% •§. £L pU ^Itbarg ■North Carolina State OloUeae QH3G& H5 j^_L S00260311 D This BOOK may be kept out TWO WEEK ONLY, and is subject to a fine of FIV: CENTS a day thereafter. It is due on ih day indicated below: 17^4. ? 1ar '36 MA I:.. . I El 1 7 ii»/a MR a*** ^AUG31 2001 -3 " vS £S 'JUL 14 199]^ V0C1 ^ m»i THE EVOLUTION OF MAN Vol. I. -HUMAN EMBRYOLOGY OR ONTOGENY THE EMRRYONIC DEVELOPMENT OF THE FACE The Evolution ai \d Fifth Edition Vol. I.— EMBRYOLOGY (ONTOGENY) I. I'm Fundamental Law of Organic Evolution - - i II. The Older Embryology ..... 21 III. Modern Embryology -.-... 37 IV. The Older Phylogeny ------ w V. The Modern Science of Evoli tion - 75 VI. Till Om M VND THE AMCEBA ----- q(, VII. Conception - - - - - - - U4 VIII. Tin Gastrjea Theory ------ [^g IX. The Gastrulation of rHE Vertebrate - - - 174 X. The Ccelom Theory ------ >i& XI. The Vertebrate Character 01 Man ... - 24(1 XII. Embryonic Shield and Germinative Area - - - 273 XIII. Dorsal Body wd Ventral Body - .2^4 XIV. The Articulation of the Body .... 330 XV. FtETAL Membranes and Circulation ... - 361 *^ 'oe LIST OF PLATES IX Vol. I. Plate I. Embryology >>i iiii Him an FaCE. (Frontispiece to firsl volume) - - Explanation see Chapter XXV., Vol. II. rial.- II. Gastrulation 01 Holoblastic Animals (with total seg- mentation) ---... Explanation p. 170 Plate III. Gastrixation >>i Mesoblasth Animals (with partial seg- mentation) ----.. Explanation p. 17.1 Plate IV. Sandal-Shaped Embryos 01 Three Sauropsids (lizard, tortoise, hen) at three different stages - - Explanation p. ;,, ; Plate V. Sandal-Shaped Embryos of Threi Mammals (pig, hare, man) at three different stages - Explanation p. ;.,.; Plate VI. Transversi mmiunm'! V'er ;\ii Embryos. Diagram- matic, germ-layers coloured - - -Explanation pp. 326 j2g Plate VII. Longitudinal Sections .>i V'ertebrati Embryos. Dia- grammatic, germ-layers coloured - - -Explanation pp. 326-329 Plate \ III. Embryos Of Three Reptiles (lizard, serpent, crocodile) at three different stages - Explanation p. 359 Plate IX. Embryos 01 Threi Sacropsids (tortoise, hen, ostrich) at three different stages - Explanation p. 359 Plate X. Embryos 01 Three Sai ropsids (stem-reptile, river-tortoise, kiwi) at three different stages - - - Explanation p. 359 Plate XI. Embryos 01 Three Mammals (hedge-hog, dolphin, gibbon) ai three different stages - Explanation p. 359 Plate XII. Embryos 01 Four Mammals (opossum, pig, goat, ox) al three different stage - Explanation p. 359 Plate XIII. Embryos 01 Four Mammals (dog, bat, hare, man) at three different stages ..... Explanation p. J59 Plate XIV. Human Embryos From iiii Fourth ro 1111: Eighth " ' ' K ----.-_ Explanation pp. 5(14 ^ Plate XV. Human Embryo in iiii Fojtal Membranes (from the second to the twelfth week) .... Explanation p. 411 Plate XVI. Human Embryo, Five Months Old, in iiii, I-'h-iai. Membranes --.... Explanation p. 411 LIST OF FIGURES IN THE TEXT The human ovum Stem-cell of one of the echinoderms - Throe epithelial cells - Five spiny or grooved cells Ton liver cells - Nine star-shaped bone-cells Eleven star-shaped tells - Unfertilised ovum of an echinoderm - Large branching nerve-cell Blood-cells - Indirect or mitotic cell- division - Mobile cells from the in- flamed eye of a frog Ova of various animals The human ovum Fertilised ovum from the oviduct of a hen A creeping amoeba - Division of a unicellular amoeba - Ovum of a sponge Blood-cells, or phagocytes Spermia or spermatozoa - Spermatozoa of various animals - A single human spermato- zoon - Fertilisation of the ovum - Unfertilised ovum of an echinoderm - Impregnated echinoderm ovum - Impregnation of the ovum of a star-fish - 28. Impregnation of the ovum of the sea-urchin - Stem-cell of a sea-urchin - Stem-cell of a hare - Gastrula of a coral - Gastrula of a gastraead Gastrula of a worm - Gastrula of an echinoderm PICt 97 35- 36. 99 .;:■ 100 38. 1O0 39- 58. 129 '3° 59- 60. l33 61. '33 62. '35 63- 136 '37 64. '43 'S° 65- '53 66. '53 67. '53 68. Gastrula of an arthropod - Gastrula o( a mollusc Gastrula of a vertebrate - Gastrula of a lower sponge Cells from the primary ger- minal layers - - - Gastrulation of the amphi- oxus .... Gastrula of the amphioxus Cleavage of the frog's ovum 46. Sections of the fertilised ovum of the toad - Embryonic vesicle of the water-salamander - Embryonic vesicle of triton Sagittal section of a hooded- embryo of triton - Section of the gastrula of triton - Segmentation in the lamprey Gastrulation of the lamprey Gastrulation of ceratodus - Ovum of a pelagic bony fish Segmentation of a bonv fish Discoid gastrula of a bony fish Section of the blastula of a shark - Section of the blastula of a shark - -Mature ovum of a hen Diagram of discoid seg- mentation - - - Section of the blastula of a hen ---.. Germinal disk of the hen's ovum - - _ _ Section of the germinal disk of a siskin Section of the discoid gas- trula of the nightingale Germinal disk of the lizard Ovum of the opossum Blastula of the opossum Blastula of the opossum - '53 '53 '53 '56 '58 '59 160 176 181 1S2 '83 184 184 186 1 88 189 190 '93 '94 196 '97 '99 '99 199 200 202 204 2°5 LIST OF FIGURES IN THE TEXT 76. Gastrula of the opossum - Section of the gastrula of the opossum - - - Stem-cell of th" mammal ovum - - - - Incipient cleavage of the mammal ovum First segmentation-cells ol the mammal ovum - Mammal ovum with eight segmentation-cells - Gastrula of the placental mammal - - - - Gastrula of the hare - 78. Diagram of the four secondary germ-layers - 80. Ccelomula of sagitta - Section of a young sagitta 83. Section of amphioxus- larvs - - - - 85. Section of amphioxus embryo - - - - 87. Chordula of the amphi- oxus - - - - Sg. Chordula of the am- phibia - 91. Section of coelomula- embryos of vertebrates - 93. Section of ccelomula- triton - - - - Section of three triton- embryos - - - - Section of the chordula- embrvo of a bird - Section of the vertebrella- embryo of a bird - 9S. Section of the primitive streak of the chick - Section of the primitive groove of a hare Section of the primitive mouth of a human embryo -105. The ideal primitive vertebrate - - - Instances of redundant mammary glands - A Greek gynecomast , Severance of the discoid mammal embryo - , no. The visceral embryonic vesicle of a hare Four entodermic cells Two entodermic cells -[17. Ovum of a hare - >o6 118. 208 208 123. 208 124. J 09 >-5- 210 126. 219 127. 227 129. 130. 22S '3-- 230 ^3° '34- '35- ,38. ■39' 1 40. 240 ^53 '43- 266 268 148- 277 152. 2S1 282 282 28s '53- '54- '55- Round germinative area of the hare - - - - 286 Oval area - 286 Oval germinal disk of the hare - 288 Pear-shaped germinal shield of the hare - 288 Section of the gastrula of four vertebrates - - 291 Embryonic vesicle of a hare 295 Oval embryonic shield of the hare - 295 Dorsal shield of a hare- embryo - - - - 296 Embryonic shield of a hare 297 Section of the ccelomula of amphioxus - 297 Section of the chordula of a frog - - - - 298 Section of a frog-embryo - 298 131. Dorsal shield of the chick - - - - ^99 Section of the hinder end of a chick - - - - 300 Germinal area of the hare - 300 Embryo of the opossum - 301 Sandal-shaped embryonic shield of a hare - - 301 Human embryo at the sandal-stage - - - 3°' Sandal-shaped embryonic shield of a hare - - 302 Sandal-shaped embryonic shield of an opossum - 304 Section of the embryonic shield of a chick - - 306 Section of the embryonic disk of a chick - - 306 Section of the embryonic shield of a chick - - 307 Sections of the embryonic disk of the higher verte- brates - - - - 309 147. Sections of the maturing mammal embryo and its envelopes - - - 310 151. Sections of chick-em- bryos - - - - 3 '4 Section of the embryo of a chick - - - -317 Section of the fore-half of a chick-embryo- - - 318 Section of a human embryo 320 Section of a human embryo 321 LIST OF FIGURES IX THE TEXT F1GUKE 156. Section of a shark embryo 157. Section of a dink embryo - 158 [60. Embryonic disk of the chick - 161. Embryo of the amphioxus - 162, 163. Embryo of the amphi- oxus - - - . 164-160. Embryo of the amphi- oxus - 167. Section of an amphioxus embryo - 168, 169. Section of shark em- bryos - - - - 170. Frontal section of a triton embryo - - - - 171. Section of a chick embryo - 172. Section of a chick embryo - 173. The third cervical vertebra 174. Tin' sixth dorsal vertebra - 175. The .second lumbar vertebra 176. Section of the trunk of a primitive vertebrate 177. Section of the primitive vertebrate - 17S. Head of a shark embryo - 179, tSo. Head of a chick em- bryo .... 181. Head of a dog embryo 1S2. Human embryo ofthefourth week _ _ - - 183. Section of shoulder of chick embryo ... - 184. Section of pelvic region of chick embryo ... 185. Development of the lizard's legs - 186. Human embryo, five weeks old - [87 189. Embryos of the bat 190. Sandal-shaped human em- bryo . . _ . 191. Human embryos from second to fifteenth week 192. Human embryo of fourth « eek - - - - 193. Human embryo of fifth week .... 194. Section of tail of a human embryo - 195. Tail of a six-months'-old boy - - - - - 196. Human embryo, four weeks old - -' - 197. Human embryo, five weeks 3a> IE old 37o J-- 1 98. Head of Miss Julia Pastrana 372 199. Human ovum (twelve days) 374 334 200. Human ovum (ten days) 374 336 201. Human foetus (ten days) - 374 202. Human ovum (twenty to 337 twenty-two days) - 374 203. Human foetus (twenty to 338 twenty-two days) - 374 204. Human embryo of sixteen 339 to eighteen days - 375 205. Human embryo of fourth 34° week ... - 376 206. Human embryo of fourth 34' week - 376 34' 207. Human embryo with its 342 membranes - 377 344 208. Section of embryo of a chick 377 344 209. Embryonic organs of the 344 mammal - - - - 37S 210. Embryo of a dog 379 347 211. Embryo of a dog 380 212. Section of the pregnant 348 human womb - 3s ' 35° ^'3- 215. Embryos ofthekalawet- gibbon - - - - 382 35° 216. Male embryo of the sia- 35° mang-gibbon - - - 3S3 217. Section of pregnant human 35^ womb - - - - 384 218. Human foetus, twelve weeks 353 old ----- 385 219. Mature human foetus - 386 354 220. Section of the lower half of the trunk of a woman in 355 advanced pregnancy 3S7 221- 225. Sections of the maturing 356 mammal embryo 389 357 226. Section of the embryo of a chick - 39' 3<>4 227. Section of the embryo of a chick - - ' - 39' 365 22S. Section of the embryo of a chick - 392 366 229. Human embryo (fourteen to eighteen days) 393 366 230. Section of the bead of a mammal embryo 394 368 23'- Vitelline vessels in the ger- minative area of a chick 369 embryo - - - - 395 232- Boat-shaped embryo of the 37° dog - - - - 396 -Wv Embryonic shield of a hare 397 LIST OF FIGURES IN THE TEXT 234- 235- 236. 237- Embryonic shield of a hare Lar or white-handed gibbon Young orang - - - Wild orang - - - Head of an old male orang 403 The bald-headed chimpanzee 404 I M , B FILL RE 1 \1.1-- 398 J 4O Female chimpanzee - - 405 41 !<) 241 Female mafuka - - 406 40I 242 Female gorilla - - 4°7 402 243 Male giant-gorilla - 408 403 244 Giant-gorilla - 40b LIST Ob' GENETIC TABLES i. Composition of the organic cell - - ,,- 2. Differences in segmentation and gastrulation ,-, 3. Four embryonal stages in animals - - - - . _ ,-, 4. Chief variations in segmentation ,«, 5. Phytogeny of vertebrate gastrulation - ------ ,,, 6. Four types of vertebrate gastrulation ------ 21s 7. Names of the germinal layers --- ->,•> S. Origin and function of the fundamental organs of the chordula - 243 9. Fundamental organs and body-cavities of the chordula - - - 244 0. Four chief groups of the metazoa ----... 24- 1. Chief organs of the provertebrates - - - - - . 1-, 2. Composition of the amniote embryo 2q? 3. Composition of the vertebrate body ------ n2. 4. Organisation and articulation of the vertebrates and articulates - 360 5. Embryonic plates of the vertebrates - 410 PREFACE TO THE FOURTH EDITION' WHEN the first edition of this work appeared in 1874, and the third edition followed three years afterwards, the circum- stances of biologv were very different from what they are to-day. It is true that the struggle for the recognition of the great truths of science, which Darwin had initiated by the publication of his epoch-making work in 1859, had already been decided in his favour on the main issue. But the most important consequence of the new evolutionary doctrine (now firmly established for the first time through his theory of selection) — that is to say, its application to man— still met with the most spirited and widespread opposition. I had in my Generelle Morphologies published in 1866, made the first attempt to trace the series of man's ancestors, and to indicate the several stages of animal organisation which led up to his appearance ; and I had continued this task in my History of Creation, published in 1868. The profound importance that the facts of human embryology have in the attempt to construct our ancestral tree became more and more evident to me. A prolonged study of human embryology, and the giving of university lectures on this first base of physical anthropology, emboldened me to attack the difficult task of applying it to the history of our species. The complete application to man of the first law of biogeny seemed to me the more useful and desirable as the great majority of embrvologists at that time knew nothing about it. The only work that dealt comprehensively with human embryo- logy after 1859 — namelv, Albert Kolliker's widely-circulated Manual — took an entirely opposite view ; even in the latest edition (1884) the distinguished author adheres to the opinion 1 Nol translated into English. PREFACE TO THE FOURTH EDIT/OX that " the laws governing the evolution of living things are still wholly unknown ; it is believed that the development took place by abrupt stages rather than by a continuous growth, as the Darwinians imagine." In opposition to this dualistic idea that was then prevalent on all sides, I attempted in 1874 to obtain a hearing for my monistic conception of the embryological phenomena. I started from the following general principles : — 1. There is a direct causal connection between the observed facts of human embryology and the theoretical ancestry of our race, which, for obvious reasons, for the most part lies outside our sphere of observation. 