The larg st glands of the alimentary canal open into it-1 — the liver, the chief digestive gland, that secretes the gall, and the pancreas, which secretes the pancreatic juice The two glands pour their secre- tions, the bile and pancreatic juice, close together into the duodenum ( 1). The opening of the gall-duct is of particular phylogenetic importance, as it is the same in all the Vertebrates, and indicates the principal point of the hepatic or trunk-gut (Gegen'baur). The liver, phylogeneticallv older than the stomach, is a large gland, rich in blood, in the adult man, imme- diately under the1 diaphragm on the left 3*° THE r I '< )L f -Tin V f)E THE M f \fF.XT. \RV S I 's TF \t side, and separated bj it From the lungs. The pancreas lies a link- further back and more to the left. The remaining pan of the small intestine is so long that it lias to i oil itself in main folds in order to find room in the narrow space ofthe abdominal cavity It is divided mio the jejunum above and the ileum below. In the lasl section o\ it is the part ofthe small intes- tine at which in the embryo the yelk-sac opens into the i^ut. This long and thin intestine then passes into the large intes- tine, from which it is cut o\'\ by a special valve. Immediately behind this-"Bauhin- valve " the first part ofthe large intestine forms a wide, pouch-like structure, the ccecum. The atrophied end ofthe ccecum is the famous rudimentary organ, the Fig. 350.— Median section of the head of a hare- embryo, one-fourth of an inch in length. (From Mihalcovics. ) The deep mouth-cleft (hp) is separated by the membrane of the throat (rlij from the blind cavity of the head-gut ( kd). hz heart, ch chorda, A^the point at which the hypophysis developesfrom the mouth-cleft, vh ventricleof thccerebrum.t's third ventricle (intermediate brain), vt fourth ventricle (hind brain), ck spinal canal. vermiform appendix. The large intestine (colon) consists of three parts — an^ascend- ing part on the right, a transverse middle part, and a descending part on the left. The latter finally passes through an S-shaped bend into the last section of the alimentary canal, the rectum, which opens behind by the anus. Both the large and small intestines are equipped with numbers of small glands, which secrete mucous and other fluids. For the greater part of its length the J alimentary canal is attached to the inner dorsal surface ofthe abdominal cavity, or to the lower surface of the vertebral column. The fixing is accomplished by means ofthe thin membranous plate that we call the mesentery. Although the fully-formed alimentary canal is thus a very elaborate organ, and although in detail it has .1 quantity of complex structural features into which we cannoi enter lure, nevertheless the whole complicated structure has been histori- cally evolved from the very simple form of the primitive (jut that we find in our gastraead-ancestors, and that every gas- tru la brings before us to-day. We have already pointed out (Chapter IX.) how the epigastrula o\ the mammals (Fig. 67) can be reduced to the original type of the bell-gastrula, which is now preserved by the amphioxus alone (Fig. 35). Like (he latter, the human gastrula and that ol all other mammals must be regarded as the ontogenetic reproduction of the phylogenetic form that we call the Gastrsea, in which the whole body is nothing but a double-walled gastric sac. We already know from embryologv the manner in which the gut developes in the embryo of man and the other mammals. From the gastrula is first formed the spherical embryonic vesicle filled with fluid (gastrocystis, Fig. 106). In the dorsal wall o\ this the sole -shaped embryonic shield is developed, and on the under-side of this a shallow groove appears in the middle line, the first trace of the later, secondary alimentary tube. The gut-groove becomes deeper and deeper, and its edges bend towards each other, and finally form a tube. As we have seen, this simple cylindrical gut-tube is at first completely closed before and behind in man and in the Vertebrates generally (Fig. 148) ; the permanent openings of the alimentary canal, the mouth and anus, are only formed later on, and from the outer skin. A mouth-pit appears in the skin in front (Figf* 35° hp), ar|d this grows towards the blind fore-end of the cavity of the head- gut ( kd), and at length breaks into it. In the same way a shallow anus-pit is formed in the skin behind, which grows deeper and deeper, advances towards the blind hinder end ofthe pelvic gut, and at last connects with it. There is at first, both before and behind, a thin partition between the external cutaneous pit and the blind end of the gut— the throat- membrane in lront and the anus-mem- brane behind ; these disappear when the connection takes place. Directly in front of the anus-opening the allantois developes from the hind gut; this is the important embryonic structure the Evoi.rrmx of the aumext.xry system 3" that forms into the placenta in the Placentals (including man). In tin'-; more advanced form the human alimentary canal (and that of all the Other mammals) is a slightly bent, cylindrical tube, with an opertmg at each end, and two appen- dages growing from its lower wall: the anterior one is the umbilical vesicle or yelk-sac, and the posterior the allantois or urinary sac (Fig. 195). The thin wall of this simple alimentary tube and its ventral appendages is found, on microscopic examination, to consist of two Strata oi cells. The inner stratum, lining the entire cavity, consists o( larger and darker cells, and is the gut-gland htyer The outer stratum consists o\ smaller and lighter cells, and is the gUt- tibre layer. The only exception is in the cavities of the mouth and anus, because these originate from the skin. The inner coat of the mouth-cavity is not provided by the gut-gland layer, but by the skin- sense layer; and its muscular substratum is provided, not by the gut-fibre, but the skin-fibre, laser. It is the same with the wall of the small anus-cavity. If it is asked how these constituent layers of the primitive gut-wall are related to the various tissues and organs that we find afterwards in the fully- developed system, the answer is ver) simple. It can be put in a single sen- tence. The epithelium of the gut — that is to say, the internal soft stratum of cells that lines the cavity of the alimentary canal and all its appendages, and is immediately occupied with the pn o( nutrition— is formed solely from the gut-gland layer ; all other tissues and organs that belong to the alimentary canal and its appendages originate from the gut-fibre layer From the latter is also developed the whole t>f the outer envelope of the gut and its appendages ; the fibrous connective tissue and the smooth muscles that compose its muscular layer, the cartilages that support it (such as the cartilages o\ the larynx and the trachea), the blood-vessels and lymph— vessels that absorb the nutritive fluid from the intestines in a word, all that there is in the alimentary system besides the epithelium of the gut. From the same layer we also gel the whole of the mesentery, with all the organs embedded in it — the heart, the large blood-vessels o\' the body, etc. Let us now leave this original structure of the mammal gut ior a moment, in order to compare it with the alimentary canal of the lower Vertebrates, and of those Invertebrates that we have recog- nised as man's ancestors. We find, first of all, in the lowest Metazoa, die Gastneads, that the gut remains perma- nently in the very simple form in.which we find it transitorily in the palingenetic gastrula ol the Other animals ; it is thus in the Gasiremaria fPemmotodtSCUt), the Physemaria (PrvpkysemaJ^ the simplest Sponges ( Olyntkus), the fresh- water Polyps (Hydra), and the ascula- embryos of many other t'celenteria (Figs. Fu. 351 —Scales or cutaneous teeth of a shark (Centrophoms cnlcens). A three-pointed tooth risca obliquely on each of the quadrangular bony plates that licjn the corium (From Gigenbaur.) 233-238). Even in the simplest forms of the Platodes, the Rhabdocula (Fig. 240), the gut is still a simple Straight tube, liwcd with the entoderm ; but with the important difference that in this i.iseits single opening, the primitive mouth ( m ), ha- formed a muscular gullet ( sd ) by invagination of the skin We have the same simple form in the gut of the lowest Vermalia (Gastrotricha, Fig. 242, Nematodes, Sagitta, etc.). Hut in these a second important opening ol the L,rut has been formed at the Oppo- site end to the mouth, the anus (Fig. 242 a). J13 '/'///•; EVOLUTION OF THE ALIMENTARY SYSTEM We see .i great advance in the struc- ture ol ihe vermalian gut in the remark- able Bafamotfossus (Fig. 245), the sole survivorof the Enteropneust class. Here we have ihe lirsi appearance o( the division of the alimentary tube into two sections that 1 liaracterises the Chordonia. The tore half, the head-gut ( cephalo- gastcr ), becomes the organ of respiration (branchial gut. Fig-. 245 k) , the hind half, the trunk-gut ftruncQgnsterJ, alone acts as digestive organ (hepatic gut, d). respiratory branchial gut. the posterior the digestive hepatic gut. In both it developes palingenetieally from the primi- tive gut of the gastrul.i, and in both the hinder end of the medullary tube covers the primitive mouth to such an extent that Ihe remarkable medullary intestinal duct is formed, the passing communication between the neural and intestinal tubes (canalis neurentertcus, Figs. 83, 85 nej. In the vicinity of the closed primitive mouth, possibly in its place, the later Epiglottis Tongue - Hypophysis l;'/S$r- V-— J ■• Pancreas Tail-gut Umbilical *W^E| cord \ Larynx Rudimentary lungs V- Stomach Tail Allantoic / duct Bladder Wolffian duct Rudimentary kidneys Fig. 352. — Gut Of a human embryo, -one-sixth of an inch long, magnified fifteen times. (From His.) The differentiation of these two parts of j the gut in the Enteropneust is just the same as in all the Tunicates and Verte- | brates. It is particularly interesting and in- structive in this connection to compare the Enteropneusts with the Ascidia and the Amphioxus (Figs. 220. 210) — the remarkable animals that form the con- necting link between the Invertebrates and the Vertebrates. In both forms the g^ut is of substantially the same-construc- tion ; the anterior section forms the anus is developed. In the same way the mouth is a fresh formation in the Amphioxus and the Ascidia. It is the same with the human mouth and that of iheCraniotes generally. The secondary formation of the mouth in the Chordonia is probably connected with the develop- ment of the gill-clefts which are formed in the guf-wall immediately behind the mouth. In this way the anterior section of the gut is converted into a respiratory organ. I have already pointed out that this modification is distinctive of the THE EVOLUTION OF THE ALIMENTARY SYSTEM :>'3 Vertebrates and Tunicates The phylo- genctic appearance of the gill-clefts indicates the coniniencement oi a new epoch in the stem-history of the Verte- brates. In the further ontogenetic development of the alimentary canal in the human embryo the appearance of the gill-clefts is the most important process. At a very early stage the gullet-wall joins with the external body-wall in the head of the human embryo, and this is followed by the formation of four clefts, which lead directly into the gullet fiom without, on the right and left sides of the neck, behind the mouth These are the gill or gullet clefts, and the partitions that separate them are the gill or gulJet- arches (Fig 171) These are most inter- esting embryonic structures. They show us that all tlie higher Vertebrates repro- duce in their earlier stages, in harmony with the biogenetic law, the process that had so important a part in the rise of the whole I'hordonia-stem. This process was the differentiation of the gut into two sections an anterior respiratory section, the branchial gut. that was restricted to breathing, and a posterior digestive section, the hepatic gut As we find this highly characteristic differentiation of the gut into two different sections in all the vertebrates and all the Tunicates, we may conclude that it was also found in their common ancestors, the Prochor- donia especially as even the Entero- pneusts have it. (Cf. pp. 119. 151, 227. and Figs. 210, 220, 245.) It is entirely wanting in all the other Invertebrates. There is at first only one pair of gill- clefts in the AmphioxUs, as in the Ascidia and Enteropneusts ; and the Copelata (Fig. 225) have only one pair throughout life. But the number present Iv increases in the former. In the Craniotes, however, it decreases still further The C'yclostomes have six to eight pairs (Fig. 247) ; some o\ the Selachii six or seven pairs, most of the fishes only four or five pairs In the embryo of man, and -the higher Vertebrates generally, where tlu\ make an appearance at an early Stage, only three or four pairs are developed. In the fishes they remain throughout life, and form an exit for the water taken in at the mouth (Figs. 249 251). Hut they are partly lost in the! amphibia, and entirely in the higher Vertebrates. In these nothing is left but a relic of the first gill-cleft. This is formed into a part of the organ of hearing ; from it arc developed the external meatus, the tympanic cavity, and tlie Eustachian tube. We have already considered these remarkable structures, and need only point here to the interesting fact that our middle and external ear is a modified inheritance from the fishes. The branchial an Ins also, which separate the clefts, develop into very different parts. In the fishes they remain ^ill-arches, supporting the respiratory gill-leaves. It is the same with the lowest amphibia, but in the higher amphibia they undergo various A F'G. 353- F,G- 354 f'g- 353— Gut of a dog-embryo (shown in Fi^. 202. from Rischoff), seen from the ventral side, a gill- arches (four pairs), b rudiments of pharynx and larynx, r lungs, d stomach./' liver, g walls of the open yellf-sac (into which the middle gut opens with a wide aperture), A rectum. Fig. 354— The Same gilt seen from the right. a lungs, b stomach, r liver, d yelk-sac, c rectum. modifications j and in the three higher classes of Vertebrates (including man) the hyoid bone and the ossicles of the can develop from them. (Cf. p. 291.) From the first gill-arch, from the inner] surface of which the muscular tongue proceeds, we get the first structure of the maxillary skeleton — the upper and lower jaws, which surround the mouth and support the teeth. These important parts are wholly wanting in the two lowest classes of Vertebrates, the Acrania and Cyclostoma. They appear first in the earliest Selachii (Figs. 248-251), and have been transmitted from this stem- group of the.tinathostomes to the higher 314 THE EVOLUTION OF THE ALIMENTARY SYSTEM Vertebrates. Hence the original forma- tion of -the skeleton of the mouth can he traced to these primitive fishes, from which we have inherited it. The teeth are developed from the skin thai clothes Fig. «s. -Median section of the head of a Petromyzon-larva. (From (irgrnbaur.) h h\ pobranchial groove (above it in the gullet We MC the internal openings ot tin- seven gill-clefts), v velum, o mouth, c heart, a auditory vesicle, « neural lube, ch chorda. the jaws. .\s tlic wliolc moutli cavity originates from the outer integument (Fig. 350), the teeth also must come from it. As a fact, this is found to be the case on microscopic examination of the development and finer structure of the teeth. The scales of the fishes, especially of the shark type (Fig. 35'), are in the same position as their teeth in this it resembles (he gu( of the earliest Yermalia Gastrotricha). It then divides into two sections, a fore or branchial gut and a hind or hepatic gut, like the alimentary canal of the BalanoglossUS, the Ascidia, and the Amphioxus. The for- mation of (he jaws and the brant hial arches changesit into a real fish-gul ( Sela- chii J. Hut tlie bran- chial gut, the one reminiscence of our fish - ancestors, is afterwards atrophied as such. "The parts of it that remain are converted into en- tirely different struc- tures. But, although the anterior section of our alimentary canal thus entirely loses its original character of branchial gut, it retains the physiological character of respiratory gut. We are now astonished to find that the permanent respiratory organ of the higher Vertebrates, the air- breathing lung, is developed from this first part of the alimentary canal. Our lungs, trachea, and larynx are formed from the ventral wall of the branchial gut. respect (Fig. 252). The osseous matter of the tooth (dentine) developes from the I The whole of the respiratory-apparatus; • its enamel covering is a secretion which occupies the greater 'part of the corium of the epidermis that covers the corium. It is the same with the cutaneous teeth or placoid scales of the Selachii. At first the whole of the mouth was armed with these cutaneous teeth in the Selachii and in the earliest amphibia. Afterwards the formation of them was restricted to the edges of the jaws. Hence our human teeth arc, in relation to their original source, modified fish- scales. For the same reason we must regard the salivary glands, which open into the mouth, .as epidermic glands, as they are formed, not from the glandular layer of the gut like the rest of the alimentary glands, but from the epidermis, from the horny plate of the outer germinal layer. Naturally, in harmony with this evolution of the mouth, the salivary glands belong genetically to one series with the sudoriferous, sebaceous, and mammary glands. Thus the human alimentary canal is as simple as the primitive gut of the gaslrula in its original structure. Later pectoral cavity in the adult man, is at first merely a small pair of vesii les or sacs, which grow out of the floor of the head- Fir.. ts6 — Transverse section of the head of a Petromyzon-larva. (From Grgenbaur.) Be- neath the pharynx (d) we see the hypobranctlial groove; above it the chorda and neural tube. A, B, C stages of constriction. gut immediately behind the gills (Figs. 354 r, 147 /). These vesicles arc found in all the Vertebrates except the two lowest classes, the Acrania and Cycloslomes. In the lower Vertebrates they do not develop THE EVOLUTION OF THE ALIMENTARY SYSTEM 1'5 into lungs, but into a large air-filled bladder, which occupies a good deal ol the body-cavitj and has a quite different purport. It serves, not for breathing, but to effect swimming movements up and down, and so is a sorl of hydrostatic apparatus the floating bladder of t lu- fishes (nec/ocystis, p. 233). However, the hum. in lungs, and those of all air-breath- ing Vertebrates, develop from the same simple vesicular appendage of the head- gut that becomes the floating bladder in the fishes. At first this bladder has no respirator) function, but merel) .uts .is hydrostatic apparatus for the purpose ol increasing or lessening the specific gravity of the body. The fishes, which have a fully-developed floating bladder, can press it together, and thus condense the air it contains. The air also escapes sometimes from the alimentary canal, through an air-duct that connects the floating bladder with the pharynx, and is ejected by the mouth. This lessens the size of the bladder; and so the fish becomes heavier and sinks. When it wishes to rise again, the bladder is expanded by relaxing the pressure. In many of the Crossopterygii the wall of the bladder is covered with bony plates, as in the Triassic Undina (Fig. 254). This hydrostatic apparatus begins in the Dipneusts to change into a respiratory organ ; the blood-vessels in the wall of the bladder now no longer merely secrete air themselves, but also take in fresh air through the air-duct. This process reaches its full development in the Am- phibia. In these the floating bladder has turned into lungs, and the air-passage into a trachea. The lungs of the Am- phibia have been transmitted to the three higher > l.i^^ of Vertebrates. In the lowest Amphibia the lungs on either side are still very simple transparent sacs with thin walls, as in the common water- salamander, the Triton. It still entirely resembles the floating bladder of the fishes. It is true that the Amphibia have two lungs, right and left. Hut the float- ing bladder is also double In man} o\ the fishes (such as the early Ganoids), and divides into right and left babes. On the other hand, the lung is single in 1 1 ratodus ( big. 257). In the human embryo and that of all the other Amniotes the lungs develop from the hind part oi the ventral wall Ol the head-gut (Fig, [49). Immediately behind the single structure of the thyroid gland a median groove, the rudiment ot the trachea, is detached bom the gullet. From its hinder end a couple of vesicles develop the simple tubular- rudiments o( the 1 ignt and kit lungs. They- afterwards increase considerably in size, fill the greater pari oi the thoracic cavity, and take the heart between them. Even in the frogs we find that the simple sax has developed into a spongy bod) oi peculiar froth-like tissue. The originally short connection of the pulmonary sacs with the head-gut extends into a long, thin tube. This is the wind-pipe (trachea); it opens into the gullet above, and d\\ ides below into two branches which go to the two lungs. In the wall of the trachea Fig. 357.— Thoracic and abdominal viscera of a human embryo of twelve weel^s, natural size, < From Kdlliker.) The head is omitted. Neutral and pec- toral walls are removed. The greater part of the body-cavity is taken up with the liver, from the middle part of which the ccecum and the vermiform appendix protrude. Above the diaphragm, in the middle, is the conical heart ; to the right and left of it are the two small lungs. circular cartilages develop, And these keep it open. At its upper end, under- neath its pharyngeal opening, the larynx is formed — the organ of voice and speech. The larynx is found at various stages of development in the Amphibia, and com- parative anatomists are in a position to trace the progressive growth of this im- portant organ from the rudimentary structure of the lower Amphibia up to the elaborate and delicate vocal apparatus that we have in the larynx oi man and of the birds. We must refer here to an interesting rudimentary organ of the respiratory gut, the thyroid gland, the huge-gland in front of the larynx, that lies below the " Adam's 316 THE EVOLUTION OF THE M.1UEXTARY SYSTEM apple," and is often especially developed in the male sex. It has a certain function — not yet fully understood in the nutri- tion of the body, and arises in the embryo by constriction from the lower wall of the pharynx. In many mining districts the thyroid gland is peculiarly liable to morbid enlargement, and then forms goitre, a growth that hangs at the front of the neck, lint it is much more interest- ing phylogenetically. AsWiihelm Miiller, of Jena, has shown, this rudimentary organ is the List relic o\ the hypobranchial groove, which we considered in a previous chapter, and which runs in the middle line o\ the gill-crate in the Ascidia and Amphioxus, and conveys food to the stomach. (Cf. p. 184, Fig. 246). We still find it in its original character in the larvae of the Cyclostomes (Figs. 355 and 356) The second section of the alimentary canal, the trunk or hepatic gut, undergoes not less important modifications among our vertebrate ancestors than the first section. In tracing the further develop- ment of this digestive part of the gut, we find that most complex and elaborate organs originate from a very rudimentary original structure. For clearness we may divide the digestive gut into three sections: the fore gut (with oesophagus and stomach), the middle gut (duodenum, with liver, pancreas, jejunum, and ileum), and the hind gut (colon and rectum). Here again we find vesicular growths Or appendages of the originally simple gut developing into a variety of organs. Two of these embryonic structures, the yelk-sac and allantois, are already known to us. The two large glands that open into the duodenum, the liver and pancreas, are growths from the middle and most im- portant part of the trunk-gut. Immediately behind the vesicular rudi- ments of the lungs comes the section of the alimentary canal that forms the stomach (Fig*. 