2. This mechanical causal nexus finds its simplest expression in the fundamental law of biogeny: "Ontogeny is a brief and imperfect recapitulation of phylogeny." ' 3. The phylogenetic process, or the gradual development of man's higher vertebral ancestors through a long series of lower animal forms, is a very complex historical fact, due to a manifold play of heredity and adaptation. 4. Each one of the processes involved depends on the physiological functions of the organism, and can be traced to the action of either reproduction (heredity) or nutrition (adaptation). 5. The fact of human embryology can only be explained as the inheriting of phylogenetic (ancestral) forms, in which the palingenetic phenomena are to be carefully distinguished from the cenogenetic.2 6. Only the palingenetic phenomena (that is to say, such reminiscences of earlier stages as the temporary formation of the spinal cord, the primitive kidneys, or the gill-clefts) are of direc' interest in the tracing of our animal ancestors, because they are due to the inheritance of adaptive structures in earlier animals. 1 Biogeny is the general science ot the development of lite ; ontogeny is the genesis of the individual (or the science dealing with this — embryology); and phytogeny the genesis of the species. Further explanation will be given presently. — Trans. - Palingenesis = " repeated " or inherited evolutionary phenomena : eeno- genesis = ''foreign," or more recently acquired phenomena. — TRANS. PREFACE TO THE FOURTH EDITIOX -. On the other hand, the cenogenetic phenomena (such as, for instance, the embryonic formation of the foetal membranes, the allantois, the dual structure of the heart, etc.) have only a subordinate and indirect interest for phytogeny, as they have arisen later by the adaptation of the foetus to its embryonic conditions. S. The many gaps in phytogeny, which are due to the lack of empirical material in embryology, may be remedied for the most part from paleontology and comparative anatomy. The application of these general principles of biogeny to the particular case of the evolution of man, as I first attempted it in my Anthropogenie, was bound, oi course — being the earliest independent advance into a fresh field of investiga- tion— to be imperfect. At the most it could only hope to attract attention to this new inquiry, and to induce other Students to test the results in their special provinces. When we compare the condition of our science at that time with its situation to-day, I think we must admit that my Anthropo- genie fully achieved its aim in this respect. Most men of science who have since worked in the field of comparative evolution are convinced to-dav that the two chief sections of it which I was the first to distinguish — Ontogeny and Phytogeny — have a causal connection of the closest character, and that the one cannot be understood apart from the other. The great majority of the useful results which their sedulous and searching inquiries have yielded can only be thoroughly appreciated when we recognise that the facts of ontogeny have found an explanation in phytogeny. Twenty-five vears ago, when my Generelle Morphologic appeared, human embryology was generally looked upon as a sort of fairyland, in which a number of most extra- ordinary and enigmatic processes were linked together without any visible ground in the shape of causal connection. To-day, on the contrary, we see in this chain of wonderful processes an historical document of the first importance, a chapter of the story of creation, which gives us most valuable information as to the chief features of the bodily structure and mode of life of our animal ancestors. PREFACE TO THE FOURTH EDITION The brilliant progress that comparative embryology has made during the last few decades is often attributed to extrinsic considerations — to the great number of fresh workers in this field of research, and to the improvement in the technical methods of investigation and the instruments used in the study. Certainly we must not fail to appreciate these advantages, especially the improvement of the micro- scope and microtome ; but the chief cause of progress has been the application of phylogenetic methods. It is to this we owe that immense enlargement of our intellectual horizon which enables us to regard the whole story of organic life, from the earliest beginning to the present day, as a vast mechanical process. It is reserved for phylogeny " to reduce the constructive forces of the animal body to the general forces or life-tendencies of the universe." No sooner does the science of the evolution of species shed its light on the dark puzzles of embryology than the true laws of develop- ment take definite shape. It is becoming clearer every year that this alone is the right path; that the facts of ontogeny can only be really explained by the theories of phylogeny. Moreover, the number and importance of the facts which we borrow from two other fields of research, the cognate sciences of paleon- tology and comparative anatomy, also grow every year. The profound and intimate connection of the historical documents furnished by these two sciences with those of ontogeny is growing clearer and more impressive the more we penetrate to these three sources of history. The need for using the three classes of documents in equal measure and with discrimination in the tracing of our ancestral tree is more evident every day. These leading principles, which I had presented and followed in the first edition of the Anthropogenie, have been applied far more thoroughly and comprehensively in the fourth edition, as our biological knowledge has been great* enlarged in all three fields of inquiry during the last fifteen years. In thus recognising and appreciating these general biogenetic principles, I find myself completely opposed to the PREFACE TO THE FOURTH EDITION / the Modern Science of Evolution (1875) and The Origin and Development of Animal Tissues (1884), pointed out the chief errors of this pretentious " mechanical science of embryology," and shown its radical opposition to our phylogenetic method. Surprise has often been expressed that so superficial a method, directed solely to the external appearance of the embryonic PREFACE TO THE FOURTH EDITIOX processes, and ignoring their historic nature, should have attained such considerable results. It is due mainly to the restriction of its aim. This narrowness of the pseudo- mechanical school is, in fact, three-fold. Firstly, it restricts itself in the use of its empirical material, as it only uses one of the three great documents — ontogeny — and ignores the other two — paleontology and comparative anatomy. Secondly, it restricts itself in its scientific method, in assuming as its sole aim the exact determination, with rule and compasses, of the embryonic forms. And, thirdly, it restricts itself in its philosophic insight, since it excludes all comparison with cognate phenomena and all correlation of the parts with the whole. However, this concentration — in itself a most prolific source of error — is welcomed in many quarters to-day, at a time when the narrowest specialism obtains its greatest triumphs, when the study of history is reversed, and when every thoughtful scientist who looks to the connection of phenomena is tabooed as "a natural philosopher." For all that, the scienc'e of evolution is an historical, and not an " exact," inquiry. Convinced that this method of anthropogenetic research is the method of the future, I conclude with the hope that this enlarged fourth edition of the Anthropogenic may, like its predecessors, contribute towards the enkindling of a deeper interest in the most important basis of anthropology. " Know thyself": that is the source of all wisdom. But it is impos- sible for a man to have real self-knowledge unless he is acquainted with the story of his development. Ernst Haeckel. Jena. Angus/ 18th, iSiji. PREFACE TO THE FIFTH EDITION Nearly thirty years have elapsed since the appearance of the first edition of the Anthrqpogente, and twelve years since the publication of the fourth edition. In the long interval scientific research into the subject of the work lias made extraordinary progress, not only in the great enlargement of the field of inquiry and the multiplication of workers, but also by the improvement of methods and greater thorough- ness in the treatment of the most important questions. Hence I found it no light task to undertake a new issue of my work after such a lapse of time, and in advanced age. But, after long hesitation, I was moved to do so by the following con- siderations. My Anthropogenic was in a twofold sense a " first attempt " when it appeared in 1874. In the first place, I approached the difficult and hitherto neglected task of applying to man the chief law of biogeny in all its force, and of giving a hypothetical sketch of the course of his ancestral develop- ment founded on the observed facts of embryology. But I also made the still more difficult attempt to render these com- plicated embryological facts, and the cognate theories of phytogeny, intelligible, not merely to the small circle of my scientific colleagues, but also, by a popular presentation, to the general public. In both respects my work lias remained for thirty years the only one of its kind ; and on this account I deemed it my duty, in spite of its great defects, of which I am not unconscious, to undertake a revision of the book. Many disapproved of the presentation of so difficult and delicate a subject to the general reader. A number of my colleagues expressed the opinion that it was impossible and undesirable to give a popular treatment of so obscure and unfamiliar a study as human embryology ; and that it was PREFACE TO THE FIFTH EDITIOX still more regrettable to associate with these facts of embryo- logy the airy and precarious hypotheses of phylogeny. This academic view, which is widely shared in learned circles, was extended to the popularisation of the whole science of evolu- tion and the monistic conception "of life which is founded thereon. I have never been able to accept this opinion of the German professors ; I share, on the contrary, the view of the learned among our neighbours, that the whole educated world has a right to be acquainted with the most important advances of science, even when their general results are only matters of theory and are opposed to the prevailing beliefs. It is enough to quote the instance of geology. With this conviction I undertook, in my History of Creation, in 1868, the difficult task of introducing the modern science of evolu- tion, founded bv Darwin, to the general reader, and to win for phylogeny the general recognition which its sister-science, geology, had long enjoyed. The immense correspondence I have had in connection with the ten editions of this book has proved to me that it met a real want on the part of the public. The same may be said of my work, The Riddle of the Universe, in which I gathered together the conclusions of fifty years of study in 1899. I attribute the remarkable success of this " popular study of the monistic philosophy " to no special merit of my book, but to the eagerness of the majority of educated people to acquaint themselves with the results of progressive science and cast off the superstitions of conven- tional theology and metaphysics. Interest in the embryology of plants and animals — that is, in the experimental study of these mysterious processes — has increased during the last ten years to an extent that was undreamt-of fifty years ago. Every year a number of specialist publications are issued which deal with one or other subject in this very attractive and most fruitful field of research. An introduction to this wonderful study, once so remote and exclusive, is provided by well-illustrated manuals and text-books. Unfortunately, many of these works show a lack of general morphological (or anatomical) knowledge, and of the indispensable method of comparison with related PREFACE TO THE FIFTH EDITION xxiii phenomena — not only of " comparative embryology," but also "comparative anatomy "; that is to say, of a discerning- and philosophical study of the complicated conditions of the whole series of tonus, or the stem, to which the organism in question belongs. It is also necessary to have a thorough preparatory training in classification, or an acquaintance with the relations of affinity, on the ground of which our "natural system " arranges the classes, orders, families, and so on. I have shown in my Systematische Phylogenie* (1894-6 — three volumes) how profound an insight this " phyletic classifica- tion " gives us into the history of the stem. Paleontology is even more neglected than comparative anatomy and classification by most of our modern embrvo- logists. Many of them are totally ignorant of it. Never- theless, the fossils, the historical succession and systematic arrangement of which are taught in paleontology, are just as important documents for the history ot the stem as the embryos which are taken by these one-sided embryologists to be the only fitting subject of research. We must, it is true, grant that most of the paleontologists are equally narrow ; they commonly lack the necessary preliminary training in comparative anatomv and embryology which is indispensable for the correct appreciation of the fossilised remains and their phvlogenetic significance. It was my chief and constant care, in the heavy task of preparing this fifth edition of my Anthropogenic, to avoid this narrowness, and to use all three documents bearing on our ancestral history in even greater force and harmony than in the preceding editions. Paleontology, comparative anatomy, and ontogeny must complete each other's work, and give to the historical hypotheses of ph\ logeny that firmness and fullness which they are bound to secure. In order to make this work accessible to a wider class of readers, I have considerably increased the number of illustrations in the present edition. The number of plates (originally twelve) is now thirty, and the illustrations in the text have been 1 Not translated into English. PREFACE TO THE FIFTH EDITIOX increased from 210 to 512 ; the number of genealogical tables is raised from thirty-six to sixty. The text has also been much extended ; the forty-six sheets of the first edition, and fifty-seven of the fourth, have now grown to sixty-two. I have, nevertheless, left unchanged the general arrangement of the thirty chapters. I must express my gratitude to the house of Wilhelm Engelmann for the excellent production of the work and assistance in preparing its many illustrations ; and to my pupil, Heinrich Schmidt, for his aid in correcting proofs and revision of the index. To speak of the alterations in detail, most of the chapters have been substantially improved, and some of them have been entirely re-written. I thought it necessary to include at least the most important advances that have been made in each branch from the vast and increasing literature of the subject. I fear that many errors may have been overlooked. That was inevitable in view of the intricacy of the work and the defects of the craftsman. Yet I hope the book will attain its chief purpose of introducing the thoughtful reader into the great and wonderful realm of the evolution of man, and stimulate him to reflect on its significance. I would include especially teachers, doctors, and students, among these "thoughtful readers"; but I appeal also to the many educated men and women who desire to know the full truth as to the origin and development of their individual being and the place of man in nature. Ernst Haeckel. Jena, September yth, igoj. CHAPTER I. THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION1 General importance of the science of human evolution. Ignorance of it among educated people Tin- two sections of the science of evolution : Ontogeny or embryology, and Phytogeny or stem-history. Causal connec- tion between the two sections. Phytogeny is the cause of ontogeny. Ontogeny as a summary or recapitulation of Phytogeny. Incompleteness of this summary. The chief law of biogeny. Heredity and adaptation are the two constructive functions, or the mechanical causes, of evolution. Exclusion of final causes. Sole validity of mechanical causes. Supplant- ing of the dualistic by the monistic philosophy. Great importance of tin' facts of embryology for the monistic philosophy. Palingenesis and cenogenesis. Evolution of structure and function. Necessary connection ofphysiogeny and morphogeny. Evolutionary science hitherto an achieve- ment of morphology, not physiology. The evolution of the central nervous system (the brain and spinal cord) proceeds step by step with that of the psychic or mental life. The field of natural phenomena into which I would introduce my readers in the following chapters has a quite peculiar place in the broad realm of scientific inquiry. There is no object of investigation that touches man more closely, and the knowledge of which should be more acceptable to him, than his own frame. But among all the various branches of the natural history of mankind, or anthropology, the story of his development bv natural means must excite the most lively interest. It gives us the key of the great world- riddles at which the human mind has been working for thousands of years. The problem of the nature of man, or the question of man's place in nature, and the cognate inquiries as to the past, the earliest history, the present situation, and the future of humanity — all these most important questions are directly and intimately connected with that branch of study which we call the science of the ' The English works recommended by Professor llaeekel are : Chap. xiii. of Darwin's Origin of Species, Spencer's Principles »/' Biology, and Haeckel's Riddle of the Universe. — Trans. THE FUXDAMEXTAI. LA W OF ORGAXIC EVOLUTIOX evolution of man, or, in one word, " Anthropogeny " (the genesis of man). Yet it is an astonishing but incontestable fact that the science of the evolution of man does not even yet form part of the scheme of general education. In fact, educated people even in our day are for the most part quite ignorant of the important truths and remarkable phenomena which anthropogeny teaches us. As an illustration of this curious state of things, it may be pointed out that most of what are considered to be " educated" people do not know that every human being is developed from an egg, or ovum, and that this egg is one simple cell, like any other plant or animal egg. They are equally ignorant that in the course of the development of this tiny, round egg-cell there is first formed a body that is totally different from the human frame, and has not the remotest resemblance to it. Most of them have never seen such a human foetus or embryo in the earlier period of its develop- ment, and do not know that it is quite indistinguishable from other animal embryos. At first the embryo is no more than a globular group 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 lancelot, 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 into doubt or denied outright as fairy-tales. Everybody knows that the butterfly emerges from the pupa, and the pupa from a quite different thing called a larva, and the larva from the butterfly's egg. But few besides medical men are aware that man, in the course of his individual formation, passes through a series of transformations which are not less THE FUNDAMENTAL I A II' OF OKCAX/C EVOLUTION surprising and wonderful than the familiar metamorphoses of the butterfly. The mere description of these remarkable changes through which man passes during his embryonic life should arouse considerable interest. But the mind will experience a far keener satisfaction when we trace these curious facts to their causes, and when we learn to behold in them natural pheno- mena which are of the highest importance throughout the whole field of human knowledge. The}' throw light first of all on the " natural history of creation," then on psycho- logy, or •• the science of the soul," and through this on the whole of philosophy. And as the general results of every branch of inquiry are summed up in philosophy, all the sciences come in turn to be touched and influenced more or less by the study of the evolution of man. But when I say that I propose to present here the most important features of these phenomena and trace them to their causes, I take the term, and I interpret my task, in a very much wider sense than is usual. The lectures which have been delivered on this subject in the universities during the last half-century are almost exclusively adapted to medical men. Certainly, the medical man has the greatest interest in Studying the origin of the human body, with which he is daily occupied. But I must not give here this special descrip- tion of the embryonic processes such as it has hitherto been given, as most of my readers have not studied anatomy, and are not likely to be entrusted with the care of the adult organism. 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 are found in the story of the development of the human frame. To understand these fully a knowledge of anatomy is needed. I will endeavour to be as plain as pos- sible in dealing with this branch of science. Indeed, a sufficient general idea of the course of the embryonic develop- ment of man can be obtained without going too closely into the anatomic details. I trust we may be able to arouse the same interest in this delicate field oi inquiry as has been 4 THE FUNDAMENTAL LA W OF ORGANIC EVOLUTION excited already in other branches of science ; though we shall meet more obstacles here than elsewhere. The story of the evolution of man, as it has hitherto been expounded to medical students, has usually been confined to embryology — or, more correctly, ontogeny — or the science of the development of the individual human organism. But this is really only the first part of our task, the first half of the story of the evolution of man in that wider sense in which we understand it here. We must add as the second half — as another and not less important and interesting branch of the science of the evolution of the human stem — phylogeny : this may be described as the science of the evolution of the various animal forms from which the human organism has been developed in the course of countless ages. Everybody now knows of the great scientific activity that was occasioned by the publication of Darwin's Origin of Species in 1859. The chief direct consequence of this publication was to provoke a fresh inquiry into the origin of the human race, and this has proved beyond question our gradual evolution from the lower species. We give the name of " Phylogeny " to the science which describes this ascent of man from the lower ranks of the animal world. The chief source that it draws upon for facts is "Ontogeny," or embryology, the science of the development of the individual organism. More- over, it derives a good deal of support from paleontology, or the science of fossil remains, and even more from comparative anatomy, or morphology. These two branches of our science — on the one side ontogeny or embryology, and on the other phylogeny, or the science of race-evolution — are most vitally connected. The one cannot be understood without the other. It is only when the two branches fully co-operate and supplement each other that " Biogeny " (or the science of the genesis of life in the widest sense) attains to the rank of a philosophic science. The connection between them is not external and superficial, but profound, intrinsic, and causal. This is a discovery made by recent research, and it is most clearly and correctly expressed in the comprehensive law which I have called THE FUNDAMENTAL LA W OF ORGANIC EVOLUTION 5 "the fundamental law o( organic evolution," or " the funda- mental law of biogenv." This general law, to which we shall find ourselves constantly recurring, and on the recogni- tion of which depends one's whole insight into the story of evolution, may be briefly expressed in the phrase: "The history of the fcetus is a recapitulation of the history of the race"; or, in other words, "Ontogeny is a recapitulation o( phylogeny." It may be more fully stated as follows: The series of forms through which the individual organism passes during its development from the ovum to the complete bodily structure is a brief, condensed repetition of the long scries of forms which the animal ancestors of the said organism, or the ancestral forms of the species, have passed through from the earliest period of organic life down to the present day. The causal character of the relation which connects embrvology with stem-history is due to the action of heredity and adaptation. When we have rightly understood these, and recognised their great importance in the formation of organisms, we can go a step further and say: Phylogenesis is the mechanical cause of ontogenesis.1 In other words, the development of the stem, or race, is the cause, in accor- dance with the phvsiological laws of heredity and adaptation, of all the changes which appear in a condensed form in the evolution of the fcetus. The chain of manifold animal forms which represent the ancestrv of each higher organism, or even of man, according to the theory of descent, always form a connected whole. We may designate this uninterrupted series of forms with the letters of the alphabet: A, B, C, D, E, etc., to Z. In apparent contradiction to what I have said, the story ot the development of the individual, or the ontogeny of most organisms, only offers to the observer a part of these forms; 1 The term "genesis," which recurs throughout, means, of course, "birth " or "origin.'1 From this we get: Biogeny=the origin of life (.bios) \ Anthro- pogenj : the origin of taan(anthropos) ; Ontogeny the origin of the individual ( on j; Phylogeny = the origin of the species (phulon ) -, and so on. In each case the 1. -rin may refer to tin- process itself, or to the science describing the process. — Trans. THE FVXDAMEXTAL LA W OF ORGAXIC EYOLVTIOX so that the defective series of embryonic forms would run: A, B, D, F, H, K, M, etc.; or, in other cases, B, D, H, L, M, N, etc. Here, then, as a rule, several of the evolutionary- forms of the original series have fallen out. Moreover, we often find — to continue with our illustration from the alphabet — one or other of the original letters of the ancestral series represented by corresponding letters from a different alphabet. Thus, instead of the Roman B and D, we often have the Greek B and A. In this case the text of the biogenetic law has been corrupted, just as it had been abbreviated in the preceding case. But, in spite of all this, the series of ancestral forms remains the same, and we are in a position to discover its original complexion. In reality, there is always a certain parallel between the two evolutionary series. But it is obscured from the fact that in the embryonic succession