353 d, 354 b). This sac- shaped organ, which is chiefly responsible for the solution and digestion of the food, has not in the lower Vertebrates the great physiological importance and the complex character that it has in the higher. In the Acrania and Cyclostomes and the earlier fishes we can scarcely dis- tinguish a real stomach ; it is represented merely by the short piece from the bran- chial to the hepatic gut. In some of the other fishes also the stomach is only a very simple spindle-shaped enlargement at the beginning of the digestive section of the gut, running straight from front to back in the median plane of the body, underneath the vertebral column. In the mammals its first structure is just as rudimentary as it is permanently in the preceding. But its various parts soon begin to develop. As the left side of the spindle-shaped sac grows much more quickly than the right, and as it turns considerably o\\ its axis at the same time, it soon comes to lie obliquely. The upper end is more to the left, and the lower end more to the right. The foremost end draws up into the longer and narrower canal of the oesophagus. Underneath this on the left the blind sac (fundus) of the stomach bulges out, and thus the later form gradually developes (Figs. 349, 184 e). The original longitudinal axis becomes oblique, sinking below to the left and rising to the right, and approaches nearer and nearer to a transverse position. In the outer layer of the stomach-wall the powerful muscles that accomplish the digestive movements develop from the gut-fibre layer. In the inner layer a number of small glandular tubes are formed from the gut-gland layer ; these are the peptic glands that secrete the gastric juice. At the lower end of the gastric sac is developed the valve that separates it from the duodenum (the pylorus, Fig. 349 d). Underneath the stomach there now developes the disproportionately Jong stretch of the small -intestine. The development of this section is very simple, and consists essentially in an extremely rapid and considerable growth length- ways. It is at first very short, quite straight, and simple. But immediately behind the stomach we find at an early stage a horseshoe-shaped bend and loop of the gut, in connection with the sever- ance of the alimentary canal frpm the yelk-sac and the development of the first mesentery. The thin delicate membrane that fastens this loop to the ventral side of the vertebral column, and fills the inner bend of the horseshoe formation, is the first rudiment of the mesentery (Fig. 147^). We find at an early stage a con- siderable growth of the small intestine ; it is thus forced to coil itself in a number of loops. The various sections that we have to distinguish in it are differentiated in a very simple way — the duodenum (next to the stomach), the succeeding long jejunum, and the last section of the small intestine, the ileum. THE EVOLUTION OF THE ALIMENTARY SYSTEM -From the duodenum are developed the two large glands that we have already mentioned the liver and pancreas. The liver appears first in the shape of two small sacs, that are found to the right and left immediately behind the stomach ( Figs. 353 /< 354')- *n manyofthe lower Verte- brates they remain separate for a long time (in thie Myxinoides throughout life), or are only imperfectly joined. In the higher Vertebrates they soon blend more 01 kss completely to form a single large organ. The growth of the liver is very brisk at first. In the human embryo it giows so much in the second month of development that in the third it occupies by far the greater part of the bodv-cavity (^'K- 35?) At first the two babes develop equallj ; afterwards the left falls far behind the right. In consequence of the unsymmeti ical development and turn- ing of the stomach and other abdominal viscera, .the whole liver is now pus'hed to the right side. Although the liver does not afterwards grow so disproportionately, it is comparatively larger in the embryo at the end of pregnane) than in the adult. Its weight relatively to that of the whole body is 1 : 36 in the adult, and 1 : 18 in the embryo. Hence it is very important physiologically during embryonic life ; it is chiefly concerned in the formation of blood, not so much in the secretion of bile. Immediately behind the liver a second large visceral gland developes from the duodenum, the pancreas or sweetbread. It is wanting in most of the lowest classes of Vertebrates, and is first found in the fishes. This organ is also an outgrowth from the gut., The last section of the alimentary canal, the large intestine, is at first in the embryo a very simple, short, and straight tube, which opens behind bv the anus. It remains thus throughout life in the lower Vertebrates. But it grows considerably in the mammals, coils into various folds, and divides into two sections, the first and longer of which is the colon, and the second the rectum. At the beginning of the colon there is a valve (valvula Hau- hini ) that separates it from the small intestine. Immediately behind this there is a sac-like growth, which enlarges into the ccecum (Fig. 357 7'). In the plant- eating mammals this is very large, but it is very 'small or completely atrophied in the flesh-eaters. In man, and most of the apes, only the first portion of the COKUm is wide ; the blind end-part of it is very narrow, and seems later to be merely a useiesvappendage of the former. This " vermiform appendage " is very interesting as a rudimentary organ. The only significance of it in man is that not infrequently a cherry-stone or some other hard and indigestible matter penetrates into its narrow cavity, and by setting up inflammation and suppuration causes the death of otherwise sound men. Teleo- logy has great difficulty in giving a rational explanation of, and attributing to a beneficent Providence, this dreaded appendicitis. In our plant-eating ances- tors this rudimentary organ was much larger and had a useful function. Finally, we have important appendages of the alimentary tube in the bladder and urethra, which belong to the alimentary system. These urinary Organs, acting as reservoir and duct for the urine excreted by the kidneys, originate from the inner- most part of the allantoic pedicle. In the Dipneusts and Amphibia, in which the allantoic sac first makes its appearance, it remains within the body-cavity, and functions entirely as bladder. But in all the Amniotes it grows far outside of the body-cavity of the embryo, and forms the large embryonic "primitive bladder," from which the placenta developes'in the higher mammals. This is lost at birth. But the long stalk or pedicle of the allan- tois remains, and forms with its upper part the middle vesico-umbilical ligament, a rudimentary organ that goes in the shape of a solid string from the vertex of the bladder to the navel. The lowest part of the allantoic pedicle (or the " urachus ") remains hollow, and forms the bladder. At first this opens into the last section of the gut in man as in the lower Verte- brates ; thus there is a real cloaca, which takes off both urine and excrements. But among the mammals this cloaca is only permanent in the Monotremes, as it is in all the birds, reptiles, and amphibia. 1 n all the other mammals (marsupials and placentals) a transverse partition is after- wards formed, and this separates the uro- genital aperture in front from the anus- opening behind. (Cf. p. 249 and Chapter XXIX.) 3i8 EVOLUTIOX OF THE VASCULAR SYSTEM Chapter XXVIII. EVOLUTION OF THE VASCULAR SYSTEM The use that we have hitherto made ol' our biogenetic law will give the reader an idea how far we may trust its guidance in phylogenetic investigation. This differs considerably in the various systems of organs ; the reason is that heredity and variability have a very different range in these systems. While some of them faith- fully preserve the original palingenetic development inherited from earlier animal ancestors, others show little trace of this rigid heredity ; they are rather disposed to follow new and divergent cenogerietic lines of development in consequence of adaptation. The organs of the first kind represent the conservative element in the multicellular state of the human frame, while the latter represent the progressive element. The course of historic develop- ment is a result of the correlation of the two tendencies, and they must be care- fully distinguished. There is perhaps no other system of organs in the human body in which this is more necessary than in that of which we are now going to consider the obscure development — the vascular system, or apparatus of circulation. If we were to draw our conclusions as to the original features in our earlier animal ancestors solely from the phenomena of the develop- ment of this system in the embryo of man and the other higher Vertebrates, we should be wholly misled. By a number of important embryonic adaptations, the chief of which is the formation of an extensive food-yelk, the original course of the development of the vascular system has been so much falsified and curtailed in the higher Vertebrates that little or nothing now remains in their embryology of some of the principal phylogenetic features. We should be quite' unable to explain these if comparative anatomy and ontogeny did not come to our assis- tance. r The vascular system in man and all the Craniotes is an elaborate apparatus of cavities tilled with juices Or cell-containing fluids. These " vessels " (vascu/aj play an important part in the nutrition of the body. They partly conduct the nutritive red blood to the various parts of the body (blood-vessels) ; partly absorb from the gut the white chvle formed in digestion (chyle-vessels) ; and partly collect the used-up juices and convey them away from the tissues (lymphatic vessels). With the latter are connected the large cavities of the body, especially the body- cavity, orcoeloma. The lymphatic vessels conduct both the colourless lymph and the white chyle into the venous part of the circulation. The lymphatic glands act as producers of new blood-cells, and with them is associated the spleen. The centre of movement for the circulation of the fluids is the heart, a strong muscular sac, which contracts regularly and is equipped with valves like a pump. This constant and steady circulation of the blood -makes possible the complex meta- bolism of the higher animals. But, however important the vascular system may be to the more advanced and larger and highly-differentiated animals, it is not at all so indispensable an element of animal life as is commonly supposed. The older science of medicine regarded the blood as the real source of life. Even in the still prevalent confused notions ©f heredity the blood plays the chief part.' People speak generally of full blood, half blood, etc., and imagine that the here- • ditary transmission of certain characters " lies in the blood." The incorrectness of these ideas is clearly seen from the fact that in the act of generation the blood of the parents is not directly transmitted to tin offspring, nor does the embryo possess blood in its early stages. We have already seen that not only the differentia- tion of the four secondary germinal layers, but also the first structures of the princi- pal organs in the embryo of all the Verte- brates, take place long before there 15 any EVOLUTION OF THE VASCULAR SYSTEM 3i9 trace of the vascular system— the heart and the blood. In accordance with this ontogenetic fact, we must regard the vascular system as one of the latesl organs from the phylogenetic poinl of view, just as we baye round the alimentary canal to ho one of the earliest. In an) case, the vascular system is much later than the alimentary. The important nutritive fluid that circulates as blood and lymph in . the elaborate canals of our vascular system is not a clear, simple fluid, hut a \ery complex chemical juice with millions of The red colour o\ the hlood is caused by the great accumulation o( the former, tin* others circulate among them in much smaller quantity. When the colourless cells increase at the expense of the red we get anaemia (or < blorosis). The lymph-cells f feUCOCytes), commonly called the "white corpuscles" o\ the hlood, are phylogcnetically older and more widely distributed in the animal world than the red. The great majority of the Invertebrates that have acquired an independent vascular system have only colourless lymph-cells in the circulating Fie. 358. F'fi. 359 . Fig. 358. — Red blood-cells of various Vertebrates (equally magnified). /. of man, !. camel, 3. dove, 4. proteus, •;. water-salamander ( Triton), 6. frog, 'y. merlin ( Cobilis), 8. lamprey ( Pctromyzon). a. surface- view, b edge-view. (From Wagner.) Fig. 3$q.^VaSCUlar tissues or endothelium (vasnlium). A capillary from the mesentery, a vascular cells, b their nuclei. cells Boating in it. These blood-cells are just as important in the complicated life of the higher animal body as the circula- tion of money is to the commerce of a civilised community. Just as the citizens meet their needs most conveniently by means of a financial circulation, so the various tissue - cells, the microscopic citizens of the multicellular human body, have their food conveyed to them best In the circulating tells in the blood. These blood cells ( keptnocytes ) are oi' two kinds in man and all the other Craniotes — red c< Us (rhodocytes or erythrocytes) and Colourless or lymph cells (leucocytes.). fluid. There is an exception in' the Nemertines (Fig. 35X) and some groups of Annelids. When we examine the colourless blood of a cray-fish or a snail (Fig. 358) under a high power of the microscope, we find in each drop numbers of mobile leucocytes, which behave just like independent Amoebae (Fig. 17). Like these unicellular Protozoa, the colourless blood-cells creep slowly about, their un- shapely plasma-body constantly changing its form, and stretching out finger-like processes first in otiv direction, then another. Like the Amoebae, they take particles into their cell-body. On account EVOLUTION OF THE VASCULAR SYSTEM of this feature these amoeboid pdasticU are j called "eating colls" ( 'phagocytes ) \ and ! ow account of their motions " travelling | colls" (piano, vtcs ). It Has boon shown bj tlio discoveries of the last few decades that these leucocytes are of the greatest physiological and pathological conse- quence to the organism. They can absorb either solid or dissolved particles from the wall of the gut, and convey them to the blood in the chyle ; they can absorb and remove unusable matter from the tissues. When they pass in large quanti- ties through the tine pores of the capil- laries and accumulate at irritated spots, they cause inflammation. They can con- sume and destroy bacteria, the dreaded vehicles of infectious diseases ; hut they can also transport these injurious Monera to fresh" regions, and so extend the sphere g-fobfnjis regularly distributed in the pores of their protoplasm. The red cells of most of the Vertebrates are elliptical Rat disks, and enclose a nucleus of the same shape ; they differ a good deal in size (Fig. 358). The mammals are distin- guished from the other Vertebrates by the circular form of their biconcave red cells and by the absence of a nucleus (Fig. 1) ; only a few genera still have the elliptic form inherited from the reptiles (Fig. 2). In the embryos of the mammals the red cells have a nucleus and the power of increasing by cleavage (Fig. 10). The origin of the hlood-cells and Vessels in the embryo, and their relation to the germinal layers and tissues, are among the most difficult problems of ontogeny — those ohscure questions on which the most divergent opinions are still advanced Mc. Fig. .560. -Transverse ssction of the trunk ot a chick-embryo, forty-five nours old. (From Balfour.) A ectoderm (horrfy-plate), Mr medullary tube, ch chorda, C entoderm (gut-gland layer), Pv primitive segment (episomite). 11',/ prorenal duct. />/> cceloma (secondary body-cavity), So skin-fibre layer, S/> gut-fibre layer, '.■ blood-vessels in latter, ao primitive aortas, containing rod blood-cells. of infection. It is probable that the sensitive and travelling leucocytes of our invertebrate ancestors have powerfully co-operated for millions of years in the phylogenesis of the advancing animal organisation. The red hlood-cells have a much more re- stricted sphere of distribution and activity. But they also are very important in con- nection with certain functions of the cranfole - organism, especially the ex- change of gases or respiration. The cells of the dark red, carbonised or venous,. blood, which have absorbed carbonic acid from the animal tissues, give this off in the respiratory organs ; they receive instead of it fresh oxygen, and thus bring about the bright red colour that distin- guishes oxydised or arterial blood. The red colouring matter of the blood ChtBHlO- by the most competent scientists. In general, it is certain that the greater part of the cells that compose the vessels and their contents come from the mesoderm — in fact, from the gut-fibre layer ; it was on this account that Baer gave the name of " vascular layer " to this visceral layer of the cceloma. But other important observers say that a part of these cells come from other germinal layers, espe- cially from the gut-gland layer. It seems to be true that blood-cells may be formed from the cells of the entoderm before the development of the mesoderm. If we examine sections of chickens, the earliest and most familiar subjects of embryology, we find at an early stage the " primitive aortas'1 we have already described (Fig. 360^0) in the ventral angle between the episoma ( Pv ) and liyposoma ( SpJ. The EVOLUTION OF '/'///■: VASCULAR SYSTEM 3*i thin wall of these first vessels of the amniote embryo consists of il.it cells (endothelia or vascular epithelia)-; the fluid within already contains numbers of rod blood-cells : both have been developed from the gut-fibre layer. It is the same with the vessels of the germinative area (Fig. 361 v), which lie on the entoderm ic membrane of the yelk-sac ( c ). These features are seen --till more clearly in the transverse section of the duck-embryo in Fig. 152 (p. 141). In this we see clearl) how a number of stellate cells proceed from the "vascular layer" and spread in all directions in the "primary body-cavity"1 — i.e., in the spaces between the germinal layers. A part of these travelling cells come together and line the wall of the larger spaces, and thus form the Hist vessels; others enter into the cavity, live in the fluid that fills it, and multiply by cleav- age the first blood-cells. But, besides these niesodermic cells of the "vascular layer" proper, other travelling cells, of which the origin and purport are still obscure, take pari in the formation of blood in the mero- blastic Vertebrates (especially fishes). The chief of these are those that Ruckert has most aptly denominated "merocytes." These "eating yelk-cells " are found in large numbers in the food-yelk of the Selachii, especially in the yelk- wall — the border zone of the germinal disk in which the em- bryonic vascular net is first developed. The nuclei of the merocytes become ten times as large as the ordinary cell-nucleus, and are distinguished by their strong capacity for taking colour, or their special richness in chromatin. Their protoplasmic body resembles the stellate cells of osseous tissue (astro- cytes), and behaves just like a rhizo- pod (such as Gromia) ; it sends out numbers of stellate processes all round, which ramify and stretch into the sur- rounding food-yelk. These variable and wry mobile processes, the pseudopodia of the merocytes, serve both for locomotion and for getting food ; as in the real rhizopods, they surround the solid particles of food (granules and plates of yelk), and accumulate round their nucleus the food they have received and digested. Hence we may regard them both as eating-cells (phagocytes) and travelling-cells f pluna- cytes ). Their lively nucleus divides quickly and often repeatedly, so that a number of new nuclei are formed in a short time ; as each fresh nu< leus sur- rounds itself with a mantle of protoplasm, it provides a new cell for the construction of the embryo. Their origin is still much disputed. Half of the twelve stems of the animal world have no blood-vessels. They make their firsl appearance in the Vermalia. Their earliest source is the primary bod\- Cavity, the simple space between the two primary germinal layers, which is either a relic of the segmentation-cavity, or is a subsequent formation. Amoeboid plano- Flu .161.— Merocytes of a sh^rk-embryo, rhizopod-like yelk-cells underneath tin- embryonic cavity//?/ (From Ruckrrt.) z two embryonic cells, k nuclei of the merocytes. winch wander about in the yelk and eat small y elk-plates ( d ), £ smaller, more superficial, lighter nuclei, k' a deeper nucleus, in the act of cleavage, b* chromatin-fillcd border-nucleus, freed from the surrounding yelk in order to show the numerous pseudopodia of the protoplasmic cell-body. cytes, which migrate from the entoderm and reach this fluid-filled primary cavity, live and multiply there, and form the first colourless blood-cells. We find the vascular system in this very simple form to-day in the Bryozoa, Rotatoria, N'ema- toda, and other lower Vermalia. The first step in the improvement of this primitive vascular system is the formation of larger canals or blood- conducting tubes. The spaces filled with blood, the relics of the primary body- cavity, receive a special wall. " Blood- vessels " of this kind (in the narrower sense) are found among the higher worms in various forms, sometimes very simple, at other times very complex. The form 3" EVOU'T/O.X OF THE VASCULAR SYSTEM that was probably the incipient structure of the elaborate vascular system of the Vertebrates (and of the Articulates) is found in two primordial principal vessels — a dorsal vessel in the middle line o\' the dorsal wall Of the gut, and a ventral vessel tli.u runs from front to rear in the middle line of its ventral wall. From the dorsal vessel is evolved the aorta (or principal artery), from the ventral vessel the principal or subintestinal vein. The fpfl- Vo two vessels are connected vsJl^A^/ in front and behind by a loop that runs round nW^ \ the gut. The blood con- tained in the two tubes is propelled by their peri- ls L-^p/ staltic contractions. I I _ The earliest VermaHa CT^SiZp in which we first find this il> — &c independent vascular system are the Nemertina (Fig. 244). As a rule, they have three parallel longitudinal vessels con- nected by loops, a single dorsal vessel above the gut and a pair of lateral vessels to the right and left. In some of the Nemertina the blood is already coloured, and the red colouring matter is real haemoglobin, con- nected with elliptical dis- coid cells, as in the Verte- brates. The further evo- lution of this rudimentary vascular system can be gathered from the class of the Annelids in which we find it at various stages of develop- ment. First, a number oi transverse connections are formed between the dorsal and ventral_ vessels, which pass round the gut ring-wise (Fig. 362). Other vessels grow into the body-wall and ramify in order to convey blood to it. In addition to the two large vessels of the middle plane there are often two lateral vessels, one to the right and one to the left ; as, for instance, in the leech. There are four of these parallel longitudinal vessels in the Enteropncusts (Balano- flossus, Fig. 245). In these important Uermalia the foremost section of the gut Fig. 362- Vas- cular system of an Annelid (See- nufis ). foremost section, d dorsal vessel, v ventral vessel, c transverse connection of two (enlarged in shape of heart). The arrows indicate the direction of the flow of blood. (From Gegcn- has already been converted into a gill- I crate, and the vascular arches that rise in the wall of this from the ventral to the dorsal vessel have become branchial vessels. We have a further important advance in the Tunicatcs, which we have recog- nised as the nearest blood-relatives of our early vertebrate ancestors. Here we find for the first time a real heart — i.e., a central organ of circulation, driving the blood mto the vessels by the regular, contractions of its muscular wall. Jt is) of a very rudimentary character, a spindle- shaped tube, passing at both ends into a principal vessel (Fig. 221). By its original position behind the gill-crate, on the Ventral side of the Tunicates (sometimes more, sometimes less, forward), the heart shows clearly that it has been formed by^ the local enlargement of a scctionpf the ventral vessel. We have already noticed the remarkable alternation of the direc- tion of the blood stream, the heart driving it first from one end, then from the other (p. 190). This is very instructive, because in most of the worms (even the Entero- pneust) the blood in the dorsal vessel travels from back, to front, but in the Vertebrates in the opposite direction. As the Ascidia-heart alternates steadily from one direction to the other, it shows us permanently, in a sense, the phylogenetic transition from the earlier forward direc- tion of the dorsal current (in the worms) to the new backward direction (in the Vertebrates). Fig. .163 —Head of a fish-embryo, with rudimen- tary vascular system, from the loft, ac Cuvier s duct (juncture of the anterior and posterior principal veins). •>- venous sinus (enlarged end ot Cuvier s duct), a auricle, V ventricle, abr trunk of branchial artery, s tjili-iletts (arterial arches between!. «%. s subclavian arteries, / pulmonary artery, p' branches of same, the capillary system of the large 01 body- circulation. It is only in the two highest classes of Vertebrates— the birds and mammals -that we find a complete divi- sion of the circulations. Moreover, this complete separation has been developed quite independently in the two classes, as the dissimilar formation of the aortas shows of itself. In the birds the right half of the fourth arterial arch has become the permanent arch (Fig. 3^5). In the mammals this has been developed from the left half of the same fourth arch (Fitf. 366). If we compare the fully-developed arterial system of the various classes of Craniotes, it shows a good deal of variety, yet it always proceeds from the same fundamental type. Its development is Fig. Fig. 373.