much is wanting that certainly existed in the earlier ancestral succession. If the parallel of the two series were complete, and if this great fundamental law affirming the causal nexus between ontogeny and phylogeny in the proper sense of the word were directly demonstrable, we should only have to determine, by means of the microscope and the dissecting knife, the series of forms through which the fertilised ovum passes in its development; we should then have before us a complete picture of the remarkable series of forms which our animal ancestors have successively assumed from the dawn of organic life down to the appearance of man. But such a repetition of the ancestral history by the individual in its embryonic life is very rarely complete. We do not often find our full alphabet. In most cases the correspondence is very imperfect, being greatly distorted and falsified by causes which we will con- sider later. We are thus, for the most part, unable to determine in detail, from the study of its embryology, all the different shapes which an organism's ancestors have pre- sented ; we usually — and especially in the case of the human 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 embryological THE FUNDAMENTAL LA II' OF ORGANIC EVOLUTION 7 observation. Hence it is important that we find a large number of lower animal forms to be still represented in the course of man's embryonic development. In these cases we may draw our conclusions with the utmost security as to the nature of the ancestral form from the features of the form which the embryo momentarily assumes. To give a few examples, we can infer from the fact that the human ovum is a simple cell that the first ancestor of our species was a tiny unicellular being, something like the amoeba. In the same way, we know, from the fact that the human foetus consists, at the first, of two simple cell-layers (the gastrula J, that the gas trcea, a form with two such layers, was certainly in the line of our ancestry. A later human embryonic form (the chordula) points just as clearly to a worm-like ancestor (the prochordouici J, the nearest living relation of which is found among the actual ascidia. To this succeeds a most important embrvonic stage ( ' acnuiia J, in which our headless fcetus presents, in the main, the structure of the amphioxus. But we can only indirectly and approximatelv, with the aid of comparative anatomy and ontogeny, conjecture what lower forms enter into the chain of our ancestry between the gastrasa and the chordula, and between this and the amphioxus. In the course of the historical development (by means of heredity in a condensed form) many intermediate structures have gradually fallen out, which must certainly have been represented in our ancestry. But, in spite of these many, and sometimes very appreciable, gaps, there is no contradiction between the two successions. In fact, it is the chief purpose of this work to prove the real harmony and the original parallelism of the two. I hope to show, on a substantial basis of facts, that we can draw most important conclusions as to our genealogical tree from the actual and easily-demonstrable series of embryonic changes. We shall then be in a position to form a general idea of the wealth of animal forms which have figured in the direct line of our ancestry in the lengthy history of organic life. In this phylogenetic appreciation of the facts of embryo- logy we must, of course, take particular care to distinguish 8 THE FUNDAMENTAL LA W OF ORGAXIC EVOLUTION sharply and clearly between the primitive, palingenetic (or ancestral) evolutionary processes and those due to ceno- genesis. ' By palingenetic processes, or embryonic recapitu- lations, we understand all those phenomena in the development of the individual which are transmitted from one generation to another by heredity, and which, on that account, allow us to draw direct inferences as to corresponding structures in the development of the species. On the other hand, we give the name of cenogenetic processes, or embryonic variations, to all those phenomena in the foetal development that cannot be traced to inheritance from earlier species, but are due to the adaptation of the foetus, or the infant-form, to certain condi- tions of its embryonic development. These cenogenetic phenomena are foreign or later additions ; they allow us to draw no direct inference whatever as to corresponding processes in our ancestral history, but rather hinder us from doing so. This careful discrimination between the primary or palingenetic processes and the secondary or cenogenetic is of great importance for the purposes of the scientific history of a species, which has to draw conclusions from the available facts of embryology, comparative anatomy, and paleontology, as to the processes in the formation of the species in the remote past. It is of the same importance to the student of evolution as the careful distinction between genuine and spurious texts in the works of an ancient writer, or the purging of the real text from interpolations and alterations, is for the student of philology. It is true that this distinction has not yet been fully appreciated by many scientists. For my part, I regard it as the first condition for forming any just idea of the evolutionary process, and I believe that we must, in accordance with it, divide embryology into two sections — palingenesis, or the science of repetitive forms ; and ceno- genesis, or the science of supervening structures. ' Palingenesis = new birth, or re-incarnation (palin = again, genesis or genea = development) ; hence its application to the phenomena which are recapitulated by heredity from earlier ancestral forms. Cenogenesis = foreign or negligible development (kenos and genea) ; hence, those phenomena which come later in the story of life to disturb the inherited structure, by a fresh adaptation to environment. — Trans. THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION 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 oi man, and of all the higher vertebrates, as palingenetic : the formation of the two primary germinal layers and of the primitive gut, the undivided structure of the dorsal nerve- tube, the appearance of a simple axial rod between the medullary tube and the gut, the temporary formation of the gill-clefts and arches, the primitive kidneys, and so on. 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 changes, 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 corresponding changes in the blood vessels. Further instances are : the dual structure of the heart cavity, the temporary division of the plates of the primitive vertebra? and lateral plates, the secondary closing of the ventral and intestinal walls, the formation of the navel, and so on. All these and many other phenomena are certainly not traceable to similar structures in any earlier and completely-developed ancestral form, but have arisen simply by adaptation to the peculiar conditions of embryonic life (within the fcetal membranes). In view of these facts, we may now give the following more precise expression to our chief law of hiogeny : — The evolution of the foetus (or ontogenesis) is a condensed and abbreviated recapitulation of the evolution of the stem (or phylogenesis) ; and this recapitulation is the more complete in proportion as the original development (or palingenesis) is preserved by a constant heredity ; on the other hand, it becomes less complete in proportion as a varying adaptation to new conditions increases the disturbing factors in the development (or catagenesis). io THE FUNDAMENTAL LA W OF ORGANIC EVOLUTION The cenogenetic alterations or distortions of the original palingenetic course of development take the form, as a rule, of a gradual displacement of the phenomena, which is slowly effected by adaptation to the changed conditions of embryonic existence during the course of thousands of years. This displacement may take place as regards either the locality or the time of a phenomenon. The first is called heterotopism, the second heterochronism. Heterotopisms, or variations in locality, affect, in the first place, the cells, or elementary parts of which the organs are composed ; but they also affect the organs themselves. Thus, for instance, the sexual glands in the human embryo, and most of the higher animals, arise out of the middle germinal layer. On the other hand, the comparative embry- ology of the lower animals shows us that originally they did not arise from this, but from one of the primary germinal layers. However, the germ-cells have gradually changed their position, and passed over at so early a period from their original situation into the middle layer that they now seem really to arise from it. A similar heterotopism is observed in the case of the primitive renal (kidney) passages of the higher vertebrates, which originally took their rise in the external skin. Even in the case of the origin of the mesoderm (middle-skin) itself heterotopism, in connection with a removal of embryonic cells from one skin layer to another, plays an important part. Heterochronism, or variation in time, is not less instruc- tive. It consists in the fact that the series of forms in which the organs successively appear is different in embryology from what the stem history leads us to expect. Just as the spatial disposition is falsified in heterotopism, so we find the time arrangement altered in heterochronism. This may appear either as an acceleration or a delay in the rise of an organ. As cases of ontogenetic acceleration we may instance, in the embryonic development of man, the early appearance of the heart, the gill-clefts, the brain, the eyes, etc. These organs clearly arise much earlier, in comparison with others, than was originally the case with our ancestors. THE FUNDAMENTAL /../ II' OF ORGANIC EVOLUTION We find the reverse of this in the retarded formation of the gut, the ventral cavity, and the sexual organs. These are clear instances of ontogenetic retardation. The great importance and strict regularity of these time variations in embryology have been carefully studied recently by Ernest Mehnert, in his Biomcchanik (Jena, 1898). He formulates his "chief law of organogenesis" in the following words : " The rapiditv of the embryonic development of an organ is in proportion to its stage of evolution, which has been retarded for a time. It rises with the increase and falls with the diminution of the stage of evolution once attained." Mehnert contends that our biogenetic law has not been impaired by the attacks of its opponents, and goes on to say : " Scarcelv any piece of knowledge has contributed so much to the advance of embryology as this ; its formulation is one of the most signal services to general biology. It was not until this law passed into the flesh and blood of investigators, and they had accustomed themselves to see a reminiscence of ancestral history in embryonic structures, that we witnessed the great progress which embryological research has made in the last two decades." The best proof of the correctness of this opinion is that now the most fruitful work is done in all branches of embryology with the aid of this biogenetic law, and that it enables students to attain every year thousands of brilliant results that they would never have reached without it. It is only when one appreciates the cenogenetic processes in relation to the palingenetic, and when one takes careful account of the changes which the latter may suffer from the former, that the radical importance of the biogenetic law is recognised, and it is felt to be the most illuminating principle in the science of evolution. In this task of discrimination it is the silver thread in relation to which we can arrange all the phenomena of this realm of marvels — the "Ariadne thread," which alone enables us to find our way through this labyrinth of forms. Hence the brothers Sarasin, the zoologists, could say with perfect justice, in their study of the evolution of the Ichthyopliis, that " the great biogenetic law is THE FUXDAMEXTAL LA IV OF ORGAXIC EVOLUTIOX just as important for the zoologist in tracing long-extinct 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 astonishment was felt at the remarkable similarity observed between the embryonic forms, or stages of 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 of members of lower groups. But it was impossible fully to understand and appreciate this remarkable resem- blance at that time. We owe our capacity to do this to the theory of descent; it is this that puts in their true light the action of heredity on the one hand and adaptation on the other. It explains to us the vital importance of their constant reciprocal action in the production of organic forms. Darwin was the first to teach us the great part that was played in this by the ceaseless struggle for existence between living things, and to show how, under the influence of this (by natural selection), new species were produced and maintained solely by the interaction of heredity and adaptation. It was thus Darwinism that first opened our eyes to a true comprehension of the supremely important relations between the two parts of the science of organic evolution — Ontogeny and Phylogeny. Heredity and adaptation are, in fact, the two constructive physiological functions of living things : unless we understand these properly we can make no headway in the study of evolution. Hence, until the time of Darwin no one had a clear idea of the real nature and causes of embryonic THE FUNDAMENTAL /..I II' OF ORGANIC EVOLUTION 13 development. It was impossible to explain the curious series of forms through which the human embryo passed ; it was quite unintelligible why this strange