— Heart and head of a dog-embryo, I from the front, a fore brain, /• eyes, c middle I rain . d primitive lower jaw. e primitive upper jaw. j gill- arches, g right auricle, h left auricle, / left ventricle, k right ventricle. (From Bischoff.) Fig. 374. — Heart of the same embryo, from 1 behind, a inosculation of the vitelline veins, 6 left ! auricle, c right auricle, d auricle, e auricular canal./" left ventricle, g right ventricle, h arterial bulb. (From Bischoff.) :ust the same in man as in the other mammals ; in particular, the modification of the six pairs. of arterial arches is the same in both (Figs, 367-370). At first there is only a single pair of arches, which 326 EVOLCTIOX OF THE VASCULAR SYSTEM tie on the inner surface of the first pair o\ gill-arches. Behind this there then develop a second ami third pair of arches (lying on the inner side of the second .md third gill-arches, Fig. 367). Finally, wo F'G- 375- Fig. 377. P'g- 375-— Heart of a human embryo, four weeks old; / front view, i. back view, j. opened, and upper half of the atrium removed, a' left auricle, a" right auricle, v left ventricle, v" right ventricle, ao arterial bulb, c superior vena cava (cd right, cs left), s rudiment of the interventricular wall. (From Kolliker.) Fig. 376.— Heart of a human embryo, six weeks old, front view, r right ventricle, _ t left ventricle, s furrow between ventricles, ta arterial bulb, af furrow on its surface; to right and left are the two large auricles. (From Ecker.) Fig. 377— Heart of a human embryo, eight weeks old, back view, a' left auricle, a" right auricle, v' left ventricle, v" right ventricle, erf" right superior vena cava, ci inferior vena cava. (From Kelliker.) get a fourth, fifth, and sixth pair. "Of the six primitive arterial arches of the Amniotes three soon pass away (the first, second, and fifth) ; of the remaining three, the third gives the carotids, the fourth the aortas, and the sixth (number 5 in Figs. 364 and 368) the pulmonary arteries. The human heart also developes in just the same way as that of the other mam- mals (Fig. 378). We have already seen the first rudiments of its embryology, which in the main corresponds to its phylogeny(Figs. 201, 202). We saw that the palingenetic form of the heart is a spindle-shaped thickening of the gut-fibre layer in the ventral wall of the head-gut. The structure is then hollowed out, forms a simple tube, detaches from its place of origin, and henceforth lies freely in the cardiac cavity. Presently the tube bends into the shape of an S, and turns spirally on1 an imaginary axis in such a way that the hind pari comes td lie on the dorsal surface of the lore part The united vitelline veins open into the posterior end. From the anterior end spring the aortic arches. This first structure of the human heart, enclosing a very simple cvitv, corre- sponds to the tunicate-heart, and is a reproduction of that o( the Prochordoftia, but it now divides into two, and subse- quently into three, compartments ; this reminds us for a time of the heart of the Cyclostomes and fishes. The spiral turn- ing and bending of the heart increases, and at the same time two transverse con- strictions appear, dividing it externally into three sections (Figs. 371, 372). The foremost section, which is turned towards the ventral side, and from which the aortic arches rise, reproduces the arterial bulb of the Selachii. The middle section is a simple ventricle, and the hindmost, the section turned towards the dorsal side, into which the vitelline veins inosculate, is a simple auricle (or atrium). The latter forms, like the simple atrium of the fish- heart, a Dair of lateral dilatations, the Fio. 378.— Heart of the adult man. fully developed, front view, natural position, a right auricle (under- neath it the ri^'lit ventricle), 6 left auricle (under it the left ventricle), C superior vena cava, (-'pulmonary veins, P pulmonary artery, d Botalli's duct, A aorta. (From Meyer. ) auricles (Fig. 371 b) ; and the constriction between the atrium and ventricle is called the auricular canal (Fig. 372 ca). The heart of the human embryo is now a com- plete fish-heart. EVOLUTION OF THE VASCULAR SYSTEM 3*7 In perfect harmony with its phytogeny, I the embryonic development of the human heart shows a gradual transition from the fish-heart, through the amphibian and reptile, to the mammal form. The most important point in the transition is the formation df a longitudinal partition — incomplete at first, hut ifterwards com- plete -which separates all three divisions of the heart into right (venous) and left (arterial) halves (cf. hi^,r-- 373 178). The atrium is separated into a right and left half, each of which absorbs the corre- sponding auricle ; into the right auricle ! open the body-veins (upper and lower vena cava, Figs. 375 < , 377 e) ; the left 1 auricle receives the pulmonary veins, in j the same way a superficial interventricular | The heai t of all the Vertebi ates belongs originally to the hyposoma of the head, and tve according!) and it in the embryo of man and all the Other Anmioles right in front on the under-side of the head ; just as m the fishes it remains perma- nently in firont of the gullet. It afterwards descends into the trunk, with the advance in the development of the neck and breast, and at last reaches the breast, between the two lungs. At first it lies symmetrically in the middle ptahe of the body, so that its long axis corresponds with that of the body. In most of the mammals it remains permanently in this position. But in the apes the axis begins to be oblique, and the apex of the heart to move towards the left side. The dis- dfr£nt fo.— Transverse section of the back of the head of a chick-embryo, forty hours old. tn medulla oblongata, ph pharyngeal cavity (head-gut). /; horny plate, h' thicker part of it. from which (From Fig. Krfliker.) the MMCultory pits afterwards develop, hp skin-fibro plate, hh cervical cavity (head-ccelom or cardioccel). hzp cardiac plate (the outermost meaodermic wall of the heart), connected by the ventral mesocardium (nhg) with the gut-hbre layer or visceral ccelom-laycr ( d/p' ). Ent entoderm, ihh inner (entodermic ?) wall of the heart ; the two endothelial cardiac tubes are still separated by the cenogenetic septum (s) of the Amniotes. tr vessels. furrow is soon seen in the ventricle (Fig. 376 s). This is the external sign of the internal partition by which the ventricle is divided into two — a right venous and left arterial ventricle. Finally a longi- tudinal partition is formed in the third section of the primitive fish-like hear!, the arterial bulb, externally indicated by a longitudinal furrow (Fig. 376 af). The Cavity of the bulb is divided into two lateral halves, the pulmonary-artery bulb, that opens into the right ventricle, and tin- aorta-bulb, that opens into the left ventricle. When all the partitions are complete, the small (pulmonary) circula- tion is distinguished from the large (body) circulation; the motive centre of the former is the ri^ht half, and that of the latter the left hall, of the heart. placement is greatest in the anthropoid apes — chimpanzee, gorilla, and orang — which resemble man in this. As the heart of all Vertebrates is origi- nally, in the light of phylogeny, only a local enlargement of the middle principal vein, it is in perfect accord with the biogenetic law that its first structure in the embryo is a simple spindle-shaped tube in the ventral wall of the head-^rut. A thin membrane, standing vertically in the middle plane, the mesocardium, connects the ventral wall of the head-gut with the lower head-wall. As the cardiac tulie extends and detaches from the gut- wall, it divides the mesocardium into an upper (dorsal) and lower (ventral) plate ( usual lj called the mesocardium anterhts and pdterius in man, Fig. 379 uhg). The .V* RVOLITTIOX OF THE VASCULAR SYSTEM mesocardium divides two lateral cavities, Remak's "neck-cavities" (Fig. 379 hh). These cavities afterwards join and form the simple pericardial cavity, and are i hoi (.'ton- called l\\ Kollikec the "primi- ti\ e pei icardial cavities." The double cervical cavity of the Amniotes i-. very interesting, both from the anatomical and Ihe evolutionary point of view ; it corresponds to a part o( the hyposomites of the head of the lower Vertebrates— that part of the ventral cuelom-pouches which comes next to Van Wijhe's " visceral cavities" below. Each of the cavities still communicates freely behind with the two ccelom-pouches of Fie. 380.— Frontal section of a human embryo, one-twelfth of an inch long in the neck, magnified forty times ; " invented" by U'ilkclm His. Seen from ventral side, mb mouth-fissure, surrounded by the branchial processes, a^bulbus of aorta, hm middle part of ventricle, hi left lateral part of same, ho auricle, d diaphragm, vc superior vena cava, iu umbilira! vein, vo vitelline space, lb liver. le~ hepatic duct. the trunk ; and, just as these afterward - coalesce into a simple body-cavity (the ventral mesentery disappearing), we find the same thing happening in the head. This simple primary pericardial cavity- has been well called by Gegenbaur the " head-ccelom 1," and by Hertwig the " pericardial breast-cavity." As it now encloses the heart, it may also be called cardioccel. The cardioccel, or head-ccelom, is often disproportionately large in the Amniotes, the simple cardiac tube growing consider- ably and lying in several folds. This causes the ventral wall of the amniote embryo, between the head and the navel, to be pushed outwards as in rupture (cf. Fig. 180 //). A transverse fold of the ventral wall, which receives all the vein- trunks that open into the heart, grows up from below between the pericardium and the stomach, and forms a transverse par- tition, which is the first structure of the primary diaphragm (Fig. 380 d). This important muscular partition, which com- pletely separates the thoracic and abdomi- nal cavities in the mammals alone, is still very imperfect here ; the two cavities still communicate for a time by two narrow canals. These canals, which belong to the dorsal part of the head-ccelom, and which we may call briefly pleural ducts, receive the two pulmonary sacs, which develop from the hind end of the ventral wall of the head-gut ; they thus become the two pleural cavities. The diaphragm makes its first appear- ance in the class of the Amphibia (in the salamanders) as an insignificant muscular transverse fold of the ventral wall, which rises from the fore end of the transverse abdominal muscle, and grows between the pericardium and the liver. In the reptiles (tortoises and crocodiles) a later dorsal part is joined to this earlier ventral part of the rudimentary diaphragm, a pair of subvertebral muscles rising from the vertebral column and being added as "columns" to the transverse partition. But it was probably in the Permian sauro- mammals that the two originally separate parts were united, and the diaphragm necame a complete partition between the thoracic and abdominal cavities in the mammals , as it considerably enlarges the chest-cavity when it contracts, it becomes an important respiratory muscle. The ontogeny of the diaphragm in man and the other mammals reproduces this phylogenetic process to-day, in accordance with the biogenetic law ; in all the mammals the diaphragm is formed by the secondary conjunction of the two origi- nally separate structures, the earlier ventral part and the later dorsal part. Sometimes the blending of the two diaphragmatic structures, and conse- quently the severance of the one pleural duct from the abdominal cavity, is not completed in man. This leads to a diaphragmatic rupture (hernia diaphrag- matica). The two cavities then remain in communication by an open pleural duct, and loops of the intestine may penetrate by this " rupture opening " into the client-cavity. This is one of those EVOLl'TIOX OF THE VASCULAR SYSTEM m fatal mis-growths that show the great part that blind chance has in organic development. Thus the thoracic ca\ itj of the mammals, with its important contents, the heart and I"k. (Si- Transverse section of the head of a chick-embryo. thirt\-si\ hours old Underneath tin' medullary tube the two primitive aortas ( />«« J CWI Ik- seen in the brad plattf ( *) at each side of the chorda. Underneath the gullet < d ) «c see the aorta-end of the heart ( ' ae >,_ hh cervical cavitv or head cosfom, hk top of heart, ks head-sheath, amniotic fold, h horn) plate. (From Ki-mak.) lungs, belongs originally to the head-part o( the vertebrate body, and its inclusion in the trunk is secondary. This instructive and very interesting fact is entirely proved bv the concordant evidence of comparative anatomy and ontogeny. The lungs are outgrowths of the head-gut ; the heart developes from its inner wall. The pleural sacs that enclose the lungs are dorsal parts o\ the head-coelom, originat- ing from the pleuroducts ; the pericar- dium in which the heart afterwards lies is also double originally, being formed from ventral halves of the head-ccejom, which only combine at a later stage. When the lung of the air- breathing Vertebrates issues from the head-cavity and enters the trunk-cavity, it follows the example of the floating bladder of the fishes, which also origi- nates from the pharyngeal wall in the shape of a small pouch-like out-growth, but soon grows so large that, in order to find room, it has to pass far hehind into the trunk- cavity. To put it more pre- cisely, the lung of the quad- rupeds retains this hereditary growth-process of the fishes ; lor the hydrostatic floating bladder of the latter is the air-filled organ from which the air-breathing organ of the former has been evolved. There is an interesting cenogenetic phenomenon in the formation of the heart o( the higher Vertebrates that deserves special notice. In it-- earliest form the heart is double, as recent observation has shown, in all the Amniotes, and the simple spindle-shaped cardiac tube, which we look as our starting- point, is only formed at a later Stage, when the two lateral tubes move hack- wards, touch each other, and at last combine in the middle line. In man, as in the rabbit, the two embryonic hearts are still far apart at the stage when there are already eight primitive seg- ments(Fig. 134 h). Soalso the two civlom-pouches of the head in which they lie arc still separated bya broad space. It is not until the permanent body of the em- bryo developes and detaches from the embryonic vesicle that the sepa- rate lateral structures join together, and finally combine in the middle line. As the median partition between the right and left caruioCOel disappears, the two cervical cavities freely communicate (Fig. 381), and form, on the ventral side of the amniote head, a horseshoe-shaped arch, the points of which advance backwards into the pleuroducts or pleural cavities, and from there into the two peritoneal sacs of the trunk. But even after the conjunction of the cervical cavities (Fig. 381) the two cardiac tubes remain separate Fk.. 38^. Transverse section of the cardiac region of the same chick-embryo (behind the preceding). In the cervical cavity ffihj the heart (h) is still connected by a mesocard (hg) with the gut-fibre la>er ((>/) ti gut-^land layer, up prevertebral plates, jb rudimentary auditory vesicle in the horn) plate. hf< first rise ot the amniotic fold. (From A'e»iak.) at first ; and even alter they have united a delicate partition in the middle of the simple endothelial tube (Figs. 379 s, }82 h) indicates the original separation. This cenogenetic " primary cardiac sep- 130 / VOLUTION OF THE SEXUAL ORGANS turn" present I j disappears, and has no relation to the subsequent penhanent par- tition between the halves of the heart, which, as a heritage from the reptiles, has a great PaJingenetic importance. Thorough opponents of the biogenetic law have laid great stress >>n these and similar cenogenetic phenomena, and endeavoured to urge them as Striking disproofs of the law. As in every other instance, careful, discriminating, com- parative-morphological examination con- verts these supposed disproofs of evolution into strong arguments in its favour. In his excellent work, On the St nature of the Heart in the Amphibia (1886),. Carl Rahl has shown how easily those curious Cenogenetic facts can he explained by the secondary adaptation of the embryonic Structure to tbe great extension of the food-yelk. The embryology of all the other parts of the vascular system also gives us abundant and valuable data for the pur- poses of phytogeny. But as one needs .1 thorough knowledge of the intricate structure of the whole vascular system in man and the other Vertebrates in order to follow this with profit, we cannot go into it further here. Moreover, many- important features in the ontogeny of the vascular system are still very obscure and controverted. The characters of the em- bryonic circulation of the Amniotes, which we have previously considered (Chapter XV.), are late acquisitions and entirely cenogenetic. (Cf. pp. 170- 171 ; Figs. 198-202.) Chapter XXIX. EVOLUTION OF THE SEXUAL ORGANS If we measure the importance of the systems of organs in the animal frame according to the richness and variety of their phenomena and the physiological interest thai this implies, we must regard as one of the principal and most interest- ing systems the one which we are now going to- examine— the system of the reproductive organs. Just as nutrition is the first and most urgent condition for the self-maintenance of the individual organism, so reproduction alone secures the maintenance of the species — or, rather, the maintenance of the long series of generations which the totality of the organic stem represents in their genea- logical connection. \o individual organ- ism has the prerogative of immortality, j To each is allotted only a brief span of personal development, an evanescent moment in the million-year course of the history of life. Heine, reproduction and the correlative phenomenon, heredity, have long been regarded, together with nutrition, as the most important and fundamental func- tions of living things, and it has been attempted to distinguish them from " life- les, bodies " on this very score. As a matter of fact, this division is not so profound and thorough as it seems to be, and is generally supposed to be. If we examine carefully the nature of the repro- ductive process, we soon see that it tan be reduced to a general property that is found in inorganic as well as organic bodies — growth. Reproduction is a nutrition and growth of the organism beyond the individual limit, which raises a part of it into the whole. This is most clearly seen when we study it in the simplest and lowest organisms, especially the Monera (Figs. 22b 22S) and the unicellular Amoeba (Fig. 17). There the simple individual is a single plastid. As soon as it has reached a certain limit of size by continuous feeding and normal growth, it cannot pass it, but divides, by- simple cleavage, into two equal halves. Each of these halves then continues its independent life, and grows on until it in (urn reaches the limit of growth, and divides. In each of these acts of self- cleavage two new lent res of attraction are formed lor the particles of bodies, (he foundations of the two new-formed indi- viduals. There is no such thing as immortality even in these unicelhilars. EVOLUTION OF THE SEXUAL ORGANS 3.1 » The individual as such is annihilated In the acl of cleavage (< f. p. 48). In in. ms other Protozoa reproduction takes place not by cleavage, but by budding (gemmation). In this case the growth thai determines reproduction is nol total (as in segmentation), but partial. Hence in gemmation also we may oppose the local growth-product, that becomes a new individual in the bud, as a child-organism to the parent-organism from which it is formed. The latter is older and larger than i lu- former. In cleavage the two products arc- equal in age and morpho- logical value. Next to gemmation we have, as other forms of asexual reproduc- tion, the forming of embryonic buds and the forming of embryonic cells. But file latter leads us al once to sexual genera- tion, the distinctive feature of which is the separation o\' the sexes. 1 have dealt fully with these various types of reproduc- tion in my History of Creation (chap, viii.) and my Wonders of Life (chap. xi.). The earliest ancestors of man and the higher animals had no faculty of sexual reproduction, but multiplied solely by asexual means — cleavage, gemmation, or the formation of embryonic buds or cells, as man) Protozoa still do. The differen- tiation of the sexes came at a later stage. We see this most plainly in the Protists, in which the union of two individuals precedes the continuous cleavage of the unicellular organism (transitory conjuga- tion and permanent copulation of the Infusoria). We may say that in this case the growth (the condition of reproduction) is attained by the coalescence of two full- grown cells into a single, dispropor- tionately large individual. At the same time, the mixture of the two plastids causes a rejuvenation of the plasm. At first the copulating cells are quite homo- geneous ; but natural selection soon brings about a certain contract between them — larger female cells ( macrospores ) and smaller male cells ( microspores J. It must be a great advantage in the struggle for life for the new individual to have inherited different qualities from the two cellular parents. The further advance of this contrast between the generating cells led to sexual differentia- tion. One cell became the female ovum fnutcrogonidion), and the other the male sperm-cell fmu rogonidion). The simplest forms of sexual reproduc- tion among the living Metazoa are seen in the Gastrasads (p. 233), the lower sponges, the common fresh-watei polyp (Hydra), and Other Culcntcria of the lowest rank. Prophysema (Fig. ■• v\), Olynthus (Fig 838), Hydra, etc., have very simple tuhulai bodies, the thin wall of which consists (as in the original gastrula) only of the two primary germinal lasers. As soon as the hod) re.ulies sexUal maturity, a number of the cells in its wall become female ova, and others male sperm-cells : the former become very Inge, as they accumulate a con- siderable quantity of yelk-granules in their protoplasm (Fig. 235 e ; the latter are very small on account of their repeated cleavage, and change into mobile cone-shaped spermatozoa (h'ig. 20). Both kinds of cells detach from their source of origin, the primary germinal layers, fall either into the surrounding water or into the cavity of the gut, and unite there by fusing together. This is the momentous process of fecundation, which we have examined in the seventh Chapter (cf. Figs. 23-29). From these simplest forms of sexual propagation, as we can observe them to-day in the lowest Zoophytes, the Gastraeads, Sponges, and Polyps, we gather most important data. In the first place, we learn that, properly speaking, nothing is required for sexual reproduc- tion except the fusion or coalescence of two different cells— a female ovum and male sperm-cell. All other features, and all the vers- complex phenomena that accompany the sexual act in the higher animals are of a subordinate and secondary character, and are later addi- tions to this simple, primary process of copulation and fecundation. But if we bear in mind hosv extremely important a part this relation of the two sexes plays in the whole of organic nature, in the life of plants, of animals, and of man ; how the mutual attraction of the sexes, love, is the mainspring of the most remarkable processes — in fact, one of the chief mechanical causes of the highest develop- ment of life — we cannot too greatly emphasise this tracing of love to its source, the attractive force of two erotic cells. Throughout the svhole of living nature the greatest effei tS proceed from this very small cause. Consider the part that the flowers, the sexual organs of the flower- ing plants, play in nature; or the exuber- ance of wonderful phenomena thai sexual selection produces in animal life ; or the 3J- /•; I ()/. l //OX OF THE SA AY '. I / t WGA MS momentolis influence of love in the lit*.' ol in. in. In every case the fusion ol two cells is the sole Original motive power ; in every ease this invisible process pro- foundly affects the development o\ the most varied structures, we may say, indeed, that no other organic process can Iv compared to it for a moment in com- prehensiveness and intensity of action. Are not the Semitic myth oi Adam and Eve, the old Greek legend of Paris and Helena, and so many other famous tradi- tions, only the poetic expression of the vast influence that love and sexual selection have exercised over the course of history ever since the differentiation of the sexes ? All the other passions that agitate the heart of man are far out- stripped in their joint influence by this sense-inflaming and mind-benumhing Eros. On the one hand, we look to love with gratitude as the source of the greatest artistic achievements — the noblest creations of poetry, plastic art, and music ; we see in it the chief factor in the moral advance of humanity, the foundation of family life, and therefore of social advance. On the other hand, we dread it as the devouring flame- that brings destruction on so many, and has caused more misery, vice, and crime than all the other evils of human life put together. So wonderful is love and so momentous its influence on the life of the soul, or on the different functions of the medullary tube, that here more than anywhere else the "supernatural " result seems to mock any attempt at natural explanation. Yet comparative evolution leads us clearly and indubitably to the first source of love — the .affinity of two different erotic cells, the sperm<-cell and ovum.' The lowest Metazoa throw light on this very simple origin of the intricate pheno- mena of reproduction, and they also teach us that the earliest sexual form was hermaphrodism, and that the separation of the sexes (by division of labour) is a secondary and later phenomenon. Hermaphrodism predominates in the most varied groups of the lower animals ; each sexually- mature, individual, each person, contains female and male sexual cells, and is therefore able fo fertilise itself and reproduce. Thus we find ova ' The sensual perception (probably related to smell) of tho two copulating sex-cells, which causes their mutual attraction, is a little understood, but very inte- resting, chemical function of the cell-soul (cf. p. 58 and The Riddle of the Universe, chap, ix.) and Sperm-Cells in the same individual, not only in the lowest Zoophytes (Cias- trseads, Sponges, and many Polyps), but also in many worms (leeches and earth- worms), many of the snails (the common garden and vineyard snails), all the Tunicates, and many other invertebrate animals. All mail's earlier invertebrate ancestors, from the Gastr.eads up to the Prochordonia, were hermaphrodites ; possibly, even the earliest Acrania. We have an instructive proof of this in the remarkable circumstance that many genera of fishes are still hermaphrodites, and that it is occasionally found in the higher Vertebrates of all classes (as atavism). Wevnvty conclude from this that gonochorisnr (separation of the sexes) was a later stage in our development. At first, male and female individuals differ only in the possession of one or other kind of gonads ; in other respects they were identical, as we still find in the Amphioxus and the Cyclostomes. After- wards, accessory organs (ducts, etc.) are associated with the primary sexual glands ; and much later again sexual selection has given rise to the secondary sexual characters — those differences between the sexes which do not affect the sexual organs themselves, but other parts of the body (such, as the man's beard or the woman's breast). The third important fact that we learn from the lower Zoophytes relates to the earliest origin of the two kinds of sexual cells. As in the Gastraeads (the lowest sponges and hydroids), in which we find the first beginnings of sexual differentia- tion, the whole body consists merely of the two primary germinal layers, it follows that the sexual cells also must have" pro- ceeded from the cells of these primary layers, either the inner or outer, or from both. This simple fact is extremely im- portant, because the first trace of the ova as well as the spermatozoa is found in the middle germinal layer or mesoderm in the higher animals, especially the Verte- brates. This arrangement is a later development from the preceding (in con- nection with the secondary formation of the mesoderm). If we trace the phylogeny of the sexual organs in our earliest Metazoa ancestors, as the comparative anatomy and ontogeny of the lowest Ccelentcria (Cnidaria, Platodaria) exhibit it to us, we find that the first step in advance is the localisation or concentration of the two kinds of sexual EVOLUTION OF THE SEXUAL ORGA VS 333 cells scattered in the epithelium into definite groups. In the Sponges and low L-t Hydropolyps isolated cells are detached from the cell-strata of the two primary germinal layers, and bet ome i< ee sexual cells ; bul in the Cnidaria and Platodes we find these associated in groups which we call sexual glands (gonads). We can now for the first time speak ol sexual organs in the mor- phological sense. The female germina- tive glands, which in this simplest form are merely groups of homogeneous colls, are the ovaries (Fig. 241 <■'). The male germinative glands, which also in their first form consist o\ a cluster of sperm- cells, are the testicles (Fig. -'41 //). In the medusae, which descend, both onto- geneticallyand phylogenetically, from the more simply organised Polyps, we lind these simple sexual glands sometimes as thai appear at the edge oi the primitive mouth (right and left), as a rule during gastrulation or immediately afterwards the important promesoblasts, or "polar Cells OI (lie mesoderm," Of "primitive cells of the middle germinal layer "(p. 194). In the real Knteio. ola, in which the mesoderm appears from the lirsi in the shape of a couple ol i oloin-pouclies, these are verj probably the original gonads (p. 194). This is seen very clearly in the arrow-worm (Sagitta). In the gastrula o\ Sagitta (Fig. 383 A) we find at an early stage a couple of entodermic cells o\ an unusual size (g) at the base of the primitive gUl (ud). These primi- tive sexual cells (Qrogonidia) are sym- metrically placed 'to the right and left of the middle plane, like the two promeso- hlasi-, o( the bilateral gastrula of the Amphioxus (Fig. ^ />, p. 66). A little Fn.. ;S.l- -Embryos Of Safiritta, in three earlier stages of development. (From ffertwif.) .1 £-astrula, B coelonnil.i with open primitive mouth, < the same with primitive mouth closed, ua primitive gfUt, /'/primitive mouth, g pro^onidia (hermaphroditic primitive sexual cells), cs coelom-pouches, pm parietal layer, vm visceral laver ol same, d permanent jjut (cnteron), st mouth-pit (stomoda?um). gastric pouches, sometimes as outgrowths of the radial canals that proceed from the stomach. Particularly interesting in con- nection with the question of the first Origin of the gonads are the lowest forms of the Platodes, the Cryptocaela that have o\ late been separated as a special clas> ( Platodaiia) from the Turbellaria proper (Pig. 239). In these very primitive Platodes tlie two pairs oi sexual glands are merely two pairs of rows oi' differen- tiated cells in the entodermic wall of the primitive gut -two median ovaries (o) within, and two lateral spermaries (s) without. The mature sexual cells are v ! by the posterior outlets ; the female (fj lies in front of the male (m). In the great majority of the Bilateria or Ccelomaria it is the mesoderm from which the gonads develop. Probably the first traces of them are the two large cells VuL. II. outwards from them the two adorn pouches ( B, cs J are developed out of the primitive gut, and each progonidion divides into a male and a female sexual cell ( 11, g). The two male cells (at first rather the larger) lie close together within, and are (he parent-cells o( the testicles ( pros f>c rm a rui ). The two female cells lie outwards from these, and are the parent-cells of the ovary ( brotovaria). Afterwards, when the orloni-pouches have detached from the permanent gut fC, d ) and the primitive mouth (A, hi ) is closed, the female cells advance towards the mouth ( C, st), and the male towards the rear. The foremost pair of ovaries are then separated by a transverse parti- tion from the hind pair. Thus the first structures of the sexual glands of the Sagitta are a couple of hermaphroditic entodermic cells; each of these divides ;>» EVOLUTION OF THE SEXUAL ORGANS into a male and i female cell ; and these tour cells are the parent-cells of the lour sexual glands. Probably the two pro- mesoblasts of the Amphioxus-gastrula (Fig. 38) are also hermaphroditic primi- tive sexual cells in the same sense, inherited by this earliest vertebrate from its ancient bilateral gastnead ancestors. Fig. 384.—./ Part of the kidneys of Bdello- Stoma. a prerenal duct ( nebhroductus), b segmental or primitive urinary canals ( pronephridia), c renal or Malpighian capsules. H Portion of same, highly magnified, r renal capsules with the glomerulus, d afferent artery, e efferent artery. From Johannes Muller (Myxinoidcs). The sexually-mature Amphioxus is not hermaphroditic, as its nearest invertebrate relatives, the Tunicates, are, and as the long-extinct pre-Silurian Primitive Verte- brate (Prospondy/us, Figs. 98-102) prob- ably was. The actual lancelet has gono- choristic structures of a very interesting kind. As we saw in the anatomy of the Amphioxus, we find the ovaries of the female and the spermaries of the male in the shape of twenty to thirty pairs of elliptical or roundish four-cornered sacs, which lie on either side o\ the gut on the parietaj surface of the respiratory pore (fig. 219^). According to the important discovery o\ Riickert >iXSS), the sexual glands of the earliest fishes, the Selachii, .ue similarly arranged. They only unite afterwards to form a pair of simple gonads. These have been transmitted by heredity to all the rest of the Craniotes. In every case they lie originally on each side ol the mesentery, underneath the chorda, at the bottom of the body-cavity. The first traces of them are found in the ccelom- epithelium, at the spot where the skin- fibre layer and gut-fibre layer meet in the middle of the mesenteric plate (Fig. 93 nip). At this point we observe at an early stage in all craniote embryos a small string-like cluster of cells, which we may caii, with Waldeyer, the "germ epithelium." or (in harmony with the other plate-shaped rudimentary organs) the sexual plate (Fig. 173 g). This germinal or sexual plate is found in the fifth week in the human embryo, in the shape of a couple of long whitish streaks-, on the inner side of the primitive kidneys (Figs. 183 /). The cells of this sexual plate are distinguished by their cylin- drical form and chemical composition from the rest of the coelom-cells ; they have a different purport from the flat cells which line the rest of the body-cavity. As the germ epithelium of the sexual plate becomes thicker, and supporting tissue grows into it from the mesoderm, it be- comes a rudimentary sexual gland. 'This ventral gonad then developes into the ovary in the female Craniotes, and the testicles in the male. In the formation of the gonidia or erotic sexual cells and their conjunction at fecundation we have the sole essential features of sexual reproduction ; but in the great majority of animals we find other organs taking part in it. The chief of these secondary sexual organs are the gonoducts, which berve to convey the mature sexual cells out of the body, and the copulative organs, which bring the fecundating male sperm into touch with the ovum-bearing female. The latter organs are, as a rule, only found in the higher animals, and are much less widely distributed than the gonoducts. Hut these also are secondary formations, and are wanting in many animals of the lower groups. In the lower animals the mature sexual cells are generally ejected directly from EVOLUTION OF THE SEXUAL ORGANS 335 the body. Sometimes they pass out immediately through the skin (Hydra and many hydroids) ; sometime? they tall into the gastric cavity, and arc evacuated by the mouth (gastrssads, sponges, many medusae, and corals) ; sometimes they fall into the body-cavity, and are ejected bv a special pore f forms genitalis) in the ventral wall. The latter procedure is found in many o\ the worms, and also in the lowest Vertebrates, Amphioxus has the peculiar feature that the mature sexual products tall first into the manlle-ca\ itv ; from there they are either evacuated by the respiratory pore, ov else the) pass through the gill-clefts into the branchial gut, and so out by the mouth (p. 185). In the Cyclostomes they tall into the body-Cavity, and are ejected by a genital pore in its wall ; so also in some of the fishes. From these we gather the fea- to convey the sexual products, and this had originally .1 totally different function — namely, the system of urinary organs. These organs have primarily the soleduty of removing unusable matter from the body in a fluid form. Their liquid excre- tory product, the urine, is either e\ acuated directly through the skin or through the last section Of the gut. It is only at a later stage (hat the tubular urinary pas- sages also convey the sexual products from the body. In this way thej become "urogenital ducts." This remarkable secondary conjunction of the urinary and sexual organs into a common urogenital System is very characteristic of the Gnathostomes, the six higher classes o\ Vertebrates. It is wanting in the lower classes. In order to appreciate it fully, we must give a comparative glance at the structure of the urinary organs. Fig. 385.— Transverse section of the embryonic shield of a chick, forty-two hours old. (From A'bllilrrr.) mr medullar) tube, < // chorda, h hornv plate (skin-sense layer), ung ncphroduct, nw episomitcs (dorsal primitive segments), hf> skin-fibre layer (parietal layer of the hypoeomitea), dft> gut-fibre layer (visceral layer of nyposomites), ao aorta, g vessels. (Cf. transverse section of duck-embryo, Fig. iji.) tures oi our earlier ancestors in this respect. On the other hand, in all the higher and most of the lower Vertebrates (and most of the higher Invertebrates) we find in both sexes Special tubular pas-ai^es o\ the sexual gland, which are called " gonoducts." In the female they conduct the ova from the ovary, and so are called "oviducts," or ."Fallopian tubes." In the male they convey the spermatozoa from the testicles, and are called " spermaducts," or VOSO Jcfocntia. The original and genetic relation of these two kinds of ducts is just the same in man a- in tin.- rest o\ the higher Vertebrates, and quite different from what we find in most of the Invertebrates. In the latter, ; s ,1 rule, the gonoducts develop direct Iv from the embryonic glands or from the outer skin ; but in the Vertebrates an independent organic system is employed The renal or urinary system is one of the oldest and most important systems oi organs in the differentiated animal bodv, as I have pointed out on several previous occasions (cf. Chapter XVII.). \Ve find it not only in the higher stems, but also very generally distributed in the earlier group of the Vermalia. Here we meet it in the lowest worms, the Rotatoria (Gas- trotricha, Fig. 242), and in the instructive stem of the Platodes. It consists of" a pair of simple or branching canals, which arc lined with one layer o\ cells, absorb unusable juices from the tissue, and eject them by an outlet in the outer skin (Fig. 240 run). Not only the free-living Turbel- laria, but also the parasitic Suctoria, and even the still more degenerate tape- worms, which have lost their alimentary canal in consequence o\ their parasitic life, are equipped with these renal canals EVOLUTION OF THE SEXUAL ORGANS ov nephridia. In the first embryonic structure they are merely a pair of simple cutaneous glands, or depressions in the ectoderm. They are generall) described as excretory organs in the wormsj but A V Qz^ Fig. 386— Rudimentary primitive kidneys of a dog-embryo. ' The hind end of the embryonic bodj is seen froin the ventral sjdjc and covered with the visceral layer of the \elU---ac, which is torn away and folded down in front in order to show the nephroducts with the primitive urinary canals (a). b primitive vertebra, e spinal cord, d entrance into the pelvic-gut cavity. (From Bischoff.) formerlj' often as " water vessels." They may be conceived as largely-developed tubular cutaneous glands, formed by invagination of the cutaneous layer. According to another view, they owe their origin to a later rupture of the body-cavity outwards. In most of the Yermalia each nebhridium has an inner opening (with cilia) into the bodv-cavilv and an outer one on the epidermis. In these lowest, unsegmented worms, and in the unsegmented Molluscs, there is only one pair of renal canals. They are more numerous in the higher Articu- lates. In the Annelids, the body of which i-. composed of a large number of joints, there is a pair of these proncphridia in each segment (hence they are called seg- mental canals or organs). Even here they are still simple tubes; on account of their coiled or looped form they are often called "looped canals." In most of the Anne- lids, and many o\~ the Yermalia, we can distinguish three sections in the nephri- dium .hi outer muscular duct, a glan- dular middle part, and an inner part that opens by a ciliated funnel into the body- cavity* Tin-- opening is furnished with whirling cilia, and can, therefore, take up the juices to he excreted directly from the body-Cavity and convey tlv»m from the bod)'. Hut in these worms the sexual cells, which develop in very primitive form on the inner surface of the body- cavity, also fall into it when mature, and are sucked up by the funnel-shaped inner ciliated openings of the renal canals, and ejected with the urine. Thus the urine- forming looped canals, or pronephridia, serve as oviducts in the female Annelids and as spermaducts in the male. The renal system of the Vertebrates is similar to, yet materially different from, these seg- mental canals of the Annelids. The peculiar develop- ment of it and its relations to the sexual organs are among the most difficult problems in the morphology of our stem. If we examine briefly the vertebrate renal system from the phylogenetic point of view, as con- firmed by recent discoveries, we may distinguish three forms of it : (1) Fore -kidneys or head-kidneys (pro- nephros ) ; (2) prim- itive or middle kid- neys (tneson'e- phros); (3) perma- nent kidneys (met- anephros). These three systems of kidneys are not fundamentally and completely distinct, 100. 387. — Primitive kidneys of a human embryo. « the urinary canals of the primitive kidneys, v> Wolffian duct, •:«•' uppermost end ot the same I Morpagni's hyda- tid I. hi jvliillcnan duct, «>' uppermost end of same (Fallopian hydatid), s gw nad (sexual gland), (From h'oMl.) as earlier students (such as Semper) wrongly supposed ; they represent three different generations ofone and the same excretory apparatus ; they correspond to three phylogenetic stages, EVOLUTION OF THE SEXUAL ORGANS and succeed each other in the stem- histor) of the Vertebrates in Buch wise tli.u each younger and more advanced generation developes farther behind in the body, and replaces the older and less f% *?n fh; mw Fig. :88.— Pig-embryo, three-fifth* of an inch long, magnified six times, seen from the ventral side, a fore leg. z hind leg. b ventral wall, f sexual prominence, :< nepnroduct, >i primitive kidneys, > of a mammal (ox-embryo). « primitive kionej ♦ sexual elafiid (rudiment of testicle and ovary 1. The primarj nephroduct (tig in Fig 390) divides fin • and 39a) into the two secondary nephroducts — the Mullcrian f m ) and Wolffian ( ug'j ducts, joined together behind in the genital cord fgj. I ligament of the primitive kidneys. (From Grgcnhour.) they open seems to correspond to the prerenal duct of the latter. The next higher Vertebrates, the Cyclo- stomes, yield some very Interesting" data. Both orders of* this class, the hags and lampreys, have still the fore kidneys inherited from the Acrania — the former permanently*! the latter in their earlier stages. Behind these the primitive kid- neys soon develop, and in a very charac- teristic form. The remarkable structure of the mesonephros of the Cyclostomes, discovered by Johannes Mullet", explains the intricate formation of the kidneys in the higher Vertebrates. We find in the hag-fishes ( Rdcllostoma ) a long tube, the prordnal duct ( nephroduct 'us, Fig. 384 a). This opens with its anterior end into the cceloma by a ciliated aperture, and ex- ternally with its posterior end by an outlet in the skin. Inside It open a large number of small transverse canals (" seg- mental or primitive urinary canals," l>). Each of these terminates blindly in a vesicular capsule fcj, and this encloses a coil of blood-vessel (glomerulus, an arterial network, Fig. 384 B, c). Afferent branches of arteries conduct arterial blood into the coiled branches of the glomerulus (d J, and efferent arterial branches con- duct it away from the net (c). The primitive renal canals ( nnsoucpliridia) are distinguished by this net-formation from their predecessors. In the Sclachii also we find a longi- tudinal row of segmental canals on each side, which open outwards into the primitive renal ducts (iitpfirotoiues, p. 149). The segmental canals (a pair i.i each segment of the middle part of the body) open internally by a ciliated funnel info the body -cayity. From the posterior Fig. 394. Figs. 303. 394.