succession of animal-like forms appeared in the series at all. It had previously been generally assumed that the man was found complete in all his parts in the ovum, and that the development consisted only in an unfolding of the various parts, a simple process of growth. This is by no means the case. On the contrary, the whole process of the development of the individual presents to the observer a connected succession of various animal-forms ; and these forms display a great variety of external and internal structure. But why each individual human being should pass through this series of forms in the course of his embryonic development it was quite impossible to say until Lamarck and Darwin established the theory of descent. Through this theory we have at last detected the real causes, the causce efficientes, of the individual develop- ment; we have learned that these mechanical causes suffice of themselves to effect the formation of the organism, and that there is no need of the final causes which were formerly assumed. It is true that in the academic philosophies of our time these final causes still figure very prominently; in the new philosophy of nature we can entirely replace them by efficient causes. Before I pass from the subject I must speak further of this, one of the most brilliant achievements of the human mind in modern times. The history of philosophy shows us that final causes are still generally regarded in philosophic circles, just as among the philosophers of antiquity, as the real sources of the phenomena of organic life, and especially o\ human life. This dominant teleology, which is largely based on Kant, assumes that the processes of organic life, especially those of development, can only be explained by final causes, and are not susceptible of a mechanical — that is to say, a really scientific — explanation. But the darkest enigmas which had hitherto beset us in this connection, and which seemed to be only approachable through teleology, have been fully solved in a mechanical sense by the theory of descent. The i4 THE FUNDAMENTAL LA W OF ORGANIC EVOLUTION reconstruction of the science of human evolution which this brought about removed the greatest impediments from the path of research. We shall see, in the course of our inquiry, how the most wonderful and hitherto insoluble enigmas in the human and animal frame have proved amenable to a mechanical explanation, by causes acting without prevision, through Darwin's reform of the science of evolution. We have everywhere been able to substitute unconscious causes, acting from necessity, for conscious, purposive causes.1 If the new science of evolution had done no more than this, every thoughtful man would have to admit that it had accomplished an immense advance in knowledge. It means that in the whole of philosophy that tendency which we call monistic, in opposition to the dualistic, which has hitherto prevailed, must be accepted.2 At this point the science of human evolution has a direct and profound bearing on the foundations of philosophy. I have dealt with this relation very fully in my Riddle of the Universe. In the first part I show how modern anthropology has, by its astounding dis- coveries during the second half of the nineteenth century, compelled us to take a completely monistic view of life. Our bodily structure and its life, our embryonic development and our evolution as a species, teach us that the same laws of nature rule in the life of man as in the rest of the universe. For this reason, if for no others, it is desirable, nay, indispen- sable, that every man who wishes to form a serious and philo- sophic view of life, and, above all, the expert philosopher, should acquaint himself with the chief facts of this branch of science. 1 The monistic or mechanical philosophy of nature holds that only uncon- scious, necessary, efficient causes are at work in the whole field of nature, in organic life as well as in inorganic changes. On the other hand, the dualist or vitalist philosophy of nature affirms that unconscious forces arc only at work in the inorganic world, and that we find conscious, purposive, or final causes in organic nature. 2 Monism is neither purely materialistic nor purely spiritualistic, but a reconciliation of these two principles, since it regards the whole of nature as one, and sees only efficient causes at work in it. Dualism, on the contrary, holds that nature and spirit, matter and force, the world and God, inorganic and organic nature, are separate and independent existences. Cf. The Riddle of the Universe, chap. xii. THE FUNDAMENTAL LA II' OF ORGANIC EVOLUTIOX The facts of embryology have so great and obvious a significance in this connection that even in recent years dualist and teleological philosophers have tried to rid them- selves of them by simply denying them. This was done, for instance, as regards the fact that man is developed from an egg, and that this egg or ovum is a simple cell, as in the case of other animals. When I had explained this pregnant fact and its significance in my Natural History of Creation, it was described in many of the theological journals as a dishonest invention of my own. The fact that the embryos of man and the dog are, at a certain stage of their develop- ment, almost indistinguishable, was also denied. When we examine the human embryo in the third or fourth week of its development, we find it to be quite different in shape and structure from the full-grown human being, but almost identical with that of the ape, the dog, the hare, and other mammals, at the same stage of ontogeny. We find a bean- shaped body of very simple construction, with a tail below and a pair of fins at the sides, something like those of a fish, but very different from the limbs of man and the mammals. Nearly the whole front half of the body is taken up by a shapeless head without face, at the sides of which we find gill-clefts and arches as in the fish (see the thirteenth plate at the end of Chapter xiv.). At this stage of its development the human embryo does not differ in any essential detail from that of the ape, dog, horse, ox, etc., at a corresponding period. This important fact can easily be verified at any moment by a comparison of the embryos of man, the dog, hare, etc. Nevertheless, the theologians and dualist philo- sophers pronounced it to be a materialistic invention ; even scientists, to whom the facts should be known, have sought to denv them. There could not be a clearer proof of the profound importance of these embrvcilogical facts in favour of the monistic philosophy than is afforded by these efforts of its opponents to get rid of them by silence or denial. The truth is that these facts are most inconvenient for them, and are quite irreconcilable with their views. We must be all the 16 THE FUNDAMENTAL LA W OF ORGANLC EVOLUTION more pressing on our side to put them in their proper light. I fully agree with Huxley when he says, in his Man's Place in Nature : " Though these facts are ignored by several well- known popular leaders, they are easy to prove, and are accepted by all scientific men ; on the other hand, their importance is so great that those who have once mastered them will, in my opinion, find few other biological discoveries to astonish them." We shall make it our chief task to study the evolution of man's bodily frame and its various organs in their external form and internal structures. But I may observe at once that this is accompanied step by step with a study of the evolution of their functions. These two branches of inquiry are inseparably united in the whole of anthropology, just as in zoology (of which the former is only a section) or general biology. Everywhere the peculiar form of the organism and its structures, internal and external, is directly related to the special physiological functions which the organism or organ has to execute. This intimate connection of structure and function, or of the instrument and the work done by it, is seen in the science of evolution and all its parts. Hence the story of the evolution of structures, which is our immediate concern, is also the history of the development of functions; and this holds good of the human organism as of anv other. At the same time, I must admit that our knowledge of the evolution of functions is very far from being as complete as our acquaintance with the evolution of structures. One might say, in fact, that the whole science of evolution, or biogeny (both in ontogeny and phylogeny), has almost confined itself to the study of structures ; the biogeny of functions hardly exists even in name. That is the fault of the physiologists, who have as yet concerned themselves very little about evolution. It is only in recent times that physio- logists like W. Engelmann, W. Preyer, M. Verworn, and a few others, have attacked the biogeny of functions. For a long time now the two great branches of biological research, morphology and physiology, have pursued separate ways. That is quite natural. The aims and methods of the THE FUNDAMENTAL LA W OF ORGANIC EVOLUTIOX 17 two are very different. Morphology (anatomy), or the science of forms, seeks a scientific knowledge of organic struc- ture, internal and external. On the other hand, physiology, or the science of functions, studies the vital phenomena. The two together make up biology. But the development of physiology during the last fifty years has been much more one- sided than that of morphology. It has not only failed to make much use of the comparative method, which has given such great results in morphology, but it has also neglected evolu- tionarv principles. Hence in the last few decades morpho- logy has far outrun physiology, though the latter is apt to put on superior airs in regard to its rival. Morphology has achieved its finest results in the way of comparative anatomy and ontogeny, and nearly all that I shall put before the reader in this work as to the evolution of man has been obtained by the labours, not of the physiologist, but of the morphologist. In fact, the one-sidedness of modern physio- logy is so great that it has hitherto neglected the study of the most important evolutionary functions, heredity and adapta- tion, and abandoned even these purely physiological subjects to the morphologist. We owe nearly all that we know about them to the morphologist, not to the physiologist. The latter concerns himself little more with the functions (or agencies) of evolution than with the evolution of functions. It will be the task of some future physiologist to engage in the study of the evolution of functions with the same zeal and success as has been done for the evolution of structures in morphogeny (the 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 con- struction, such as we find in permanent form among the ascidia and other low organisms; with this is associated a very simple system of circulation of the blood. Now, when we find that with the full-grown heart there comes a totally different and much more intricate circulation, our inquiry into the development of the heart becomes at once, not only a morphological, but also a physiological, study. Thus it is clear that the ontogeny of the heart can only be understood in THE FVXDAMEXTAL LA W OF ORGA.XIC EVOLUTIOX the light of its phylogeny (or development in the past), both as regards function and structure. The same holds true of all the other organs and their functions. For instance, the science of the evolution of the alimentary canal, the lungs, or the sexual organs, gives us at the same time, through the exact comparative investigation of structure-development, most important information with regard to the evolution of the functions of these organs. This significant connection is very clearlv seen in the evolution of the nervous system. This system is in the economy of the human body the medium of sensation, will, and even thought, the highest of the psychic functions ; in a word, of all the various functions which constitute the proper object of psychology. Modern anatomy and physiology have proved that these psychic functions are immediately dependent on the fine structure and the composition of the central nervous system, or the internal texture of the brain and spinal cord. In these we find the elaborate cell-machinery, of which the psychic or soul-life is the physiological function. It is so intricate that most men still look upon the mind as something supernatural that cannot be explained on mechanical principles. But embryological research into the gradual appearance and the formation of this important system of organs yields the most astounding and significant results. The first sketch of a central nervous system in the human embryo presents the same very simple type as in the other vertebrates. A spinal tube is formed in the external skin of the back, and from this first comes a simple spinal cord without brain, such as we find to be the permanent psychic organ in the lowest type of mammal, 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 developes step by step, successively assuming forms which correspond to those of the amphibia, the reptiles, the duck- bills, and the prosimias. Only in the last stage does it reach the highly organised form which distinguishes the THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION apes from the other vertebrates, and which attains its full development in man. Comparative physiology discovers a precisely similar growth. The function of the brain, the psychic activity, rises step bv step with the advancing development of its structure. Thus we are enabled, by this story of the evolution of the nervous system, to understand at length the natural develop- ment of the human mind and its gradual