— Urinary and sexual organs of an Amphibian (water salamander or Triton). Fig. 393 of a female, 194 of a male, r primitive kidney, OV ovary, od oviduct and e Rathke's duet, both developed from the Mullerian duet, u primitive ureter (also acting as sperrnaduet fve] in the male, opening below into the Wolffian duet [ u / /. ms mes- ovarium. (From Gegefliattr.) group of these organs a compact primitive kidney is formed, the anterior group taking part in. the construction of the sexual organs. In the same simple form that remains EVOLUTION OF THE SEXUAL ORGANS XW throughout life in the Myxinoides and panl\ in the Selachii we find the primi- tive kidney first developing in the embryo of man and the higher Craniotes (Figs, 386, 387). Of the two parts tliat compose the comb-shaped primitive kidney the longitudinal channel, or nephroduct) is always the first to 'appear; afterwards the transverse "-canals," the excreting nephridia, are formed in the mesoderm ; and after this again the Malptghian cap- sule-with theirartci ial.coil- are associated with these as ccelous outgrowths. The primitive renal Juct, which appears firsts i- found in all craniote embryos at t lie early stage in which the differ- entiation of the medullary tube take- place in the ectoderm, the severance oi the chorda from the visceral layer in the entoderm, and I he first trace of the COelora-pOUCheS arises between the limiting layers (Fig. 385). The nephroduct ( utig ) i- seen on each side, directly under the horny plate, in the shape oi a long, thin, thread-like string of cells. It presently hollows out and be- come- a canal, running straight from front to back, and clearly showing in the transverse section 01 the em- bryo its original position in tile space between horny plate (h), primitive segments {Wl>), and lateral plate- ( hplj. As • he originally very short urinary canal- lengthen and multiply, each of the two primitive kidneys assumes the form o\ a half-feathered leaf < Pig. 387 ). The lines of the leaf are represented by the urinary Canals ( u ). and the rib by the outlying ' nephroduct (to). At the inner edge of: the primitive kidneys the rudiment of the ventral sexual gland fg) can now be seen as a body of some size. The hinder- , most end of the nephroduct opens right behind into the last section of the rectum, thus making a cloaca of it. However, this opening oi the nephroducts into the intestine mu-t be regarded as a secondary formation. Originally they open, as the Cyclostomes clearly -how, quite inde- pendently of the gut, in the external skin of the abdomen. In the Myxinotdes the primitive kidneys retain this simple comb-shaped structure, and a part of it i- preserved in tlu- Selachii ; but in all the other Craniote- it i- only found for a short lime in the embryo, as aw ontogenetic reproduction of the earlier phylogenetic structure. In these the primitive kidney soon a-siinu- the lorni [by the rapid growth, lengthening, in- crease, and serpentining 01 the urinary canals) o\ a large compact gland, of a long, oval or spindle-shaped character, which passes through the greater part of the embryonic body-cavitv (Figs. iX\ in, 1X4;;/, ;,Stf ;;). It lies near the middle line, directly under the primitive vertebral column, and readies from the cardiac Fig. 395— Primitive kidneys and germinal glands of a human embryo, three inches injength (beginning ot' the sixth week). magnified fifteen times k germinal gland. » primitive -kidney, * diaphragmatic -ligament of same, w Wolffian duct (opened on the tight), f directing ligament (gubemaculum), a allantoic duct. (From Kollmantx.) region to the cloaca. The right and left kidneys are parallel to each other, quite J close together, and only separated by the mesentery— the -thin narrow layer that attaches the middle gut to the under surface of the vertebral column. The passage of each primitive kidney, the nephroduct, runs towards the back on the lower and outer side of the gland, and opens in the cloaca, close to the starting-point of the allantois ; it after- wards opens into the allantois itself. The primitive or primordial kidneys of ; the amniote embryo vv ei e formerly called the "Wolffian bodies," and sometimes " Oken's bodies." They act for a time as 34<> EVOLUTION Of THE SEXUAL ORGANS kidneys, absorbing unusable juices from- the embryonic body and conducting them to the cloaca aftei wards lo the allantois. There the primitive urine accumulates, and thus the allantois acts as bladder or urinary sac in the embryos of man and thi- pneusls, Amphibia, and Amniotes is ,m entodermic blind sac of the rectum. In all the Anamnia (the lower anmionless Craniotes, Cyclosto- mes, Fishes, Dipne- usts, and Amphibia) the urinary organs remain at a lower stage of development to this extent, that the primitive kidneys ( protoinphri Jact per- manently as urinary glands. This is only so as a passing phase of the early embryonic life in thethree higher classesofVertebrates, the Amniotes. In these the permanent or after or secondary (really tertiary) kid- neys ( renes or mcta- nephri) that are dis- tinctive of these three classes soon make their appearance. They represent the third and last gene- ration of the verte- brate kidneys. The permanent kidneys do not arise (as was long supposed) as independent glands from the alimentary tube, but from the last section of the primitive kidneys and the nephroduct. Here a simple tube, the secondary renal duct, developes, near the point of its entry into the cloaca; and this tube grows con- siderably forward. With its blind upper or anterior end is connected a glandular renal growth, that owes its origin to a differentiation of the last part of the primitive kidneys. This rudiment of the EVOLU TIOS OF THE SEXUAL ORGANS 34" permanent kidneys consists ol coiled urinarj canals with Malpighian capsules and vascular .oils (without Ciliated funnels), of the same structure as the segmental mesonephridiu of tho primitive kidneys. The further growth of these metanephridia gives rise to the compact permanent kidneys, which have the familial bean-shape in man and most of the higher mammals, but consist of .1 number of separate folds in the lower mammals, birds, and reptiles. As the permanent kidneys grow rapidly and advance forward., their passage, the Ureter, detaches altogether from its birth- pi. uv, the posterioi end of the nephro- duct ; it passes to the posterioi surface of the allantois. At first in the oldest Amniotes this uretei opens into the cloaca together with the last section of the neph- roduct, but afterwards separately from this, and finally into the permanent bladder apart from the rectum altogether. The bladder originates from the hindmost and lowest part of the allantoic pedicle ( urackus J, which enlarges in spindle shape before the entry into the cloaca. The anterior or upper part of the pedicle, which runs to the navel in the ventral wall of the embryo, atrophies subse- quently, and only a useless string-like relic of it is left as a rudimentary organ ; that is the single vesico-'umbilical liga- ment. To the right and left of it in the adult man are a couple of other rudi- mentary organs, the lateral vesico- umbilical ligaments. These are the de- generate string-like relics of the earlier umbilical arteries. Though in man and all the other Amniotes the primitive kidneys are thus early replaced by the permanent kidneys, and these alone then act as urinary organs, all the parts of the former are by no means lost. The nephroducts become very im- portant physiologically by being con- \erted into the passages of the sexual glands. In all the Gnathostomes — or all the Vertebrates from the fishes up to man .1 second similar canal dcvelopes beside the nephroduct at an early stage of em- bryonic evolution. The latter is usually called the Mullerian duct, after its dis- coverer, Johannes Muller, while the former is (ailed the Wolffian duct. The origin of the Mullerian duct is still obscure ; comparative anatomy and onto- geny seem to indicate that it originates bv differentiation from the Wolffian duct. Perhaps it would be best to say: "The original primary nephroduct divides by differentiation (or longitudinal cleave into two secondary nephroducts, the Wolffian and the Mullerian ducts." The latter (Fig. 3*7 '") lies just o\\ the inner side of the former (Fig. 387 B I. Moth open behind into the cloaca. However uncertain the origin of the nephroduct and its two products, the Mullerian and the Wolffian ducts, mav be, its later development is clear enough. In all the Gnathostomes the Wolffian duct is converted into the spermaduct, and the Mullerian dud into the oviduct. Only one of them i> retained in each sex ; the other either disappears altogether, or Only leaves relics in the shape o( rudimentary organs. In the male sex, Fii.. 39.) -Female sexual organs of a Mono- treme t (>ni:t/u>rfiynrhus. Fig. 269). 0 ovaries, ' ovi- ducts, u womb, sug urogenital sinu> ; at 1. is the- outlet of the two wombs. and between them the bladder ( iuj. <7 cloaca. (V rom Grgen/>aur.) in which the two Wolffian ducts become the spermaducis, we often find tracer of the Mullerian ducts, which I have called " Kathke's canals" (Fig. 394 c). In the female sex, in which the two Mullerian ducts form the oviducts, there are relics of the Wolffian ducts, which are called " the ducts of Ciaertner." We obtain the most interesting in- formation with regard to this remarkable evolution of the nephroducts and their association with the sexual glands from the Amphibia (Figs. 390-395)- Tin firsl structure of the nephroduct and its differ- entiation into Mullerian and Wolffian duds arc just the same irl lx>th sexes in the Amphibia, as in the mammal embryos (Figs. 392, 396). In the female Amphibia 34* EVOLUTION OF 1 HE SEXUAL ORGANS the Mullerian duct developes on either si Jo into a large oviduct (Fig: 393 oct), while the Wolffian dud acts permanently as ureter fu). In the male Amphibia the Mullerian duct only remains .is .1 Mh \ Fig. 400. 401. Pius.' 40a 401 —Original position of the sexual glands in the ventral cavity of the human embryo (three months old). Fig. 400 male (natural size), h testicles, gh conducting ligament of the testicles, tag spermaduct, h hladder, uh inferior vena cava, nn accessory kidneys, n kidneys. Fig. 401 female, slightly magnified, r round maternal ligament (underricath it the bladder, over it the ovaries). r> kidneys, g accessory kidneys, c caecum, o small reticle. 0111 large reticle (stomach between the two), / spleen. tFroin KbUiker.) rudimentary. organ without any functional significance, as Rathke's canal (Fig. 394 c) ; the Wolffian duct serves also as ureter, but at the same time as sperma- duct, the sperm-canals (ve ) that proceed from the testicles ft) entering the fore part of the primitive kidneys and com- bining there with the urinary canals. In the mammals these permanent amphibian features are only seen as brief phases of the earlier period of embryonic development (Fig. 392). Here the primi- tive kidneys, which act as excretory organs of urine throughout life in the amnion-less Vertebrates, are replaced in the mammals by the permanent kidneys. The real primitive kidneys disappear for the most part at an early stage of develop- ment, and only small relics of them remain. In the male mammal the epidi- dymis developes from the uppermost part of the primitive kidney ; in the female a useless rudimentary organ, the epovarium, is formed from the same part. The atrophied relic of the former is known as the paradidymis, that of the latter as the parovarium. The Mullerian ducts undergo very important changes in the female mammal. The oviducts proper are developed only from their upper part ; the lower part dilates into a spindle-shaped tube with thick muscular wall, in which the im- pregnated ovum developes into the em- bryo. This is the womb (utans). At firs! the two wombs (Fig. }<)() u) are com- pletely separate, and open into the cloaca Oil either side o\' the bladder fvuj, as is still the case in the low est living mammals, the Monotremes. But in the Marsupials a communication is opened between the two Mullerian ducts, and in the Placentals they combine below with the rudimentary Wolffian ducts to form a single "genital cord." The original independence of the. two wombs and the vaginal canals formed from their lower ends are retained in many of the lower Placentals, but in the higher they gradually blend and form a single organ. The conjunction proceeds from below (or behind) upwards (or forwards). In many of the Rodents (such as the rabbit and squirrel) two separate wombs still open into the simple and single. vaginal canal ; but in others, and in the Carnivora, Cetacea, and Ungulates, the Fig. 402.— Urogenital system of a human em- bryo of three inches in length, double natural si/.-. h testicles, ug spermaducts, gh conducting ligament, p processus vaginalis, b bladder, an umbilical arteries, in mc sorchium, d intestine, u ureter, n kidney, «« accessory kidney. (From Kallmann.) lower halves of the wombs have already fused into a single piece, though the upper halves (or " horns ") are still sepa- rate (" two-horned " womb, ute> us bicomis). In the bats and lemurs the " horns " are EVOLUTION OF THE SEXUAL ORGANS 343 very short, and the lower common part is longer. Finally, in the apes and in man the blending of the two halves is com- plete, and there is onl] the one simple, dllCt, and is found only in the ape and man. In the male mammals there is the same fusion of the Mullcrian and Wolffian Fig. 403. Fig. 404. Fig. 405 Fig. 406. Figs. 403-406. —Origin of human ova In the female ovary. Fig. 403 Vertical section of the OVary of ■ new-born female infant, a ovarian epithelium, i rudimentary string ol ova, < young ova in the epithelium, d long string of ova with follicle-formation (Pfluger's tube), e group of young follicles. J isolated young follicle, g olood-vcssels in connective tissue (stroma) of the ovary. In the strings the \oung ova arc distinguished b> their considerable m/.c from the* surrounding iollicle-cells. (From Waldeyer.) Fig. 404.— Two young Graafian follicles, isolated. In / the follicle-cells still form a simple, and in 2 a double, stratum round the \oung ovum ; in i they are beginning to form the ovolcmma or the zona pellucida ( <* ) liaped uterine pouch, into which the oviducts open on each side. This simple uterus i>, a late evolutionary pro- ducts at their lower ends. Here again they form a single genital cord (Fig. 397^), and this opens similarly into the EVOLUTION OF THE SEXUAL ORGANS 344 origins! urogenital sinus, which developes kidneys lo the inguinal region of the from the lowest section of the bladder (v). j ventral wall. This is the inguinal liga- Bul while in the male mammal the- ment of the primitive kidneys, known in Wolffian ducts develop into the permanent ' the male .is the Hunterian ligament (Fig. spermaducts, there are only rudimentary 400 gk), and in the female as the " round relics left of the Mullerian ducts. The maternal ligament" (Fig. 401 r). most notable o\ these is the " male womb " (uterus masculinus), which originates from the lowest fused part o( the ducts, and corresponds to the female uterus. Ii is a small, flask-shaped vesicle without any physiological significance, which opens into the ureter between the two spermaducts and the prostate folds ( vfsicula pmstatica). Fie 407.— A ripe human Graafian follicle, a the mature ovum. * the MirriniiuiirfK follicle-cells, f the epithelial cells of the follicle, d the fibrous membrane of the follicle, e its outer surface. The internal sexual organs of the mam- mals undergo very distinctive changes of position. At first the germinal glands of both sexes lie deep inside the ventral cavity, at the inner edge of the primitive kidneys (Figs. 386 jr, 392 £), attached to the vertebral column by a short mesentery {tnesdrchium in the male, mesovaiium in the female). But this primary arrange- ment is retained permanently only in the Mohotremes (and the lower Vertebrates). In all other mammals (both Marsupials and Placentals) they leave their original cradle and travel more or less far down (or behind), following the direction of a ligament that goes from the primitive In worn. m the ovaries travel more or less towards the small pelvis, or enter into it altogether. In the male the testicles pass OUl of the ventral cavity, and penetrate by the inguinal canal into a sac-shaped fold o\ the outer skin. When the right and left folds ("sexual swellings") join to- gether they form the scrotum. The various ' mammals bring before us the successive stages o( this displacement. In the elephant and the whale the testiclesdescend very little, and remain under- neath the kidneys. In many o\ the rodents and carnassia they enter the inguinal canal. In most of the higher mammals they pass through this into the scrotum. As a rule, the inguinal canal closes up. When it re- mains open the testicles may periodically pass into the scrotum, and withdraw into the ven- tral cavity again in time o\ rut (as in many of the marsupials, rodents, bats, etc.). The structure of the external sexual organs, the copulative organs that convey the fecun- dating sperm from the male to the female or- ganism in the act of copulation, is also peculiar to I he mammals. There are no organs of this character in most of the other Vertebrates. In those that live in water (such as the Acrania and Cydostomes, and most of the fishes) the ova and sperm-cells are simply ejected into the water, where their conjunction and fertilisation are left to chance. But in many of the fishes and amphibia, which are viviparous, there is a direct convey- ance of the male sperm into the female body ; and this is the case with all the Amniotes (reptiles, birds, and mammals). In these the urinary and sexual organs always open originally into the last section of the rectum, which thus forms a cloaca EVOLUTION OF THE SEXUAL ORGANS 345 (p. 249). Among the mammals this arrangement is permanent only in the Monotremes, which take their name from it (Fig 199 ,/). In .ill the other mammals a frontal partition is developed in the cloaca (in the human embryo about the beginning of the third month), and this divides it into \.\o cavities. The anterior cavity receives the urogenital canal, and is the sole outlet of the urine and the sexual products ; the hind or anus-cavity passes the excrements only. Even before this partition has been formed in the Marsupials and Placentals, we see the first trace of the external sexual organs. Firstaconical protuberance rises at the anterior border of the cloaca-outlet the . r.C- , sexual prominence ( pltal- /"VJ\ : for, l-'ig. 402 A, e, 6,eJ. t-\-' H At the tip it is swollen in /«.<£ the shape of a club (" acorn glans). On its under side there is a furrow, the sexual groove (sulcus genitalis, J ), and on each side of this a fold of -.Uin, the "sexual pad" ( torus genitalis, h I). The sexual protuberance or phallus is the chief organ of the sexual sense (p. 282) , the sexual nerves spread on it, and these are the principal organs of the specific sexual sensation. As erectile bodies (corpora cavernosa ) are developed in the male phallus by peculiar modifications of the blood-vessels, it becomes capable of erecting periodi- cally on a strong accession of blood, becoming stiff, so as to penetrate into the female vagina and thus effect copulation. In the male the phallus becomes the penis ; in the female it becomes the much smaller clitoris ; this is only found to be very large in certain apes f A teles J. A prepuce ("fore- skin ") is developed in both sexes as a protecting 'fold on the anterior surface of the phallus. Tile external sexual member (phallus ) is found at various stages of development the phallus, the gUMS, both the larger glans penis of the male Ano\ the smaller glans clitoridU of the female. The pail ot the cloaca from the upper wall of which it forms belongs tO the pn>cto (especial!) the carnassia and 'rodents) by the- ossification of a part of the fibrous body (corpus fibrosa in ). This penis-bone (os priapi) is very large in the badger and dog, and bent like a hook in the marten ; it is also ver\ large in souk- of the lower apes, and protrudes far out into the glans. It is wanting in most of the anthropoid apes ; it seems to have been lost in their case (and in man) by atrophy. The sexual groove on the under side of the phallus receives in the male the mouth of the urogenital canal, and is changed into a continuation of this, becoming a closed canal by the juncture of its parallel edges, the male urethra. In the female this only lakes place in a few cases (some of the lemurs, rodents, and moles) ; as a rule, the groove remains open, and the borders of this " \ estibule of the \ agina '" develop into the smaller labia (nympha). The large labia of the female develop from the sexual pads (tori gciiitales ), the two parallel folds of the skin that are found i-'ii each side of the genital groove. They join together in the male, and form the closed scrotum. These striking differ- ences between the two sexes cannot yet lie delected in the human embryo of the ninth week. We begin to trace them in the tenth week of development, and they are accentuated in proportion as the difference of the sexes developes. Sometimes the normal juncture of the two sexual pads in the male fails to take place, and the sexual groove may also remain open (hypospadia). In these cases l he external male genitals re- semble the female, and they are often wrongly regarded as cases of hermaphro- dism. Other malformations of various kinds are not infrequently found i'i 'be human external sexual organs, and some of them have a great morphological interest. The reverse of hypospadia, in Which the penis is split open below, is seen in , pisfxu/iu , in which the urethra is Open above. In this case the urogenital canal opens above at the dorsal root of I he penis; in the former case down below. These and similar obstructions interfere with a man's generative power, and thus prejudicially affect his whole develop- ment. They clearly prove that our history is not guided by a " kind Provi- ded e," but left to the play of blind chance. We must carefully distinguish the rarer Cases of ie.il hcrmaphrodisin from the preceding. This is only found when the essential organs of reproduction, the genital glands of both kinds, are united in one individual. In these cases either an Ovary is developed on the right and a testicle on the left (ov vice versa) ; or else there are testicles and ovaries on both sides, some more and Others less developed. As hcrmaphrodism was pro- bably the original arrangement in all the Vertebrates, and the division of the sexes only followed by later differentiation of this, these CUnOUS ' cases of\\v no theoretical difficulty. Hut they are rarely found in man and the higher mammals. Oil the ol her hand, we constantly find the original hermaphrodism in some of the lower Vertebrates, such as the iMyxinoides, many fishes of the perch-type (serranus), and some of the Amphibia (ringed snake, toad). In these cases the male often has a rudimentary ovary at the fore end of the testicle ; and the female sometimes has a rudimentary, inactive testicle. In the carp also and some other fishes this is found occasionally. We have already seen how traces of the earlier herm- aphrodism can be traced in the passages of the Amphibia. Man has faithfully preserved the main features of his stem-history in the onto- geny of his urinary and sexual organs. We can follow their development step by step in the human embryo in the ^.ame advancing gradation that is piesented to us by the comparison of the urogenital organs in the Aerania, Cyclostomes, Fishes, Amphibia, Reptiles, and then (within the mammal series) in the Mono- genics, .Marsupials, and the various I'lacentals. All the peculiarities of uro- genital structure that distinguish the mammals from the rest of the Vertebrates are found in man ; and in all special structural features he resembles the apes, particularly the anthropoid apes. In proof of the fact that the special features qf the mammals have been inherited by man, I will, in conclusion, point out the identical way in which the ov a are formed in the ovary. In all the mammals the mature ova are contained in special cap- sules, which are known as the Graafian r.VOU'TIOS <)/■' THE SEXUAL ORGANS .V47 follicleSt after their discoverer, Rogci de Graaf (1^77) They were Formerlj ->np- posed to be the ova themselves ; but Baei discovered the ova within t ho follicles (p. i()i. Each follicle (Fig. 107) consists *->t" .1 round< fibrous capsule ( >i ), which contains MuiJ and is lined with several strata of cells (i J. The layer is thickened like . 