unfolding. It is onlv with the aid of embryology that we can grasp how these highest and mjst striking faculties of the animal organism have been historicallv 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 phylogenv) 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. Happilv, our embrvological knowledge of man's central nervous svstem is now so adequate, and agrees so thoroughly with the complementary results of comparative anatomy and phvsiologv, that we are thus enabled to obtain a clear insight into one of the highest problems of philosophy, the phy- logenv 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 psychology owes its origin especially to W. Prever, in his interesting works, The Mind of the Child (English translation) and Spezielle Physio/ogie des Embryo. The Biography of a Baby (1900), of Milicent Washburn Shinn, also deserves mention. |See also Preyer's Mental Development in the Child (translation), and Sully's Studies of Childhood and Children's Ways. ] In this way we follow the only path along which we may hope to reach the solution of this difficult problem. Thirty-six years have now elapsed since I established phylogeny as an independent science and showed its intimate causal connection with ontogeny in my (ienere/le Morphologie ; thirty years have passed since I gave in my gastraea-theory THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION 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 satis- factory 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 validity of phylogeny generally, and would explain the complicated embryonic processes without going beyond by simple physical changes — such as the bending and folding of leaves by electricity, the origin of cavities through unequal strain of the tissues, the formation of processes by uneven growth, and so on. But the fact is that these embryological phenomena themselves demand explanation in turn, and this can only be found, as a rule, in the corresponding changes in the long ancestral series, or in the physiological functions of hereditv and adaptation. Heinrich Schmidt (of Jena) has given a good account and criticism of the many attacks on the biogenetic law in his interesting pamphlet, Haeckel's biogenetisches Gmndgesetz und seine Gegner (Odenkirchen, 1902). He shows that not only distinguished zoologists, but botanists also, have recog- nised it, and made profitable use of it ; it holds good of the evolution of plants no less than of animals. On the other hand, none of its critics has offered anything better to replace it. Many of the criticisms, in fact, arise from pure mis- understanding, as is quite to be expected in so difficult and complicated a subject, or from a wrong idea of the relation of cenogenesis and palingenesis. But, in spite of all this, our knowledge of the mutual relations of these two series of phenomena grows every day, and our conviction increases that " Phylogenesis is the mechanical cause of ontogenesis." CHAPTER II. THE OLDER EMBRYOLOGY Aristotle's Generation of Animals. His acquaintance with the embryology of lower animals. Arrest of scientific research during the Middle Ages. The ris,- of embryology at the beginning of the seventeenth century. Fabricius ab Aquapendente. Harvey. Marcello Malpighi. The significance of the hatched egg. The theory of Pre-formation and Scatulation (Evolution and Pre-delineation). The unfolding of parts already formed. The theory of Scatulation for male and female. Either the spermatozoon or the egg is the pre-formed individual. Animaleulists or Spermatists (Leeuwenhock, Hartsoeker, Spallanzani). Ovulists (Haller, Leibnitz, Bonnet). A calcula- tion of the germs stored in Eve's ovary. Discovery of parthenogenesis by Bonnet. Victory of the Pre-formation theory owing- to the authority of Haller and Leibnitz. Caspar Friedrieh Wolff. His life and works. The theoria generationis. New formation, or epigenesis. The evolution of the alimentary canal. First beginnings of the theory of germinal layers. The metamorphosis of plants. Germs of the cell theory. Wolff's monistic philosophy. It is in many ways useful, on entering upon the study of any science, to cast a glance at its historical development. The saying that "everything is best understood in its growth" has a distinct application to science. While we follow its gradual development we get a clearer insight into its aims and objects. Moreover, we shall see that the present condition of the science of human evolution, with all its characteristics, can only be rightly understood when we examine its historical growth. This task will, however, not detain us long. The study of man's evolution is one of the latest branches of natural science, whether you consider the embryological or the phylogenetic section of it. Apart from the few germs of our science which we find in classical antiquity, and which we shall notice presently, we may say that it takes its definite rise, as a science, in the yeat 1 759, when one of the greatest German scientists, Caspar Friedrieh Wolff, published his Theoria generationis. That was the foundation-stone of the science of animal embryology. It was not until fifty years later, in 1809, that Jean Lamarck THE OLDER EMBRYOLOGY published his Pliilosophie Zoologique — the first effort to provide a base for the theory of evolution ; and it was another half-century before Darwin's work appeared (in 1859), which we may regard as the first scientific attainment of this aim. But before we go further into this solid establishment of evolution, we must cast a brief glance at that famous philosopher and scientist of antiquity, who stood alone in this, as in many other branches of science, for more than 2,000 years : the " father of natural history," Aristotle. The extant scientific works of Aristotle deal with many different sides of biological research ; the most comprehensive of them is his famous History of Animals. But not less interesting is the smaller work, On the Generation of Animals {Peri zoon geneseos). This work treats especially of embryonic development, and it is of great interest as being the earliest of its kind and the only one that has come down to us in any completeness from classical antiquity. Like Aristotle's other scientific writings, this substantial little work has dominated the whole of science for 2,000 years. The philosopher was as keen in observation as he was profound in thought. Nevertheless, while his philosophic distinction has never been questioned, it is only in recent years that his worth as an observer has been properly appreciated. The men of science who turned to his scientific writings about the middle of the nineteenth century were astonished at the amount of information and the notable discoveries that they found. In connection with embryological questions, we must particularly note that Aristotle studied them in various classes of animals, and that among the lower groups he learned many most remarkable facts which we only re-dis- covered 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 cephalopods, in which a yelk-sac hangs out of the mouth of the foetus. 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 THE O/.OKh' EMJ1KYOLOGY time by the distinguished zoologist of Munich, Siebold. He discovered that male bees come from the unfertilised, and female bees only from the fertilised, eggs. Aristotle further states that some kinds of fishes (of the genus serranus) are hermaphrodites, each individual having both male and female organs and being able to fertilise itself ; this, also, has been recently confirmed. He knew that the embryo of many fishes of the shark family is attached to the mother's body by a sort of placenta, or nutritive organ very rich in blood; apart from these, such an arrangement is only found among the higher mammals and man. This placenta of the shark was looked upon as legendary for a long time, until Johannes Miiller proved it to be a fact in 18^9. Thus a number of remarkable discoveries were found in Aristotle's embryological work, proving a very good acquaintance of the great scientist — possibly helped by his predecessors — with the facts of ontogeny, and a great advance upon succeeding generations in this respect. In the case of most of these discoveries he did not merely describe the fact, but added a number of observations on its significance. Some of these theoretical remarks are of par- ticular interest, because they show a correct appreciation of the nature of the embryonic processes. He conceives the development of the individual as a new formation, in the course of which the various parts of the body take shape successively. When the human or animal frame is developed in the mother's body, or separately in an egg, the heart — which he regards as the starting-point and centre of the organism — must appear first. Once the heart is formed the other organs arise, the internal ones before the external, the upper (those above the diaphragm) before the lower (or those beneath the diaphragm). The brain is formed at an early stage, and the eyes grow out of it. These observations are quite correct. And, if we try to form some idea from these data of Aristotle's general conception of the embryonic process, we find a dim prevision of the theory which we now call epigenestSf and which Wolff showed 2,000 years after- wards to be the correct view. It is significant, for instance, THE OLDER EMBRYOLOGY that Aristotle denied the eternity of the individual in any respect. He said that the species or genus, the group of similar individuals, might be eternal, but the individual itself is temporary. It comes into being in the act of procreation, and passes away at death. During the 2,000 years after Aristotle no progress what- ever was made in general zoology, or in embryology in particular. People were content to read, copy, translate, and comment on Aristotle. Scarcely a single independent effort at research was made in the whole of the period. During the Middle Ages the spread of strong religious beliefs put formidable obstacles in the way of independent scientific investigation. There was no question of resuming the advance of biology. Even when human anatomy began to stir itself once more in the sixteenth century, and independent research was resumed into the structure of the developed body, anatomists did not dare to extend their inquiries to the unformed body, the embryo, and its development. There were many reasons for the prevailing horror of such studies. It is natural enough, when we remember that a Bull of Boniface VIII. excommunicated every man who ventured to dissect a human corpse. If the dissection of a developed body were a crime to be thus punished, how much more dreadful must it have seemed to deal with the embryonic body still enclosed in the womb, which the Creator himself had decently veiled from the curiosity of the scientist ! The Christian Church, then putting many thousands to death for unbelief, had a shrewd presentiment of the menace that science contained against its authority. It was powerful enough to see that its rival did not grow too quickly. It was not until the Reformation broke the power of the Church, and a refreshing breath of the spirit dissolved the icy chains that bound science, that anatomy and embryology, and all the other branches of research, could begin to advance once more. However, embryology lagged far behind anatomy. The first works on embryology appear at the beginning of the sixteenth century. The Italian anatomist, Fabricius ab Aquapendente, a professor at Padua, Efc»Xrs?:r<* THE OLDER EMBRYOLOGY opened the advance. In his two books [De formato foetu, 1600, and De format iane fee I us, 1604) he published the older illustrations and descriptions of the embryos of man and other mammals, and of the hen. Similar imperfect illustra- tions were given by Spigelius (De formato foetu, 1 631), and by Needham (1667) and his more famous compatriot, Harvey (1652), who discovered the circulation of the blood in the animal body and formulated the important principle, Omne vivum ex vivo (all life comes from pre-existing life). The Dutch scientist, Swammerdam, published in his Bible of Nature the earliest observations on the embryology of the frog and the division of its egg-yelk. But the most important embryological studies in the sixteenth century were those of the famous Italian, Marcello Malpighi, of Bologna, who led the way both in zoology and botany. His treatises, De formatione pulli and De ovo incubato (1687), contain the first consistent description of the development of the chick in the fertilised egg. Here I ought to say a word about the important part plaved by the chick in the growth of our science. The development of the chick, like that of the young of all other birds, agrees in all its main features with that of the other chief vertebrates, and even of man. The three highest classes of vertebrates — mammals, birds, and reptiles (lizards, serpents, tortoises, etc.) — have from the beginning of their embryonic development so striking a resemblance in all the chief points of structure, and especially in their first forms, that for a long time it is impossible to distinguish between them (see plates viii-xiii.). We have known now for some time that we need only examine the embryo of a bird, which is the easiest to get at, in order to learn the typical mode of development of a mammal (and therefore of man). As soon as scientists began to study the human embryo, or the mammal-embryo generally, in its earlier stages about the middle and end of the seventeenth century, this important fact was very quickly discovered. It is both theoretically and practically of great value. As regards the theory of evolu- tion, we can draw the most weighty inferences from this THE