1 knob at one point (b); this ovum- capsule encloses the ovum proper (a)-. The mammal ovan is originally a very simple oval body (Fig. 387 g), formed only of connect i> e tissue and blood-vessels, covered with a layer of cells, the ovarian. epithelium or the female germ epithelium. From this germ epithelium .strings of. cells grow oul into the connective tissue hi "stroma" of the ovary (Fig. 40$ />). Some of the cells of these strings (or Pfluger's tubes) -row larger and become Ova (primitive ova, c) ; but the -real majority remain small, and form a pro- tective and nutritive stratum of cells round each ovum the " follicle-epithe- lium " (r j. The follicle-epithelium of the mammal has at first one stratum ( Fig. 404 /), but afterwards several ( j). It is true that in all the other Vertebrates the ova are enclosed in a membrane, or "follicle," thai consists of smaller cells. But it is only in the mammals that fluid accumu- lates between the growing follicle-cells, and distends the follicle into a large round capsule, on the Inside wall of which the ovum lies, at one side (Figs. 405, 406). There again, as in the whole of his mor- phology, man proves indubitably his descent from the mammals. In the lower Vertebrates the formation of ova in the germ-epithelium of the ovary continues throughout life; but in the higher it is restricted to the earlier stages, or even to the period of embryonic develop- I ment. In man it seems to cease in the* first year; in the second year we find no new-formed ova or chains of ova (Pfluger's lubes). However, the number of o\a in the two ovaries is verj large in the young girl j there are cal< united to be 72,000 in the sexually-mature maiden. In the pro- duction ol the ova men resemble most of the anthropoid apes. Generally speaking, the natural history of the human sexual organs is one of those puts of anthropology thai furnish the most convincing proofs of the animal origin of the human race. Am man who is acquainted 'with the facts and impar- tially weighs them will conclude from them alone that we have been evolved from the lower Vertebrates. The larger and the detailed structure, the action, and the embryological development of the sexual organs are just the same in man as iii the apes. This applies equally to the male and the female, the internal and the external, organs. The differences we find in this respect between man and the anthropoid apes are much slighter than the differences between the various spe< ies of ape-. But all the apes ha\ e certainly a common origin, and have been evolved from a long-extinct early-Tertiary Stem- form, which we must trace to a branch of the lemurs. If we had this unknown pithecoid stem-form before us, we should certainly put it in the order of the true apes in the primate system ; but within this order we cannot, for the anatomic and ontogenetic reasons we have seen, separate man from the tfroup of the anthropoid apes. Here again, therefore, on the ground of the pithecometra-prin- Ciple, comparative anatomv and ontogeny. teach with full confidence the descent of man from the ape. 348 RESULTS OF ANTHROPOGENY Chapter XXX. RESULTS OF ANTHROPOGENY Now that we have traversed the wonder- ful region ol' human embryology and are familiar with the principal parts of it, it will be well to look back on the way we have come, and forward to the further path to truth to which it has led us. We started from the simplest facts of ontogeny, or the development o\ the individual — from observations that we dan repeat and verify by microscopic and anatomic study at any moment. The first and most important of these facts is that every man, like every other animal, begins his exis- tence as a simple cell. This round ovum has the same characteristic form and origin as the ovum of any other mammal. From it is developed in the same manner in all the Placentals, by repeated cleavage, a multicellular hlastula. This is converted into a gastrula, and this in turn into a blastocyst is (or embryonic vesicle). The two strata of cells that compose its wall are the primary germinal layers, the skin- layer (ectoderm), and gut-layer (ento- derm). This two-layered embryonic form is the ontogenetic reproduction of the extremely important phylogenefic stem- form of all the Metazoa, which we have called the Gastraea. As 'the human embryo passes through the gastrula-form like that of all the other Metazoa, we can trace its phylogenetic origin to the Gastnca. As we continued to follow the embry- onic development of the two-layered structure, we saw that first a third, or middle layer (mesoderm), appears between the two primary layers ; when this divides into two, we have the four secondary germinal layers. These have just the same composition and genetic significance in man as in all the other Vertebrates. From the s*kin-sensc layer are developed the epidermis, the central nervous system, and the chief part of the sense-organs. The skin-fibre layer forms the corium and the motor organs— the skeleton and the muscular system. From the gut-fibre layer are developed the vascular system, the muscular wall of the gut, and the sexual glands. Finally,, the gut-gland layer only forms the epithelium, or the inner cellular stratum of the mucous membrane of the alimentary canal and glands (lungs, liver, etc. I. The manner in which these different systems of organs arise from the secon- dary germinal layers is essentially the same Irom the start in man as in all the other Vertebrates. We saw, in studying the embryonic development of each organ, that the human embryo follows the special lines of differentiation and construction that are only found otherwise in the Verte- brates. Within the limits of this vast stem we have followed, step by step, the' development both of the body as a whole and of its various parts. This higher development follows in the human em- bryo the form that is peculiar to the mammals. Finally, we saw that, even within the limits of this class, the various phylogenetic stages that we distinguish in a natural classification of the mammals correspond to the ontogenetic stages that the human embryo passes through in the course of its evolution. We were thus in a position to determine precisely the position of man in this class, and so u> establish his relationship to the different orders of mammals. The line of argument we followed in this explanation of the ontogenetic facts was simply a consistent application of the biogenetic law. In this we have through- out taken strict account of the distinction between palingenetic and cenogenetic phenomena. Palingenesis (or " synoptic development ") alone enables us to draw conclusions from the observed embryonic form to the stem-form preserved by heredity. Such inference becomes more or less precarious when there has been cenogenesis, or disturbance of develop- ment, owing to fresh adaptations. We cannot understand embryonic develop- ment unless we appreciate this very important distinction. Here we stand at the very limit that separates the older and the new science or philosophy of nature. The whole of the results of recent mor- phological research compel us irresistibly RESULTS OF ANTHROPOGENY 349 to recognise the biogenetic law and its far-reaching consequences. These are, it is true, irreconcilable with the legends and doctrines of former days, that have been impressed on u-. by religious educa- tion. But without the friogenetit knv, without the distinction between palin- genesis and cenogenesis, and without the theory of evolution on which we base it, it is quite impossible to understand the EtU is of organic development ; without them we cannot cast the faintest gleam of explanation over tins marvellous field of phenomena. But when we recognise the causal correlation of ontogeny and phy- logeny expressed in this law, the wonder- ful facts of embryology are susceptible oi a \er\ simple explanation ; they are found to be the necessary mechanical effects of the evolution of the stem, determined by the laws of heredity and adaptation. The correlative action of these laws under the universal influence of the struggle for existence, or as we may bay in a word, with Darwin " natural selection," is entirely adequate to explain the whole process of embryology in the light of phytogeny. It is the chief merit of Darwin that he explained by his theory of selection the correlation of the l.tws of heredity and adaptation that Lamarck had recognised, and pointed out the true way to reach a causal interpretation of evolu- tion. The phenomenon that it is most impera- tive to recognise in this connection is the inheritance oi functional variations. Jean Lamarck was the first to appreciate its fundamental importance in 1809,' and we may therefore justly give the name of Lamarckism to the theory of descent he based on it. Hence the radical opponents oi the latter have very properly directed their attacks chiefly against the former. Onu of the most distinguished and most narrow-minded of these opponents, Wil- helm His, affirms very positively that "characteristics acquired in the life of the individual are not inherited." The inheritance of acquired characters is denied, not only by thorough opponents o\ evolution, but even by scientists who admit it and have contributed a good deal 1.1 it, establishment, especially VVeismann, Galton, Kay Lankester, etc'. Since it Descent (1902), he has with great success advanced the opinion that"oni\ those characters can be trans- mitted to subsequent generations that were contained in rudimentary form ill the embryo." However, this germ-plasm theory, with its attempt to explain heredity, is merely a "provisional mole- cular hypothesis , it is one of those metaphysical speculations that attribute the evolutionary phenomena exclusively to internal i.iuso, and regard the inllu- enceof the environment as insignificant. Herbert Spencer, Theodor Kimcr, Lester Ward, Hering, and Xehnderhavc pointed out the nntenable consequences ot this position. I have given my view of it in the tenth edition of the History of Creation I pp. 192, 203). 1 hold, with Lam. ink and Darwin, that the hereditary transmission of acquired characters is one of the most important phenomena in biology, and is proved by thousands of morphological and physiological experiences. It is an indispensable foundation of the theory of evolution. Of the main and weighty arguments fov the truth ot" this conception of evolu- tion I will for the moment merely point to the invaluable evidence of dysteleology, the science .of rudimentary organs. We cannot insist too often or loo strongly on the great morphological significance of these remarkable organs, which are com- pletely useless from the physiological point of view. We find some of these useless parts, inherited from our lower vertebrate ancestors, in every system of organs in man and the higher vertebrates. Thus we find at once on the skin a scanty and rudimentary coat of hair, only fully developed on the head, under the shoulders, and at a few other parts of the body. The short hairs on the greater part of the. body are quite useless and devoid oi physiological value ; they are the last relic of the thicker hairy coat of our simian ancestors. The sensory apparatus presents a series of most remarkable rudimentary organs. We have seen that the whole of the shell of the external ear, with its cartilages, muscles, and skin, is in man a useless appendage, and has not the physiological importance that was formerly ascribed to it. It is the degene- rate remainder of the pointed, (reel) moving, and more advanced mammal car, the muscles of which we still have, but cannot work them. We found at the 35° RESULTS OF ANTHROPOGENY loner corner of our eye a small, curious, semi-lunar fold thai is o\ no use whatever to us, and is only interesting as the last relicofthe nictitating membrane, the third, inner eye-lid that had a distinct physio- logical purpose in the ancient sharks, and still lias in mam of the Amniotes. The motor apparatus, in both the skeleton and muscular systems, provides a number o\ interesting dysteleological arguments. I need only recall the pro- jecting tail of the human embryo, with its rudimentary caudal vertcbr.e and muscles ; this is totally useless in man, but very interesting- as the degenerate relic of the long tail of our simian ancestors. From these we have also inherited various bony processes and muscles, which were very useful to them in climbing trees, but are useless to us. At various points ot the skin we have cutaneous muscles which we never use — remnants of a strongly-developed cuta- neous muscle in our lower mammal ancestors. This " panniculus carnosus" had the function of contracting and creas- ing the skin to chase away the flies, as 'we see every day in t he horse. Another relic in us of this large cutaneous muscle is the frontal muscle, by which we' knit our forehead and raise our eye-brows ; but there is another considerable relic of it, the large cutaneous muscle in the neck (platysma myoides), over which we have no voluntary control. Not only in the systems of animal organs, but also in the vegetal apparatus, we find a number of rudimentarv organs, many of which we have already noticed. In the alimentary apparatus there are the thymus-gland and the thyroid gland, the seat of goitre and the relic of a ciliated groove that the Tunicates and Acrania still have in the gill-pannier ; there is also the vermiform appendix to the ccecum. In the vascular system we have a number of useless cords which represent relics of atrophied vessels that were once active as blood-canals — the ductus Botalli between the pulmonary- artery and the aorta, the ductus venosus Arantii between the portal vein and the vena cava, and many others. The many rudimentarv organs in the urinary and sexual apparatus are particularly inter- esting. These are generally developed in one sex and rudimentary in the other. Thus the spermaducts are formed from the Wolffian ducts in the male, whereas in the female we have merely rudimentary traces of them in Claertnei's canals. Ow the other hand, in the female the oviducts and womb are developed from the .Mullcrian ducts, while in the male only the lowest ends of them remain as the " male womb " (vcsicula prostatica). Again, the male has in his nipples and mammary glands the rudiments of organs that are usually active only in the female. A careful anatomic study of the human frame would disclose to us numbers of other rudimentary organs, and these can only be explained on the theory of evolu- tion. Robert Wiedersheim has collected a large number o\' them in his work on The Human Frame as a Witness to its Past. They are some of the weightiest proofs of the truth of the mechanical con- ception and the strongest disproofs of the teleological view. If, as the latter de- mands, man or any other organism had been designed and fitted for his life- purposes from the start and brought into being by a creative act, the existence of these rudimentarv organs would be an insoluble enigma ; it would be impossible to understand why the Creator had put this useless burden on his creatures to walk a path that is in itself by no means easy. But the theory of evolution gives the simplest possible explanation oi' them. It says : The rudimentarv organs are parts of the body that have fallen into disuse in the course of centuries ; they had definite functions in our animal ancestors, but have lost their physiological significance. On account of fresh adap- tations they have become superfluous, but are transmitted from generation to generation by heredity, and gradually atrophy. We have inherited not only these rudi- mentary parts, but all the organs of our body, from the mammals — proximately from the apes. The human body does not contain a single organ that has not been inherited from the apes. In fact, with the aid of our biogenetic law we can trace the origin of our various systems of organs much further, down to the lowest stages of our ancestry. ~AVe can say, for instance, that we have inherited the oldest organs of the body, the external skin and the internal coat of the alimentary system, from theGastrrcads ; the nervous and mus- cular systems from the Platodes ; the vas- cular system, the body-cavitv, and the blood from the Vermalia ; the chorda and the branchial gut from the Prochordonia ; h'h.st LTS OF ANTHROPOQENY 35« the articulation of the body from ihe Acrania ; the primitive skull and the higher sense-organs from the Cyclo- stomcs; the limbs and jaws from the Selachii ; the five-toed fool from the Amphibia ; the palate from the Reptiles ; the hairy coat, the mammary glands, and the externa] sexual organs from Ihe Pro- mammals. When we formulated "the law of the ontogenetic connection of systematically related forms," an J deter- mined the relative age of organs, we saw how it was possible to maw phylogenetic conclusions from the ontogenetic succes- sion of systems of organs. With the aid of this important law and of comparative anatomy we were also enabled to determine "man's place in nature," or, as we put it, assign to man his position in the classification of the animal kingdom. In- recent zoological classification the animal world is divided into twelve stems or phyla, and these are broadly sub-divided into about sixty classes, and the-,- classes into at least 300 orders. In his whole organisation man i- mosl certainly, in the first place, a member of one of these stems, the verte- brate stem ; secondly, a member of one particular class in this stem, the Mam- mals; and thirdly, of one particular order, the order of Primates. He has all the characteristics that distinguish the Verte- brates from the other eleven animal stems, the Mammals from the other sixty classes, and the Primates from the 300 other orders of the animal kingdom. We may turn aitd twist as we like, but we cannot get over this fact of anatomy and classi- fication. Of late years this fact has given rise to a good deal of discussion, and especially of controversy as to the particular anatomic relationship of man to the apes. The most curious opinions have been advanced on this "ape-ques- tion," or " pithecoid-theory." It is as well, therefore, to go into it once more and distinguish the essentia] from the unessential. (Cf. above, pp. 261-5.) We start from the undisputed fact that man is in any case— whether we accept or reject his special blood-relationship to the apes a true mammal; in fact, a placental mammal. This fundamental fact can be proved so easily at any moment from comparative anatomy that it has been universally admitted since the separation of the Placental S fioa\ the lower mammals (Marsupials and Mono- tremes). But for every consistent sub- scriber to the theory of evolution it must follow at once that man descends from a Common stem-form with all the Other Placentals, the stem-ancestor of the Placentals, just as we must admit a common mesozoic ancestor of all the mammals. This i>, however, to settle decisively the gre.it and burning question of man's place in nature, whether or no we go on to admit a nearer or mow distant relation-hip to the apes. Whether man is or is not a member of the ape- order (or, if you prefer, the primate-order) in the phylogenetic sense, in any case his direct blood-relationship to the rest of the mammals, and especially the Placentals, is established, li is possible that the affinities of the various orders of mam- mals to each other are different from what we hypothetical ly assume to-day. Hut, in any case, the common de-cent of man and all the other mammals from on<: stem-form is beyond question. This long-extinct IYomammal was probably evolved from Proreptiles during tile Triassic period, and must certainly be regarded as the monotreme and oviparous ancestor of all the mammals. If we hold firmly to this fundamental and most important thesis, we shall see the " ape-question " in a very different light from that in which it is usually regarded. Little reflection is then needed to see that it is not nearly so important as it is said to be. The origin of the human race from a series of mammal ancestors, and the historic evolution of these from an earlier series of lower \ ertebrate ancestors, together with all the weighty conclusions that every thoughtful man deduces there- from, remain untouched ; so far as these are concerned, it is immaterial whether wc regard true "ape-" a- our nearest ancestors or not. Hut as it has, become the fashion to lav the chief stress in the whole question of man's origin on the "descent from the apes," I am compelled \ to return to it once more, and recall the facts of comparative -anatomy and onto- genv that give a decisive answer to this " ape-question." The shortest way to attain our purpose i- that followed by Huxley in 1863 in his able work, which 1 have ahead) often quoted, Man's Place in Nature— the way of comparative anatomy and ontogeny. We have to compare impartially all man's organs with the same organs in the 1 higher apes, and then to examine if the differences between the two are greater A7-..S7 7. TS OF A XTIIROPOi,!: A I than the corresponding differences between tlie higher and the lower apes. The indubitable and incontestable result of this comparative-anatomical study, conducted with the greatest care and impartiality! was the pithecometra-prin- eiple, which we have called the Huxleian law in honour of its formulator namely, that the differences in organisation between man and the most advanced apes we know .ire much slighter than the corresponding differences in organisation between the higher and lower apes. We may even give a more precise formula to this law, by excluding the Platyrrhines or American apes ,is distant relatives, and restricting the comparison to the narrower family-circle of the Catarrhines, the apes of the Old World. Within the limits of this small group of mammals we found the structural differences between the lower and higher catarrhine apes— for instance, the baboon and the gorilla — to be much greater than the differences between the anthropoid apes and man. If we now turn to ontogeny, and find, according to our " law of the ontogenetic connection of systematically related forms," that the embryos of the anthro- poid apes and man retain their resem- blance for a longer time than the embryos of the highest and the lowest apes, we are forced, whether we like it or no, to recognise our descent from the order of apes. We can assuredly construct an approximate picture in the imagination of the form of our early Tertiary ancestors from the foregoing facts of comparative anatomy ; however we may frame this in detail, it will be the picture of a true ape, and a distinct catarrhine ape. This has been shown CO well by Huxley (1863) that the recent attacks of Klaatsch, Virchow, and other anthropologists, have com- pletely failed (cf. pp. 263-264). AH the structural characters that distinguish the Catarrhines from the Platyrrhines are found in man. Hence in the genealogy of the mammals we must derive man immediately from the catarrhine group, and locate the origin of the human race in the Old World. Only the early root- form from which both descended was common to them. It is, therefore, established beyond question for all impartial scientific inquiry that the human race comes directly from the apes of the Old World ; but, at the same time, 1 repeat that this is not so important in connection with the main question of the origin of man as is com- monly supposed. Even if we entirely ignore it, all that we have lea rned from the zoological facts of comparative anatomy and ontogeny as to the placental character of man remains untouched. These prove beyond all doubt the common descent of man and all the rest of the mammals. Further, the main question is not in the least affected if it is said : " It is true that man is a mammal ; but he has diverged at the very root of the class from all the other mammals, and has no closer relationship to any living group of mammals." The affinity is more or less close in any case, if we examine the relation of the mammal class to the sixty other classes of the animal world. Quite certainly the whole of the mammals, including man, have had a common origin ; and it is equally certain that their common stem-forms were gradually evolved from a long series of lower Vertebrates. The resistance to the theory of a descent from the apes i> clearly due in most men to feeling rather than to reason. They shrink from the notion of such an origin just because they see in the ape organism a caricature of man, a distorted and unattractive image of themselves ; because it hurts man's .esthetic com- placency and self-ennoblement. It is more flattering to think we have des- cended from some lofty and god-like being ; and so, from the earliest times, human vanity has been pleased to believe in our origin from gods or demi-gods. The Church, with that sophistic reversal of ideas of which it is a master, has suc- ceeded in representing this ridiculous piece of vanity as " Christian humility"; and the very men who reject with horror the notion of an animal origin, and count themselves "children of Ciod," love to prate of their "humble sense of servi- tude." In most of the sermons that have poured out from pulpit and altar against the doctrine of evolution human vanity and conceit have been a conspicuous element ; and, although we have inherited this very characteristic weakness from the apes, we must admit that we have developed it to a higher degree, which is entirely repudiated by sound and normal intelligence. We are greatly amused at all the childish follies that the ridiculous pride of ancestrv has maintained from the Middle Ages to our own time; vet there is a large amount of this empty feeling in RESULTS OF ANTHROPOGENV 353 most men. Just M most people much prefer to trace their family back to some degenerate baron or some famous prince rather than to an unknown peasant) so most men would rather have as parent of the race B sinful and fallen Adam than an advancing and vigorous ape. It i^i matter of taste, and to that extent we cannot quarrel over these genealogical tendencies. Personally, the notion of ascent is more congenial to me than that of descent, li seetns to me a finer thing to be the advanced offspring oi a simian ancestor, that has developed progressively from the lower mammals in the struggle for life, than the degenerate descendant oi a god-like being, made from a clod, and fallen for his sins, and an Eve created from one oi his ribs. Speaking of the rib, 1 may add to what I have said about the development oi the skeleton, that the number oi ribs is just the same in man and woman. In both of them the ribs are formed from the middle germinal layer, and are, from the phylogenetic point oi view, lower ov ventral vertebral arches. But it is said : "That is all very well, as far as the human body is concerned ; on the facts quoted it is impossible to doubt that it has really and gradually been evolved from the long ancestral series of the Vertebrates. But it is quite another thing as regard > man's mind, en- soul ; this cannot possibly have been developed from the vertebrate -soul."1 Let us see if we cannot meet this grave stricture from the well-known facts oi comparative anatomy, physiology, and embryology. It will be best to begin with a comparative study of the souls oi various groups oi Vertebrates, liew we find such an enormous variety oi verte- brate souls that, at first sight, it seems quite impossible to trace them all to a common " Primitive Vertebrate." Think of the tiny Amphioxus, with no real brain but a simple medullary tube, and its whole psychic life at the very lowest stage among the Vertebrates. The following group oi the CydoStOmeS are still very limited, though they have a brain. When we pass on to the fishes, we find their intelligence remaining at a very low level. We do not see any material advance in mental development .until we ixo on to the Amphibia and Reptiles. There is '' The Bngjiah reader will recopiiM here the curious position of Ur. Wallace anU of the late Dr. Mivart.— Tkans still greater advance when we come to the Mammals, though even here the minds oi the Monotremes and of the stupid Marsupials remain at a lowr stage. But when we rise from these to the Placentals we find within this one vast group such a number of important stages of differentiation and progress that the psychic differences between the least in- telligent (such as the sloths and arma- dillos) and the most intelligent Placentals (such as the dogs and apes) are much greater than the psychic differences between the lowest Placentals and the Marsupials or Monotremes. Most cer- tainly the differences are far greater than the differences in mental power between the dog, the ape, and man. Yet all these animals are genetically-related members of a single natural class. We see this to a still more astonishing extent in the comparative psychology of another class of animals, that is especially interesting for many reasons - the insect class. It is well known that we rind in many insects a degree'of intelligence that is found in man alone among the Verte- brates. Everybody knows oi the famous communities and states oi bees and ants, and of the very remarkable social arrange- ments in them, such as we find among the more advanced races of men, but among no other group oi animals. I need only mention the social organisation and government of the monarchic bees and tlie republican ants, and their divi- sion into different conditions — queen, drone-nobles, workers, educators, soldiers, etc. One of the most remarkable pheno- mena in this very interesting province is the cattle-keeping of the ants, which rear plant-lice as milch-cows and regularly extract their honied juice. Still more remarkable is the slave-holding of the large red ants, which steal the young of the small black ants and bring them up as slaves. It has long been known that these political and social arrangements of the ants are due to the deliberate co- operation of the countless citizens, and that they understand each other. A number oi recent observers, especially Fritz Muller, Sir J. Lubbock (Lord Ave- bury), and August Ford, have put the astonishing degree of intelligence oi these tiny Articulates beyond question. Now, compare with these the mental life oi many oi the lower, especially the parasitic, insects, as Darwin did. There is, for instance, the cochineal inset I 354 R ES I rL TS OF A NTHROPOGEX 1 ' (Coccus), which, in its adult state, has a I motionless, shield-shaped body, attached to the leaves of plants. Its feet are atro- j phied. Its snout is sunk in the tissue of the plants of which it absorbs the sap, The whole psychic lite of these inert female parasites cousins in the pleasure they experience from sucking the sap of the plant and in sexual intercourse with the males. It is the same with the maggot-like females of the fan-fly ( Strep- silt ra )t which spend their lives parasi- tical ly and immovably, without wings or feet, in the abdomen of wasps. There is no question here of higher psychic action. If we compare these sluggish parasites with the intelligent and active ants, we must admit that the psychic differences between them are much greater than the psychic differences between the loVvest and highest mammals, between the Monotremes, Marsupials, and armadillos on the one hand, and the dog, ape, or man on the other. Yet all these insects belong to the same class of Articulates, just as all the mammals belong to one and the same class. And just as every consistent evolutionist must admit a common stem-form for all these insects, so he must also for all the mammals. If we now turn from the comparative study of psychic life in different animals to tJje question of the organs of this func- tion, we receive the answer that in all the higher animals they are always bound up with certain groups of cells, the gang- lionic cells or neurona that compose the nervous system. All scientists without exception are agreed that the central nervous system is the organ of psychic life in the animal, and it is possible to prove this experimentally at any moment. When we partially or wholly destroy the central nervous system, we extinguish in the same proportion, partially or wholly, the " soul " or psychic activity of the animal. We have, therefore, to examine the features of the psychic organ in man. The reader already knows the incontest- able answer to this question. Man's psychic organ is, in structure and origin, just the same organ as in all the other vertebrates. It originates in the shape of a simple medullary tube from the outer membrane of the embryo— the skin-sense layer. The simple cerebral vesicle that is formed by the expansion of the head-part of this medullary tube divides by trans- verse constrictions into five, and these pas- through more or less the same stages of construction in the human embryo as in the rest of the mammals. As these are undoubtedly of a common origin, their brain and spinal cord must also have a common origin. Physiology teaches us further, on the ground of observation and experiment, that the relation of the "soul" to its organ, the brain and spinal cord, is just the same in man as in the other mammals. The one cannot act at all without the other; it is just as much bound up with it as muscular movement is with the muscles. It can onlydevelop in connection with it. If we are evolutionists at all, and grant the causal connection of onto- genesis and plrvlogenesis, we are forced to admit this thesis : The human soul or psyche, as a function of the medullary tube, has developed along with it ; and just as brain and spinal cord now develop from tjie simple medullar) tube in every human individual, so the human mind or the psychic life of the whole human race has been gradually evolved from the lower vertebrate soul. Just as to-day the intri- cate structure of the brain proceeds step by step from the same rudiment in every human individual — the same five cerebral vesicles— as in all the other Craniotes ; so the human soul has been gradually developed in the course of millions of years from a long series of craniote-souls. Finally, just as to-day in every human embryo the various parts of the brain differentiate after the special type of the ape-brain, so the human psyche has pro- ceeded historically from the ape-soul. It is true that this Monistic conception is rejected with horror by most men, and the Dualrstic idea, which denies the in- separable connection of brain and mind, and regards body and soul as two totally different things, is still popular. But how can we reconcile this view with the known facts of evolution ? It meets with difficulties equally great and iusuperable in embryology and in phylogeny. If we suppose with the majority of men that the soul is an independent entity, which has nothing to do with the body origi- nally, but merely inhabits it for a time, and gives expression to its experiences through the brain just as the pianist does through his instrument, we must assign a point in human embryology at which the soul enters into the brain ; and at death again we must assign a moment at which it abandons the body, As, further, each human individual has inherited certain RESULTS OF ANTHROPOGENY 355 personal features from each parent, we must suppose that in the act of concep- tion pieces were detached from their souls and transferred to the embryo. A piece of the paternal soul goes with the sperma- tozoon, and a piece of the mother's soul remains in the ovum. At the moment of conception, when portions of the two nui lei of the copulating cells join together to form the nucleus of the stem-cell, the accompanying fragments of the imma- terial souls must also be supposed to coalesce. On this Dualtstic view the phenomena of psychic development are totally incom- prehensible. Everybody knows that the new-born child has no consciousness, no knowledge of itself and the surrounding world. Kvei v parent who has impartially followed the mental development of his children will find it impossible to deny that it is a ease of biological evolutionary processes. Just as all other functions of the body develop in connection with their organs, so the soul does in connection with the brain. This gradual unfolding of the soul of the child is, in fact, so wonderful and glorious a phenomenon that every mother or father who has eyes to observe is never tired of contemplating it. It is only our manuals of psychology that know nothing of this development ; we are almost tempted to think some- times that their authors can never have had children themselves. The human soul, as described in most of our psycho- logical works, is merely the soul of a learned philosopher, who has read a good many books, but knows nothing of evolu- tion, and never even reflects that his own soul has had a development. When these Pualistic philosophers are consistent they must assign a moment in the phytogeny of the human soul at which it was first " introduced" into man's verte- brate body. Hence, at the time when the human body was evolved from the anthropoid body of the ape (probably in the Tertiary period), a specific human psychic element -or, as people love to say, "a spark of divinity" — must have been suddenly- infused or breathed into the anthropoid brain, and been associated with the ape-soul already present in it. I need not insist on the enormous theo- retical difficulties of this idea. 1 will only point out that this " spark of divinity," which is supposed to distinguish the soul of man from 1li.it nf the other animate, must be itself capable of development, and has, as a mallei ^\ fact, progressively developed in the course of human history. As a rule, reason is taken to be this "spark of divinity," and is supposed to be an tXt hisivc possession oi humanity. Hut comparative psychology shows us that it is quite impossible to set up this barrier between man and the brute. Either we take the word "leason " in the wider sense, and then it is found in the higher mammals (ape, dog, elephant, hoise) just as well as in most nun; or else in the narrower sense, and then it is Licking in most men just as much as in the majority of animals. On the whole, we may still say oi man's reason what Goethe's Mcphistopheles said : — Life somewhat belter might content him Hut for the gleam of heavenly light that Thou hast given him. He calls it reason ; thence his power's increased To be still beastlier than anv beast. If, then, we must reject these popular and, in some respects, agreeable Dualistic theories as Untenable, because inconsistent with the genetic facts, there remains only tho opposite or Monistic conception, according to which the human soul is, like any other animal soul, a function of the central nervous system, and devclopes in inseparable connection therewith, we see this* ontogenetically in every child. The biogenetic law compels us to affirm it phytogeneficatty. Just as in every human embryo the skin-sense layer gives rise to the medullary tube, from the anterior end of which the five cerebral vesicles of the Craniotes are developed, and from these the mammal brain (first with the char- acters of the lower, then with those of the higher mammals); and as the whole of this ontogenetic process is only a brief, hereditary reproduction of »he same pro- cess in the phylogenesis of the Verte- brates; so the wonderful spiritual life of the human race through many thousands of years has been evolved step by step from the lowly psychic life of the lower Vertebrates, and the development of every child-soul is only a brief repetition of that long and complex phylogenetic process. From all these facts sound reason must conclude that the still prevalent belief in the immortality of the soul is an unten- able superstition. 1 have shown its in- consistency with modern seience in the eleventh chapter of The Riddle of the Universe. Here it may also be well to point out 3S6 RESULTS OF A.NTHROPOGENY the great importance of anthropogcny, in the light of the biogenetic law, for the purposes of philosophy. The speculative philosophers who take cognisance of these (Mitogenetic facts, and explain them (in accordance with die law) phylogenetically, will advance the great questions of philo- sophy far more than the most distin- guished thinkers of all ages have \et succeeded in doing. Most certainly every clear and consistent thinker must derive from the facts of comparative anatomy and ontogeny we have adduced a number of suggestive ideas that cannot fail to have an influence on the progress of philosophy. Nor can it be doubted that the candid statement and impartial appreciation of these facts will lead to the decisive triumph of the philosophic tendency that we call " Monistic " or " Mechanical," as opposed to the " Dual- istic" or "Teleological," on which most of the ancient, medieval, and modern systems of philosophy are based. The Monistic or Mechanical philosophy affirms that all the phenomena of human life and of the rest of nature are ruled by fixed and unalterable laws; that there is every- where a necessary causal connection of phenomena; and that, therefore, the whole knowable universe is a harmonious unity, a motion. It says, further, that all phenomena are due solely to mechanical or efficient causes, not to final causes. It does not admit free-will in the ordinary sense of the word. In the light of the Monistic philosophy the phenomena that we are wont to regard as the freest and most independent, the expressions of the human will, are subject just as much to rigid laws as any other natural pheno- menon. As a matter of fact, impartial and thorough examination of our "free" volitions shows that they are never really free, but always determined by antecedent factors that can be traced to either here- dity or adaptation. We cannot, there- fore, admit the conventional distinction between nature and spirit. There is spirit everywhere in nature, and we know of no spirit outside of nature. Hence, also, the common antithesis of natural science and mental or moral science is untenable. Every science, as such, is both natural and mental. That is a firm principle of Monism, which, od its reli- gious side, we may also denominate Pantheism. Man is not above, but in. nature. It is true that the opponents of evolu- tion low to misrepresent the Monistic philosophy based on it as " Materialism," and confuse the philosophic tendency of this name with a wholly unconnected and despicable, moral materialism. Strictly speaking, it would be just as proper to call our system Spiritualism as Material- ism. The real Materialistic philosophy affirms that the phenomena of life are, like all other phenomena, effects or pro- ducts of matter. The opposite extreme, the Spiritualistic philosophy, says, o\\ the contrary, that matter is a product of energy, and that all material forms are produced by free and independent forces. Thus, according to one-sided Materialism, the matter is antecedent to the livwig force; according to the equally one-sided view of the Spiritist, it is the reverse. Both views are Dualistic, and, in my opinion, both are false. For us the anti- thesis disappears in the Monistic philo- sophy, which knows neither matter with- out force nor force without matter. It is only necessary to reflect for some time over the question from the strictly scien- tific point of view to see that it is impos- sible to form a clear idea of either hypothesis. As Goethe said, " Matter can never exist or act without spirit, nor spirit without matter." The human " spirit " or " soul " is merely a force or form of energy, insepar- ably bound up with the material sub- stratum of the body. The thinking force of the mind is just as much connected with the structural elements of the brain as the motor force of the muscles with their structural elements. Our mental powers are functions of the brain as much as any other force is a function of a material body. We know of no matter that is devoid of force, and no forces that are not bound up with matter. When the forces enter into the phenomenon as movements we call them living or active forces ; when they are in a state of rest or equilibrium we call them latent or poten- tial. This applies equally to inorganic and organic bodies. The magnet that attracts iron filings, the powder that ex- plodes, the steam that drives the loco- motive, are living inorganics; they act by living force as much as the sensitive Mimosa does when it contracts its leaves at touch, or the venerable Amphioxus that buries itself in the sand of the sea, or man when he thinks. Only in the latter cases the combinations of the different forces that appear as "movement' in the res i v. rs or .1 x throw >ge \v 357 phenomenon are much more intricate .md i difficult to analyse than in the former. Our study has led us to the conclusion thai in the whole evolution oi man, in his embryology and in his phytogeny, there an' no living forces al work other than those of the icst of organic and inorganic nature. All the forces that are operative in it could be reduced in the ultimate analysis to growth, the fundamental evolutkmar) function that brings about the forms oi both the organic and the inorganic. Hut growth itself depends ow the attraction and repulsion of homo- geneous and heterogeneous particles. Seventy-five years ago Carl Krnsi \o\-\ Baer summed up the general result of his classic studies of animal development in the sentence : "The evolution oi the indi- vidual is the history of the growth of indi- viduality in every respect." And if we go deeper to the root of this law oi growth, we find that in the long run it c,o\ always he reduced to that attraction and repul- sion oi anini ited atoms which EmpedocleS vailed the "lo\e and hatred" o\' the elements. Thus the evolution oi man is directed by the same "eternal, iron laws " as the development ot" any other hod v. These laws always lead us hack to the same simple principles, the elementary prin- ciples of physics and chemistry. The various phenomena of nature only differ in the decree oi complexity in which the different forces work together. Each single process oi adaptation and heredity in the stem-history oi our ancestors is in itself a very complex physiological phe- nomenon. Far more intricate are the processes oi human embryology ; in these condensed and comprised thousands of the phylogenetic processes. In my General Morphology, which appeared in 1S06, I made the first attempt to apply the theory oi evolution, ris re- formed bv Darwin, to the whole province of biology, and especially to provide with its assistance a mechanical foundation for the science of organic forms. The intimate relations that exist between all parts oi organic science, especially the direct causal nexus bet ween the two sections of evolution ontogeny and phy- fogeny -were explained in that work for the first time by tratisformism, and were interpreted philosophically in the light of the theory of descent. The antliropo- I Igical part oi the dene nil Morphology (Hook vii.) contains the first attempt to determine the series oi mails ancestors (vol. ii., p. 428). However imperfect this attempt was, it provided a stai t ing-poinl foi further investigation. In the thirty- seven years that have since elapsed the biological horizon has been enormously widened; our enipir'ual acquisitions in paleontology, comparative anatomy, and ontogeny have grown to an astonishing extent, thanks to the united efforts of a number of able woi kers and the employ- ment of better methods. Many important biological questions that then appealed to be obscure enigmas seem to be entirely settled. Darwinism arose like the dawn oi a new day of clear Monistic science after the dark night of mystic dogmatism, and we y a\\ say now, proudly and gladly, that there is daylight in our field oi inquiry. Philosophers and others, who are equally ignorant of the empirical sources oi our evidence and the phylogenetic methods oi utilising it, have even lately claimed that in the matter of constructing our genealogical tree nothing more has been done than the discovery oi a " gallery oi ancestors," such as we find in the mansions of the nobility. This would be quite true if the genealogy given in the second pari oi this work were merely the juxtaposition of a series of animal forms, oi which we gathered the genetic con- nection from their external physiognomic resemblances. As we Have sufficiently proved already, it is for us a question oi a totally different thing — of the morpho- logical and historical proof of the phylo- genetic connection ot these ancestors the basis of their identity in internal structure and embryonic development ; and I think I have sufficiently shown in the first part oi this work how far this is calculated to reveal to us their inner nature and its historical development. I see tile essence oi its significance preci in the proof of historical connection. I am one of those scientists who believe in a real " natural history," and who think as much of an historical knowledge of the past as oi ail exact investigation oi tile present. The incalculable value oi the historical consciousness cannot be suffi- ciently emphasised at a time when historical research is ignored and neglected, and when an ** exact w school, .is dogmatic as it is narrow, would sub- stitute for it physical experiments and mathematical formulae. Historic; I know- ledge cannot be replaced by any other branch oi science. 