OLDER EMBRYOLOGY similarity between the embryos of widely different classes of animals. But for the practical purposes of embryological research the discovery is invaluable, because we can fill up the gaps in our imperfect knowledge of the embryology of the mammals from the more thoroughly studied embryology of the bird. Hens' eggs are easily to be had in any quantity, and the development of the chick may be followed step by step in artificial incubation. The development of the mammal is much more difficult to follow, because here the embryo is not detached and enclosed in a large egg, but the tiny ovum remains in the womb until the growth is com- pleted. Hence, it is very difficult to keep up sustained observation of the various stages in any great extent, quite apart from such extrinsic considerations as the cost, the technical difficulties, and many other obstacles which we encounter when we would make an extensive study of the fertilised mammal. The chicken has, therefore, always been the chief object of study in this connection. The excellent incubators we now have enable us to observe it in any quantity and at any stage of development, and so follow the whole course of its formation step by step. By the end of the seventeenth century Malpighi had advanced as far as it was possible to do with the imperfect microscope of his time in the embryological study of the chick. Further progress was arrested until the instrument and the technical methods should be improved. The vertebrate embryos are so small and delicate in their earlier stages that you cannot go very far into the study of them without a good microscope and other technical aid. But this substantial improvement of the microscope and the other apparatus did not take place until the beginning of the nineteenth century. Embryology made scarcely any advance in the first half of the eighteenth century, when the systematic natural history of plants and animals received so great an impulse through the publication of Linne's famous Systema Naturae. Not until 1759 did the genius arise who was to give it an entirely new character, Caspar Friedrich Wolff. Until then embryology THE OLDER EM 11 MYOLOGY 27 had been occupied almost exclusively in unfortunate and misleading efforts to build up theories on the imperfect empirical material then available. The theory which then prevailed, and remained in favour throughout nearly the whole of the eighteenth century, was commonly called at that time "the evolution theory"; it is better to describe it as "the preformation theory."1 Its chief point is this : There is no new formation of structures in the embryonic development of any organism, animal or plant, or even of man ; there is only a growth, or unfolding, of parts which have been constructed and ready 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 bodv, 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, o{~ parts that were pre-formed and folded up in it. So, for instance, we find in the hen's egg not merely a simple cell, that divides and sub- divides and forms germinal layers, and at last, after all kinds of variation and cleavage and reconstruction, brings forth the body of the chick ; but there is in every egg from the first a complete chicken, with all its parts made and neatly packed. These parts are so small or so transparent that the microscope cannot detect them. In the hatching, these parts merely grow larger, and spread out in the normal way. When this theory is consistently developed it becomes a "scatulation theory."- According to its teaching, there was made in the beginning one couple or one individual of each species of animal or plant ; but this one individual contained the germs of all the other individuals of the same species who should ever come to life. As the age of the earth was 'This theory is usually known as the "evolution theory" in Germany, in contradistinction to the "epigenesis theory." Hut as it is the latter that is called the "evolution theorj in England, France, and Italy, and "evolution" and "epigenesis" are taken to be synonymous, ii s,-c-ms better to call the first the " preformation theory." Kolliker has recently given the name of "evolu- tionism " to his •• theory of heterogeneous conception." Packing theory" would be the literal translation. Scatula is the Latin for a case or box.— Trans. THE OLDER EMBRYOLOGY generally believed at that time to be fixed by the Bible at 5,000 or 6,000 years, it seemed possible to calculate how many individuals of each species had lived in the period, and so had been packed inside the first being that was created. The theory was consistently extended to man, and it was affirmed that our common parent Eve had had stored in her ovary the germs of all the children of men. The theory at first took the form of a belief that it was the females who were thus encased in the first being. One couple of each species was created, but the female contained in her ovary all the future individuals of the species, of either sex. However, this had to be altered when the Dutch microscopist, Leeuwenhoek, discovered the male spermatozoa in 1690, and showed that an immense number of these extremely fine and mobile thread-like beings exist in the male sperm (this will be explained in 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 supposed to penetrate into the fertile body of the ovum and begin to develop there, as the plant seed does in the fruitful earth. Hence, every spermatozoon was regarded as a homunculus, a tiny complete man ; all the parts were believed to be pre- formed in it, and merely grew larger when it reached its proper medium in the female ovum. This theory, also, was consistently developed in the sense that in each of these thread-like bodies the whole of its posterity was supposed to be present in the minutest form. Adam's sexual glands were thought to have contained the germs of the whole of humanity. This " theory of male scatulation " found itself at once in keen opposition to the prevailing " female " theory. All that was common to them was the erroneous idea that there are in every germ the germs of innumerable organisms to come enfolded in it — an idea that served as the ground of Linne's THE OLDER EMBRYOLOGY curious " prolepsis theory." The two rival theories at once opened a very lively campaign, and the physiologists of the eighteenth century were divided into two great camps — the Animalculists and the Ovulists — which fought vigorously. The struggle rather amuses us to-day when we know that both parties were wrong. As Kirchhoff says in his admir- able biographical sketch of Wolff: "This controversy was as difficult to close as that on the question whether the angels live in the eastern or the western part of heaven." The animalculists held that the spermatozoa were the true germs, and appealed to the lively movements and the structure of these bodies. In the case of man and most of the other animals, these spermatozoa have a rather oval or pear-shaped head and a thickish stem, ending in an extremely fine and hair-like tail (Fig. 20). The whole structure is really only one cell — a ciliated cell. The head is the nucleus en- closed in a little of the cell-matter, and this is prolonged in the thick stem and fine, mobile tail; the latter is the " whip " (or cilium) by which it moves about, and corresponds to the cilium in a ciliated cell. But the animalculists believed that the " head " was a real head, and the rest of it a complete body. Leeuwenhoek, Hartsoeker, and Spallanzani were the chief champions of these fantastic speculations. 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 founda- tion. 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 Gbttingen, who is often called the father of physiology, was a man of wide and varied learning, but he does not occupy a very high position in regard to insight into natural phenomena. He has THE OLDER EMBRYOLOGY unconsciously given the best description of himself in his famous saying: " No created mind can penetrate into the heart of Nature ; happy the man to whom she does but show the outer shell." Goethe made the best reply to this " shell theory " of observation in the noble poem which closes with the words: "'Nature has neither kernel nor shell; she is all one. Try yourself whether you are either kernel or shell." Yet there has been no lack, even of late years, of attempts to defend Haller's "shell theory." Wilhelm His, especially, has made a strange effort to justify it. Haller made a vigorous defence of the "evolution theory" in his famous work, Elementa physiologiae, affirming: "There is no such thing as formation (nulla est epigenesis ). No part of the animal frame is made before another; all were made together." He thus denied that there was anv 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 that time to be 1,000 millions. And the famous Haller maintained all this non- sense, in spite of its ridiculous consequences, even after Wolff had discovered the real course of embryonic development and established it by direct observation! Among the philosophers of the time the distinguished Leibnitz was the chief defender of the " preformation theorv," and by his authority and literary prestige won many adherents to it. Supported by his system of monads, according to which body and soul are united in inseparable association and by their union form the individual, or the "monad," Leibnitz consistently extended the " scatulation theory " to the soul, and held that this was no more evolved than the body. He says, for instance, in his Theodicec: " I mean that these souls, THE OLDER EMBRYOLOGY which one day are to be the souls of men, are present in the .seed, like those of other species; in such wise that they existed in our ancestors as far back as Adam, or from the beginning of the world, in the forms of organised bodies." The theory seemed to receive considerable support from the observations of one of its most zealous supporters, Bonnet. In 1745 lie discovered, in the plant-louse, a case of parthenogenesis, or virgin-birth, an interesting form of reproduction that has lately been found by Siebold and others among various classes of the articulata, especial ly crabs and insects. Among these and other animals of certain lower species the female may reproduce for several generations without having been fertilised by the male. These ova that do not need fertilisation are called " false ova," pseudova or spores. Bonnet saw that a female plant-louse, which he had kept in cloistral isolation, and rigidly removed from contact with males, had on the eleventh day (after forming a new skin for the fourth time) a living daughter, and during the next twenty days ninety-four other daughters ; and that all of them went on to reproduce in the same way without any contact with males. It seemed as if this furnished an irrefutable proof o( the truth of the scatulation theory, as it was held by the OvulistS ; it is not surprising to find that the theory then secured general acceptance. This was the condition of things when suddenlv, in 1759, Caspar Friedrich Wolff appeared, and dealt a fatal blow at the whole preformation theory with his new theory of epigenesis. Wolff, the son of a Berlin tailor, was born in 1733, and went through his scientific and medical studies, first at Berlin under the famous anatomist Meckel, and after- wards at Halle. Here he secured his doctorate in his twenty- sixth year, and in his academic dissertation (November 2/:a:y i:\iiiryology Man and the Mammals [translations [892 and 1899) (seventh edition, 1902). This ahle anatomist lias of late often been quoted as an opponent of the biogenetic law, although he himself had demonstrated its great value thirty years ago in his Untersuchungen uber Bait und Entwickelung der Plakoid- schuppen. His recent vacillation is partly due to the timidity which our "exact " scientists have with regard to hypotheses; though it is quite impossible to make any headway in the explanation of facts without them. However, the purely descriptive part of embrvologv in Hertwig's Text-book is very thorough and reliable. A shorter account is given in his Elemente der Entwickelungslehre (Jena, 1900), and a very good summary of special work done by many authors in his Handbuch der vergleichenden und experimentellen Entwicke- lungslehre der Wirbelthiere (Jena, 1901). A new branch of embrvological research has been studied very assiduously in the last decade of the nineteenth century — namely, " experimental embryology." The great importance which has been attached to the application of physical experiments to the living organism for the last hundred years, and the valuable results that it has given to physiology in the study of the vital phenomena, have led to its extension to embryology. I was the first to make experiments of this kind during a stay of four months on the Canary Island, Lanzerote, in 1866. I there made a thorough investigation of the almost unknown embryology of the siphonophora. 