35« RESULTS OF AXT/IROPOGENY It is clear that the prejudices that stand in the way of a general recognition of this " natural anthropogeny " are still very great ; otherwise the long struggle of philosophic systems would have ended in favour oi Monism. But we may con- fidently expect that a more general acquaintance with the genetic facts will gradually destroy these prejudices, and lead to the triumph of the natural con- ception of "man's place in nature." When we hear it said, in face of this expectation, that this would lead to retro- gression in the intellectual and moral development of mankind, 1 cannot refrain from saying that, in my opinion, it will be just the reverse; that- it will promote to an enormous extent the advance of the human mind. All progress in our know- ledge of truth means an -advance in the higher cultivation of the human intel- ligence ; and all progress in its applica- tion to practical life implies a. correspond- ing improvement of morality. The worst enemies of the human race — ignorance and superstition — can only be vanquished by truth and reason. In any case, I hope and desire to have convinced the reader of these chapters that the true scientific comprehension of the human frame can only be attained in the way that we- recog- nise to be the sole sound and effective one in organic science generally— namely, the way of Evolution. I N n E X Abiogenrsis, 26 Accipmser, 234 Abortive OVa, 55 Achromaun, 42 Achromin, 42 Accela, 221 Acoustic nerve, the, 289, 290 Acquired characters, inherit- ince of, 349 Acrania, the, i8-\ --'•> Acroganglion, the, a68, -75 Adam's apple, the, 184. Adapida, 237 Adaptation. 3, 5, 27 After-birth, the, 167 Agassi z, L., 34 Ape of life, 200 Alimentary canal, evolution of the, 13, 14, 133, 308-17 structure of the, 169, 308-10 Allantoic circulation, the, 171 Allantois, development of the, 166 Allmann, 20 Amblystoma, 243 Amitotic cleavage, 40 Ammoconida, 217 Ammolynthus, 217 Amnion, the, 1 15 formation of the, 134, 244 Amniotic fluid, the, 134 Amoeba, the, 47-9, 210 Amphibia, the, 239 A mpli ich a'ru s , 221 Amphigastrula, 80 Amphjoxus, the, 105, 181-95 circulation of the, 184 - ccelomation of the, 95 embryology of the, 191-95 structure of the, 183-88 Amphirhina, 230 Anamnia, the, 1 15 Anatomy, comparative, 208 Animalculists, 12 Animal layer, the, 16 Annelids, the, 142, 219 Annelid theory, the, 142 Anomodontia, 246 Ant, intelligence of the, 353 Anthropithtcus, 174,262 Anthropogeny, 1 Anthropoid apes, the, 166, 1 73, 262 Anthropology, 1, 35 AnthropOZOIC period, 203 Antimera, 107 Anura, 243 Anus, the, 317 Anus, formation of the, 139 Aorta, the, 337 development of the, 170 Ape and man, 157, 164, 261, >*°7, 35' Ape-man! the, 263, 264 Apes, the, 257 60 Aplniiiofapsa, 210 Aphanostomum, 221 Appendicaria, 197 Appendix vermiformis, the, 32 Aquatic life, early prevalence of, »35 Ararat, Mount, 24 Archenteron, 64, 74 Archeolithic age, 203 Archicaryon, 53 Archicrania, 230 Archigastrula, 65, 193 Archiprimas, 263 Arctopitheca, 261 Area, the germinative, 121 Aristotle, 9 Arm, structure of the, 306 Arrow-worm, the, 191 Arterial arches, the, 325-26 cone, the, 324 Arteries, evolution of the, 170, 323-24 Articulates, the, 142, 219 skeleton of the, 294 Articulation, 141-42 Aryo - Romanic languages, the, 203 Ascidia, The, 181, 188-90 embryology of the, 196-98 Ascula, 217 Asexual reproduction, 51 Atlas, the, 247 Atrium, the, 183, 185' (heart), the, 326 Auditory nerve, the, 289, 290 Auricles of the heart, 325 Autolemnres, 257 Axolotl, the, 243 Bacteria, 38, 210 Beer] K. E. von, 15-17 1 Balanoglossus, 226 Balfour, F., 21 Batrachia, 241 Bdellostoma S/ou/i, 78 Bee. generation of the, 9 Beyschlag,W.,on evolution, 50 Bilateral symmetry, 66 I origin of, 221 Bimana, 258 Biogenetic law, the, 2, 21, 2^, 1 79. 349 359 Biogeny, 2 Bionomy, 33 Bird, evolution of the, 245 ovum oi the, 44-6, 80-1 Bischoff, \\'.. 17 Bladder, evolution of the, 244, 339 Blastaea, the, 206, 213 Blastoceel, the, 62, 74 Blastocrene, the. 99 Blastocysts, the, 62, 119, 120 Blastoderm, the, 62 Blastodermic vesicle, the, 119 Blastoporus, the, 64 Blastosphere, the, 62, 1 19 Blastula, the, 62, 74 the mammal, 119 Blood, importance of the, 318 recent experiments in mixture of, 172 structure of the, 319 Blood-cells, the, 319 Blood-vessels, the, 318-25 development of the, 168 of the vertebrate, no origin of the, 320-21 Boniface VIII., Bull of, 10 Bonnet, 13 Borneo nosed-ape, the, 164 Boveri, Theodor, 185 Brachytarsi, 257 Brain and mind, 278, 354-56 evolution of the, 8, 275-80 in the fish, 276 in the lower animals, 275 structure of the, 273-74 Branchial arches, evolution of the, 303 cavity, the, 183, 189 system, the, 1 10 Branchiotomes, 149 Breasts, the, 1 13 Bulbilla, 184 Calamichthys, 234 Calmly nthus, 217 Capillaries, the, 323 Caracoideum, the, 249 Carboniferous strata, 202 Curcliarodon, 234 Cardiac cavity, the, 170 Cardioccel, the, 328 Catallacta, 213 Caryobasis, 38, 54 Caryokinesis, 42 Caryolymph, 38, 54 Caryolyses, 42 Carj on, 37 Caryoplasm, 37 36o INDEX Catarrhina;, the, 173, 261 Catastrophic theory, the, ^4 Caudate cells, 53 Cell, life of the, 41-3 nature ofthe, 36-7 size of the, 38 Cell theory, the, 18, 36 Cenogenesis, 4 Cenogenetic structures, 4 Cenozoic period, the, 203 Central body, the, 3S, 4.- Central nervous system, the, Ccntrolecithal ova, 68 Centrosoma, the, 38, 42 Ceratodus, the, 76, 237 Cerebellum, the, 274 Cerebral vesicles, evolution of the, 276 Cerebrum, the, 273 Cestracion Japonicus. 75, 79 Chaetognatha, 94 Chick, importance of the, in embryology, II, 16 Child, mind ofthe, 8, 355 Chimpanzee, the, 174, 262 Chiromys, 257 Chiroptera, 258 Chirotherium, 239 Chondylarthra, 257 Chorda, the, 17, 95, 107, 183 evolution ofthe, 296 Chordcea, the, 97 Chordalemma, the, 296 Chordaria, 97 Chordula, the, 3, 96, I91 Choriata, the, 166 Chorion, the, 119 developmentof the, 165-6 frondosum, 255 laeve, 255 Choroid coat,' the, 286 Chorology, 33 Chromacea, 209 Chromatin, 42 Chroococcacea, 210 Chroococcus, the, 210 Church, opposition of to science in Middle Ages, 10 Chyle, 318 Chyle-vessels, 324 Cicatricula, the, 45, 81 Ciliated cells, 53, 193 Cinghalese gynecomast, 114 Circulation in the lancclet, 184 Circulatory system, evolution ofthe, 321-25 structure ofthe, 318 sification, 103 evolutionary value of, 33 Clitoris, the, 345 Cloaca, the, 249, 317 Cnidaria, 217 Coccyx, the, 295 Cochineal insect, the, 354 Cochlea, the, 289 Ccecilia, 241 Caecum, the, 310, 317 Coelenterata, 20, 91, 93, 104 Coslenteria, 221 Coeloma, the, 21, 64, 91 Caelormea, the, 98 Ccelomaria, 21, 91, 104, 221 Coslomation, 93-4 Coelom-theory, the, 21, 93 Coelomula, the, 98 Colon, the, 310, 317 Comparative anatomy, 31 Conception, nature of, 51 Conjunctiva, the, 286 Conocyema, 215 Convotuta, 221 Copelata, the, 197 Copulative orgr.ns, evolution ofthe, 344-45 Corium, the, 108, 268 Cornea, the, 286 Corpora cavernosa, the, 345, 346 Corpora quadrigemina, 274 Corpora striata, 274 Corpus callosum, the, 274 Corpus vitreum, the, 285 Corpuscles ofthe blood, 319 Craniology, 303 Craniota, the, 182, 229 Cranium, the, 299 Creatiofr, 23-4 Cretaceous strata, 202 Crossopterygii, 234 Crustacea, the, 142, 219 Cryptocoela, "221 Cryptorchism, 114 Crystalline lens, the, 285 development ofthe, 287 Cutaneous glands, 268 Cuttle-fish, embryology ofthe, 9 Cuvier, G. , 17, 24 Cyanophycea, 209 Cyclostoma, the, 188, 230-32 ova of the, 75 Cyemaria, 214 Cynopitheca, 262 Cynthia, ,191, 196 Cytoblastus, the, 37 Cytodes, 40 Cytoplasm, 37, 38 Cytosoma, 37 Cytula, the, 54 Dalton, 15 Darwin, C, 2, 5, 23, 28-9 E., 28 Darwinism, 5, 28 Decidua, the, 167 Deciduata, 255 Deduction, nature of, 208 Degeneration theory, the, 219 Dentition of the ape and man, 259 Depula, 62 Descent of Man, 30 Design in organisms, ^ Deutoplasm, 44 Devonian strata, 202 Diaphragm, the, 309 evolution of the, 328 Dicyema, 215 Dicyemida, 21c Didelphia, 248 Digonopora, 223 Dinosauria, 202 Dipneumones, 238 Dipneusta, 235-38 ova of the, 75 Dipnoa, 236 Directive bodies, 54 Discoblastic ova, 68 Discoplacenta, 255 Dissatyrus, 174 Dissection, medieval decrees against, 10 Dohrn, Anton, 219 Dollinger, 15 Dorsal furrow, the, 125 shield, the, 123 — — zone, the, 129 Dromatherium, 248 Dualism, 6 Dubois, Eugen, 263 Ductus Botalli, the, 350 Ductus venosus Arantii, 350 Duodenum, the, 309, 317 Duration of embryonic development, 199 of man's history, 199 Dysteleology, 32 proofs of, 349 Ear, evolution ofthe, 288-92 structure of the, 288 uselessness of the ex- ternal, 32 Ear-bones, the, 289 Earth, age of the, 200-201 Echidna hystrix, 249 Ectoblast, 20, 64 Ectoderm, the, 20, 64 Edentata, 250 Efficient causes, 6 Egg of the bird, 44 6, 81 — — or the chick, priority of the, 21 1 Elastnobranehs, the, 79 Embryo, human, development ofthe, 158 Embryology, 2 evolutionary value of, 34 Embryonic development, duration of, 199 disk, the, 121-22 spot, the, 125 Encephalon, the, 273 INDEX .tfi Endoblast, 20, 64 Endothelia, 321 Enterocoela, 93, 223 Knteropneusta, 226 Entoderm, the,. 20, 64 Eocene strata, 203 Eopitheca, 259 Epiblast, 20, 64 Epidermis, the, 108, 268 Epididymis, the, 34a Epigastrula, 80 Epigenesis, 11, 13 Epiglottis, the, 305 Epiphysis, the, 108 Episoma, 129 Episomites, 130, 194 Epispadia, 346 Epithelia, 37 Epitheria, 243, Epovarium, the, 342 Equilibrium, sense of, 291 Esthonychida, 257 Eustachian tube, the, 289 Eutheria, 253 1 2 Evolution theory, the, u, 208 inductive nature of, 30 Bye, evolution of the, 285-88 structure of the, 285 Eyelid, the third, 32 Eyelids, evolution of the, 288 Fabriciis ab Aquapendente, 10 Face, embryonic development of tin Fat glands in the skin, 269 Feathers, evolution of, 270 Fertilisation, 51 place of, 1 19 Fin, evolution of the, 239, 304 Final causes, 6 Flagellate cells, 193 Floating bladder, the, 233, 241 evolution of the, 314 Fcetal circulation, 170-71 Food-yelk, the, 67. 1 16-17 Foot, evolution of the, 241, 304-6 of the ape and man, 258- 59 Fore brain, the, 278 Fore kidneys, the, 336, 337 Fossiliferous strata, hsl of, 201 Fossils, 180 scarcity of, 208 Free will, 356 Friedenthal, experiments of, '7-' Frog, the, 241-42 ova of the, 7 1 a Frontpnia, -'-'4 Function and structure, 7 Furcation of ova, 72 ^AMh ' t, 341, 350 Cianglia, commencement of, 268 Ganglionic cell, the, 39 Ganoids, 233, 234 Gaatrsea, the, 3, 20, 206 formation of the, 213 Gastraea theory, the, 20, (>4,6q Ga«uraeads, 69, 214 Gastremaria, 214 Gastrocystis, the, 62, 119, 120 Gastrophysema, 215 Gaatrotricha, 224 Gastrula, the, 3, 20, 62 Gastmlation, 62 Gcgenbaur, Carl, 220 on evolution, 32 on the skull, 300-1 Gemmation, 331 Genera! Morphology, 8, 29 Genesis, 23 Genital pore, the, 335 Geological evolution, length of, 200 periods, 201 Geology, methods of, 180 rise of, 24 Germ-plasm, theory of, 349 Germinal disk, 46, 81 layers, the, 14, 16 scheme of the, 92 spot, the, 44 vesicle, the, 43, 54 Germinative area, the, 121 Giant gorilla, the, 176 Gibbon, the, 173, 262 Gill-clefts and arches, 110 formation ofthe, 151-2,303 Gill-crate, the, 183, 189 Gills, disappearance ofthe, 244 Glceocapsa, 210 Gnathostoma, 230, 232 Goethe as an evolutionist, 27, 299 Goitre, 1 10 Gonads, the, 1 1 1 formation ofthe, 149-50 Gonidia, 334 Gonocborism, beginning of, 3" Gonoducts, 335 Gonotomes, 146, 149 Goodsir, 189 Gorilla, the, 174, 176, a6a Graafian follicles, the, 17, 1 19, 347 Gi eganrue, 21 1 Gullet-ganglion, the, 190 Gut, evolution of the, ]\<> 17 Gyrini, 242 Gynecomastism, 114 11 u.-i isii, the, 188 Hair, evolution ofthe, 270 on the human embryo and infant, 271 Hair, restriction of, by sexual selection, j~ i Haliphysema, 215 I lalisauria, 202 Mallei , Albrecfat, 12 Halosplueru --iridis, 213 Hand, evolution of the, . 304-6 ofthe ape and man, 258 1 lapalidae, 261 Harderian gland, the, 288 Hare-lip, 284 Harrison, Granville, 161 Hartmann, 262 Harvey, 10 Hatschek, 192 Hatteria, 243, 246 Head-cavity, the, 138 Head-plates, the, 149 Heart, development of the, 7, 10, m, 151, 170, 322, 324-27 of the ascidia, the, 190 position of the, 327 Helmholtz, 207 Helminthes, 223 Hepatic gut, the, 109, 316 Heredity, nature of, 3, 5, 27, 56"7. 349 Hermaphrodism, 9, 23, 114, I 218, 322, 346 Hertwig, 2 1 1 [esperopitbeca, 259 11. s, \V., 19 1 listogeny, 18, 19 History of Creation, 6, 30 Holoblastic ova, 67, 71, 77 , Homaosaurus, 244, 246 Homology of the germinal layers, 20 Hoof, evolution ofthe, 270 Hunterian ligament, the, 344 Huxleian law, the, 171,257,362 Huxley, T. H., 7, 20, 29 1 [ydra, the, 69, 217 1 Hydrostatic apparatus in the "fish, 315 Hylobates, 173, 262 Hy lodes Mart in icen si's, 241 Hyo'.d bone, the, 299 I [ypermastism, 1 13 1 [yperthelis* , 1 13 Hypoblast, 20, 64 Hypobranchial groove, the, I IO, 184, 22(l, 316 1 1 . podermis, the, 268 I [ypopsodina, 257 I I \ posoma, the, 129 Hyposomites, 130, 194 1 [ypospadia, 346 bill HYDINA, 224 Ichthyophis gtutinosa, 80 [ctopsida, 257 Ileum, the, 310 Immortality, Aristotle o\\. 10 j62 INDEX Immortality of the soul, 58 Impregnation-rise, the, 55 Indecidua, 255 Indo-Germanic languages, 203 Induction and deduction, 31, 208 Inheritance of acquired char- acters, 349 Insects, intelligence of, 353 Interamniotic cavity, the, 165 Intestines, the, 309, 316-17 Invagination, 62 Iris, the, 286 Jacchus, 261 Java, ape-man of, 263, 264 Jaws, evolution of the, 301 Jurassic strata, 202 Kant, dualism of, 25 Kelvin, Lord, on the orierin of life, 207 Kidneys, the, 1 1 1 formation of the, 150-5^ 336 42 Klaatsch, 262 Kolliker, 21 Kowalevsky, 191 Labia, the, 346 Labyrinth, the, 290 Lachrymal glands, 269 Lamarck, J., 23,25-7 theories of, 26, 340 Lamprey, the, 230 ova of the, 75 Lancelet, the, 60, 181-95 description of the, 105 Languages, evolution of, 203 Lanugo of the embryo, 271 Larynx, the, 309 evolution of the, 314 Latebra, the, 45 Lateral plates, the, 129 Laurentian strata, 201 Lecithoma, the, 117 Leg, evolution of the, 304 structure of the, 306 Lemuravida, 257 Lemurogona, 257 Lemurs, the, 257 . Lepidosiren, 257 Leucocytes, 319 Life, age of, 200 Limbs, evolution of the, 152, .239- 304 Limiting furrow, the, 133 Linin, 42 Liver, the, 309, 317 Long-nosed ape, the, 164 Love, importance of in nature, 332 Lungs, the, no evolution of the, 241, 3'4-'5 Lyell, Sir C, 24 Lymphatic vessels, the, 318 Lymph-cells, the, 319 Macrogonidion, 331 Macrospores, 331 Magosphcpra planula, 213 Male womb, the, 344, 350 Mallochorion, the, 166 Mallotheria, 257 Malpighian capsules, 339, 341 Mammal, characters of the, 1 12 | gastrulation of the, 84 Mammals, unity of the, 247-48 ] Mammary glands, the, 1 13,269' Man and the ape, relation of, ; 262, 351 origin of, 29 Man's Place in Nature, 7, 29, 351 Mantle, the, 189 Mantle-folds, the, 185 Marsupials, the, 250-52 ova of the, 85 Materialism, 356 Mathematical method, the, 30 Mechanical causes, 6 embryology, 8, 19, 22 Meckel's cartilage, 304 Medulla capitis, the, 273 oblongata, the, 274 spinalis, the, 273 Medullary groove, the, 125 tube, the, 107, 128 formation of the, 131, 133, 227, 267, 276 Mehnert, E. , on the biogenetic law; 5 Meroblastic ova, 67, 71, 78 Merocytes, 68, 321 - Mesentery, the, 98, 109,310,316 Mesocardium, the, 327 Mesoderm, the, 20, 64, 90, 93 Mesogastria, 215 Mesonephridia, the, 338 Mesonephros, the, 336 Mesorchium, the, .344 Mesovarium, the, 344 Mesozoic period, the, 202 Metogaster, the, 64 Metagastrula, the, 67 Metamerism, 142 Metanephridia, the, 341 Metanephros, the, 336 Metaplasm, 39 Metastoma, 64, 222 Metatheria, 248 Metazoa,-2o, 62 Metovum, the, 81 Microgonidian, 331 Microspores, 331 Middle ear, the, 291 Migration, effect of, 33 Milk, secretion of the, 269 Mind, evolution of, 353-54 in the lower animals, 353 Miocene strata, 203 Mitosis, 40, 41 Monera, 40, 206, 209 Monism, 6, 356 Monodelphia, 248 Monogonopora, 223 Monopneumones, 238 Monotremes, 118, 249 ova of the, 84 Monoxenia Darwinii, 60 Moraea, the, 212 Morphology, 2, 27 Morula, the, 62, 212 Motor-germinative layer, the, '9 Mouth, development of the, 124. 139 — — structure of the, 308 Mucous layer, the, 16 Mullerian duct, the, 341 Muscle-layer, the, 16 Muscles, evolution of the, 307 of the ear, rudimentary, 292 Myotomes, 108, 146 Myxinoides, the, 188, 230 Nails, evolution of the, 270 Xasal pits, 284 Natural philosophy, 25 selection, 26, 28, 349 Navel, the, 117, 134 Necrolemurs, 257 Nectocystis, the, 314 Nemertina, 224-26 Xephroduct, evolution of the, 338-39 Xephrotomes, 149, 338 Nerve-cell, the, 39 Nerves, animals without, 267 Nervous system^ evolution of the, 7, 267 Neurenteric canal, the, 127 Nictitating membrane, the, 32, 286, 288 Nose, the, in man and the ape, 164 development of the, 282- 85 structure of the, 283 Notochorda, the, 107 Nuclein, 37 Nucleolinus, 44 Nucleolus, the, 38, 44, 54 Nucleus of the cell, 37 (Esophagus, the, 309, 316 Oken, 5, 27, 300 Oken's bodies, 339 Oligocene strata, 203 Olynthus, 217 Onthegeneration of animals.g Ontogeny, 2, 23 defective evidence of, 208 Opaque area, the, 122 tNDEX 3°3 Opossum, the, 252 ova of the, 85 Optic nerve, the, 287 Optic thalami, 274 vesicles, the, 286 Oraor, the, 174, 262 Ornithodelphia, 248 Ornithorhyncus, 85, 249 Ornithostoma, 249 Ossicles of the ear, 289 Otoliths, 289 Ova, number of, 347 of the lancelet, 192 Ovaries, evolution of the, 333- M Ol iduct, origin of the, 335, 342 Ovolemma, the, 44 Ovulists, 12 Ovum, discover)- of the, 16 nature of the, 40, 43-5 size of the, 44 1' uhyi.kmirs, the, 257 Pacinian corpuscles, 282 Paleontology, 2 — - evolutionary evidence of, incompleteness of, 208 rise of, 24 Paleozoic age, the, 202 Palingenesis, 4 Palingenetic structures, 4 PaLchatteria, 244, 246 Panniculus carnosus, the, 350 Paradidymis, the, 342 Parietal zone, the, 129 Parthenogenesis, 9, 13 Pastrana, Miss Julia, 164 Pedimana, 252 Pellucid area, the, 122 Pelvic cavity, the, 138 Pemmatodiscus gastrulaceus, 2I5 Penis-bone, the, 346 Penis, varieties of the, 345 Peramelida, 254 IVriblastic ova, 68 Peribranchial cavity, the, 185, 193 Pericardial cavity, the, 328 Perichorda, the, 108, 183 formation of the, 136 Perigastrula, 89 Permian strata, 202 Petromyzontes, the, 188, 230 Phagocytes, 49, 320 Pharyngeal ganglion, tins J75 Pn irynx, the, 309 Philology, comparison with, 203 Philosophic Zoologique, 25 Philosophy and evolution, 6 Phycochromacea, 209 Phylogeny, 2, 23 Physemaria, 214 Phvsiology, backwardness of 7 Phytomonera, 209 Pineal eye, the, 108 Pinna, the, 201 Pithecanthropus, -('3, -Oi Pithecometra - principle, the, ■7' Plac.enta, the, 166, 253-54 Placentals, the, 166 I characters. of the, iu ! gastrulation of the, 86 Planocytes, 49, 320 Plant-louse; parthenogenesis of the, 13 Planula, the, 89 Plasma-products, 38, 39 Plasson, 40, 59 Plastids, 36, 40, 209 Plastidulesj 59 ' Platodaria, 221 Platodes, the, 221 Platyrrhina?, 261 Pleuracanthida, 234 Pleural ducts, 328 Pliocene strata, 203 Polar cells, 54 Polyspermism, 58 Preformation theory, the, 11 Primary period, the, 202 Primates, the, 157, 257-60 Prirnatoid, 263 Primitive groove, the, 69, 82, 124, 125 gut, the, 20, 63, 214 kidneys, the, III, 337 mouth, the, 20, 63 segments, 143 streak, the, 100, 122 vertebrae, 144, 195, 206, 229 Primordial period, the, 201 Prochordata, 192 Prochordonia, the, 192, 218 Prochoriata, 253 Prochorion, the, 44, 119 Proctodeum, the, 345 Procytella primordialis, 210 Prodidelphia, 256 Progaster, the, 20, 63 Progonidia, 333 Promammalia, 247 Pronephridia, the, 151 Pronucleus femininus, 54 masculinus, 54 Properistoma, 69 Prorenal canals of the lance- let, 186 — — duct, the, 132, 139, 186 evolution of the, 338 Proselachii, 234 Prosimife, the, 257 Prospermaria, 333 Prospondy/us, 105, 229 Prostoma, 20, 63, 222 Protamniotes, 243-44 Protamceba, 210 Proterosaurus, the, 202, 244 Protists, 36, 38 Protonephros, m, 336 Protophyta, 210 Protoplasm, 37, 209 Protopttrtts, 238 Prototheria, 2 \B Protovertebrae, 142, 144 Protozoa, 20, 210 Provertebr.il cavity, the, 148 plates, the, 136, 144 Pseudocoela, 93, 321 Pseudopodia, 48 1'siHidova, 13 Psychic life, evolution of the, 8. Psychology, 8 Pterosauria, 202 Pylorus, the, 309 Qladratv M, the, 247 Quadrumana, 258 Quaternary period, 203 Rabbit, ova of the, 86-7 Radiates, the, 103 Rathke's canals, 341 Rectum, the, 317 Regner de Graaf, 119 Renal system, evolution of the, 335-42 Reproduction, nature of, 330- Reptiles, 245-47 Respiratory organs, evolution of the, 314-15 pore, the, 183, 189 Retina, the, 286 Rhabdoccela, 222 Rhodocytes, 319 Phopalura, 215 Khyncocephala, 243 Ribs, the, 295 number of the, 353 Rudimentary ear-muscles, 2Q2 organs, 32 list of, 349-50 toes, 306 Saccllus, the, 289 Sugitta, 65, 66, 191 ecelomation of, 93 Salamander, the, 241 ova of the, 74 Sandal-shape of embryo, 1 28- 29 Satyrus, 174, 262 Sauromammalia, 246 Sauropsida, 245 Scatulation theory, the, 12 Schizomycetes, 210 Schleiden, M., 18, 36 Schwann, T., 18, 36 Sclerotic coat, the, 286 Sclerotomes, 108, 143, 148 ;,t>4 IXDEX ttum, the, 344 ScyUium, nose of the, 283 Sea-squirt, the, 181, 188-^90 Secondary period, the, -02 Segmentation, (x>, 141-42 Segmentation-cells, 54 neotation-sphere, the, 1 7 Selachii, 22 \ — — skull of the, 301 Selection, theory of, 18 Selenka, 166, 168 Semnopitheci, 262 Sense-organs, evolution of tlu\ 151, 280 number of the, 281 origin of the, 281 Sensory nerves, 279 Seroccelom, the, 165 Serous layer, the, 16 Sex-organs, early vertebrate form of the, n 1 evolution of the, 333-47 Sexual reproduction, simplest forms of, 331 selection, 30, 271-72 Shark, the, 233 nose of the, 283 ova of the, 75 placenta of the, 9 skull of the, 301 Shoulder-blade, the, 306 Sickle-groove, the, 82, 121 Sieve-membrane, the, 167 Silurian strata, 202 Simia;, the, 257-60 Siphonophorae, embryology of the, 21 Skeleton, structure of the, 294 Skeleton-plate, the, 148 Skin, the, 151 evolution of, 266-69 function of the, 269 Skin-layer, the, 16 Skull, evolution of the, 149, 299-303 structure of the, 299 vertebral theory of the, 300 Smell, the sense of, 282 Soul, evolution of the, 353-56 nature of the, 58, 356 phylogeny of the, 8 seat of the, 278 Sound, sensations of, 289-90 Sozobranchia, 242 Space, sense of, 291 Species, nature of the, 23, 34 Speech, evolution of, 264 Spermaducts, 335, 342 Spermaries, evolution of the, 333~31 Spermatozoon, the, 52-3 discovery of the, 12, 53 Spinal cord, development of the, 8 Spinal cord, structure of the, ■7 J Spirema, the, 4.- Spiritualism, 356 Spleen, the, 318 Spondyli, 142 Sponges, classification of the, 34 ova of the, 40 Spontaneous generation, 26, 206 Stegocephala, 239 Stem-cell, the, 54 Stem-zone,- the, 129 Stomach, evolution of the, 311-14, 316 structure of the human, 309 Strata, thickness of, 200-201 Struggle for life, the, 28 Subcutis, the, 268 Sweat glands, 269 Tactile corpuscles, 268, 282 Tadpole, the, 242 Tail, evolution of the, 242-43 rudimentary, in man, '59. 295. 35o' Tailed men, 160-61 Taste, the sense of, 282 Teeth, evolution of the, 314 of the ape and man, 259 Teleostei, 234 Telolecithal ova, 67, 68 Temperature, sense of, 282 Terrestrial life, beginning ot, 235 Tertiary period, the, 203 Theoria generationis, the, 13 Theories, value of, 181 Theromorpha, 246 Third eyelid, the, 286, 288 Thyroid gland, the, no, 184. 3>5 Time-variat ions in ontogeny, 5 Tissues, primary and secon- dary, 37 Toad, the, 241 Tocosauria, 246 Toes, number of the, 240 Tori gen if ales, the, 346 Touch; the sense of, 282 Tracheata, 142, 219 Tread, the, 45, 81 Tree-frog, the, 241 Triassic strata, 202 Triton tceniatus, 74 Troglodytes, 174 Tunicates, the, 189 Turbellaria, 222 Turbinated bones, the, 28- Tympanic cavity, the, 288 Umbilical cord, the, 1 17 vesicle, the, 138 Unicellular ancestor of all animals, 47 ' animals, 38, 47 LTrachus, the, 317, 341 Urinary system, evolution of 'he, 335-42 Urogenital ducts, 335 Uterus mascu/inus, the, 344, 35° I trieulus, the, 289 Vasa deferent ia, 335 Vascular layer, the, 16, 168 system, evolution of the, 321-25 structure of the, 318 Vegetative layer, the, 16 Veins, evolution of the, 323-24 Ventral pedicle, the, 166 Ventricles of the heart, 325 Vermalia, 220, 223 Vermiform appendage, the, 32, 310, 317 \ ertebrat, 142, 294 Vertebraea, 105 Vertebral arch, the, 148, 295 column, the, 144 evolution of the, 296 structure of the, 294 Vertebrates, character of the, 104- 10 descent of the, 219-20 Vertebration, 142 Vesico - umbilical ligament, the, 341 Vesicula prostatica, the, 344, 35° Villi of the chorion, 165 Virchow, R. , 35 on the ape-man, 303 on the evolution of man, 264 Virgin-birth, 9, 13 Vitalism, 6 Vitelline duct, the, 138 Volvocina, 213 Wallace, A. R. , 29 Water, organic importance of, 200 Water vessels, 336 Weismann's theories, 349 Wolff. C. F., 13 Wolffian bodies, 339 Wolffian duct, the, 341 Womb, evolution of the,342~43 Yelk, the, 43, 45, 67 Yelk-sac, the, 117, 134 Zona pellucida, the, 44 Zonoplacenta, 255 Zoomonera, 209 Zoophytes, 20,64, 104