1 cut a number of the embryos of these animals (which develop freely in the water, and pass through a very curious transfor- mation), at an early stage, into several pieces, and found that a fresh organism (more or less complete, according to the size of the piece) was developed from each particle. I have given illustrations of the curious larva (sometimes of quite monstrous shapes) which form from them on plates 11-14 °f m.v Entwickelungsgeschichte der Siphonophoren (Utrecht, 1869). More recently some of my pupils have made similar experiments with the embryos of vertebrates (especially the frog) and some of the invertebrates. Wilhelm Roux, in SS MODERN EM BR YOLOG Y particular, has made extensive experiments, and based on them a special "mechanical embryology," which has given rise to a good deal of discussion and controversy. Roux has published a special journal for these subjects since 1895, the Archiv fur Entmickelungsmechanik. The contri- butions to it are very varied in value. Many of them are valuable papers on the physiology and pathology of the embryo. Pathological experiments — the placing of the embryo in abnormal conditions — have yielded many interesting results ; just as the physiology of the normal body has for a long time derived assistance from the pathology of the diseased organism. Other of these mechanical-evolutionary articles return to the erroneous methods of His, and are only misleading. This must be said of the many contributions of mechanical embryology which take up a position of hostility to the theory of descent and its chief embryological foundation — the biogenetic law. This law, however, when rightly understood, is not opposed to, but is the best and most solid support of, a sound mechanical embryology. Impartial reflection and a due attention to paleontology and comparative anatomy should convince these one-sided mechanicists that the facts they have discovered — and, indeed, the whole embryological process — cannot be fully understood without the theory of descent and the biogenetic law. CHAPTER IV. THE OLDER PHYLOGENY1 Evolution before Darwin. The origin of species. Carl Linne gives a defini- tion of species and genus, and associates it with the Biblical story of creation. The deluge. Paleontology. The catastrophic theory of Georges Cuvier. Repeated revolutions on earth and fresh creations. Lyell's theory of continuity. The natural causes of the gradual formation of the earth. Supernatural origin of living things. Dualistic natural philosophy of Immanuel Kant. Monistic natural philosophy of Jean Lamarck. His life. His Philosophic Zoologique. The first scientific treatment of evolution. Transformation of organs by use and habit, together with heredity. Application of the theory to man. Descent of man from the ape. Wolfgang Goethe. His scientific studies. His morphology. His studies on the formation and transformation of organic natures. Goethe's theory of the impulse to specification (heredity) and metamorphosis (adaptation). Tin-: embryology of man and the animals, the historv of which we have reviewed in the last two chapters, was mainly a descriptive science forty years ago. The earlier investigations in this province were chiefly directed to the discovery, by careful observation, of the wonderful facts of the embryonic development of the animal body from the ovum. Forty years ago no one dared attack the question of the causes of these phenomena. For fully a century, from the year 1759, when Wolffs solid Theoria generation is 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 struc- tures. 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 1 Cf. Clodd's Pioneers "/ Evolution and Packard's Lamarck ami X,-.- Lamarckism and Lamarck the Founder of Evolution. THE OLDER PHYLOGENY biology, a founder at the same time of a new period in embryology. It is true that Darwin occupied himself very little with direct embryological research, and even in his chief work he only touches incidentally on the embryonic phenomena ; but by his reform of the theory of descent and the founding of the theory of selection he has given us the means of attaining to a real knowledge of the causes of embryonic formation. That is, in my opinion, the chief feature in Darwin's incalculable influence on the whole science of evolution. When we turn our attention to this latest period of embryological research, we pass into the second division of organic evolution — stem-evolution, or phylogeny. I have already indicated in the first chapter the important and intimate causal connection between these two sections of the science of evolution — between the evolution of the individual and that of his ancestors. We have formulated this connec- tion in the biogenetic law; the shorter evolution, that of the individual, or ontogenesis, is a rapid and summary repetition, a condensed recapitulation, of the larger evolution, or that of the species. In this principle we express all the essential points relating to the causes of evolution ; and we shall seek throughout this work to confirm this principle and lend it the support of facts. When we look to its causal significance, perhaps it would be better to formulate the biogenetic law thus: "The evolution of the species and the stem (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: " Phylogenesis is the mechanical cause of ontogenesis." We owe it to Darwin that we are now in a position to trace and appreciate these hitherto obscure causes of embryonic development, and so we give his name to a new period in embryology. 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 THE OLDER PHYLOGENY have very few names to consider here. At the head of them we find the great French naturalist, Jean Lamarck, who first established evolution as a scientific theory in 1809. Even before his time, however, the chief philosopher, Kant, and the chief poet, Goethe, of Germany had occupied themselves with the subject. But their efforts passed almost without recognition in the eighteenth century. A "philosophy of nature "did not arise until the beginning of the nineteenth century. In the whole of the time before this no one had ventured to raise seriously the question of the origin of species, which is the culminating point of phylogeny. On all sides it was regarded as an insoluble enigma. The whole science of the evolution of man and the other animals is intimately connected with the question of the nature of species, or with the problem of the origin of the various animals which we group together under the name of species. Thus the definition of the species becomes impor- tant. It is well known that this definition was given by Linne, who, in his famous Systema Natures (1735), was the first to classify and name the various groups of animals and plants, and drew up an orderly scheme of the species then known. Since that time "species" has been the most important and indispensable idea in descriptive natural history, in zoological and botanical classification ; although there have been endless controversies as to its real meaning. What, then, is this "organic species"? Linne himself did not give a very clear account of it. He unfortunately relied on religious notions which the dominant creed had founded on the Mosaic story of creation, and which have not vet wholly disappeared. Linne, in fact, appealed directly to the Mosaic narrative; he believed that, as it is stated in Genesis, one pair of each species of animals and plants was created in the beginning, and that all the individuals of each species are the descendants of these created couples. As for the hermaphrodites (organisms that have male and female organs in one being), he thought it sufficed to assume the creation of one sole individual, since this would be fully competent to propagate its species. Further developing THE OLDER PHYLOGEXY these mystic ideas, Linne went on to borrow from Genesis the account of the deluge and of Noah's ark as a ground for the chorology of organisms — that is to say, for a science of their geographical and topographical distribution. 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 than 16,000 feet, and thus provides in its higher zones the several climates demanded by the various species of animals and plants : the animals that were accustomed to a cold climate could remain at the summit ; those used to a warm climate could descend to the foot ; and those requiring a temperate climate could remain half-way down. From this point the re-population of the earth with animals and plants could proceed. It was impossible to have any scientific notion of the method of evolution in Linne's time, as one of the chief sources of information, paleontologv, was still wholly unknown. This science of the fossil remains of extinct animals and plants is very closely bound up with the whole question of evolution. It is impossible to explain the origin of living organisms without appealing to it. But this science did not rise until a much later date. The real founder of scientific paleontology was Georges Cuvier, the most distin- guished zoologist who, after Linne, worked at the classifi- cation of the animal world, and effected a complete revolution in systematic zoology at the beginning of the nineteenth century. The influence of this famous scientist, which was of extraordinary service, especially in the first three decades of the century, was so great that he opened up new paths in nearly every part of scientific zoology, particularly in classification, comparative anatomy, and paleontology. It is important, therefore, to inquire what idea Cuvier had of the nature of the species. In this respect he associated himself with Linne and the Mosaic story of creation, though this was more difficult for him with his acquaintance with THE OLDER PHYLOGENY 63 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. Cuvier had, naturally, to meet the question of the origin of these different populations, and if they were connected with each other or not. He answered this question in the negative, affirming that the successive populations were quite independent of each other, and that therefore the super- natural creative act, which was demanded as the origin of the animals and plants by the dominant creed, must have been repeated several times. In this way a whole series of different creative periods must have succeeded each other ; and in connection with these he had to assume that stupendous revolutions or cataclysms — something like the legendary deluge — must have taken place repeatedly. Cuvier was all the more interested in these catastrophes or 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 paleontological and zoological research; and in his effort to get a consistent view o( the whole process of the earth's history he came to form the theory which is known as "the catastrophic theory," or the theory ot terrestrial revolutions. According to this theory, there have been a series of mighty cataclysms on the earth, and these have suddenly destroyed the whole animal and plant population then living on it ; after each cataclysm there was a fresh creation o\ living things throughout the earth. As this creation could not be 64 THE OLDER PHYLOGENY explained by natural laws, it was necessary to appeal to an intervention on the part of the Creator. This catastrophic theory, which Cuvier described in a special work, was soon generally accepted, and retained its position in biology for half a century. However, Cuvier's theory was completely overthrown sixty years ago by the geologists, led by Charles Lyell, the most distinguished worker in this field of science. Lyell proved in his famous Principles of Geology (1830) that the theory was false, in so far as it concerned the crust of the earth; that it was totally unnecessary to bring in supernatural agencies or general catastrophes in order to explain the structure and formation of the mountains; and that we can explain them by the familiar agencies which are at work to-day in altering and reconstructing the surface of the earth. These causes are — the action of the atmosphere and water in its various forms (snow, ice, fog, rain, the wear of the river, and the stormy ocean), and the volcanic action which is exerted by the glowing central mass. Lyell convincingly proved that these natural causes are quite adequate to explain every feature in the build and formation of the crust. Hence Cuvier's theory of cataclysms was very soon driven out of the province ot geology. Nevertheless, the theory remained for another thirty years in undisputed authority in biology. All the zoologists and botanists who gave any thought to the question of the origin of organisms adhered to Cuvier's erroneous idea of revolutions and new creations. It is one of the most curious instances on record of two cognate sciences pursuing for some time totally different ways from each other. Biology lagged behind on the paths of dualism, and declared it impossible to solve the problem of the formation of species on natural principles; geology, on the contrary, advanced rapidly along the monistic path, and solved the problem by the indication of the natural agencies at work. In order to illustrate the complete stagnancy of biology from 1830 to 1859, on the question of the origin of organisms, or the formation of the various species of animals and plants, THE OLDER PHYLOGENY 65 I may say, from my own experience, that during the whole of my university studies I never heard a single word said about this most important problem o( the science. I was fortunate enough at that time (1852 [857) to have the most distinguished masters for every branch o( biological science. Not one o( them ever mentioned this question o( the origin o( speeies. Not a word was ever said about the earlier efforts to understand the formation of living things, nor about Lamarck's Philosophic Zoologique which had made a fresh attack on the problem in 1S09. Hence it is easy to under- stand the enormous opposition that Darwin encountered when he took up the question for the first lime. His views seemed to float in the air, without a single previous effort to support them. The whole question o( the formation of living things was considered by biologists, until 1859, as pertaining to the province of religion and transcendentalism; even in specula- tive philosophy, in which the question had been approached from various sides, no one had ventured to give it serious treatment. This last circumstance was due to the dualistic system of Immanuel Kant, and the enormous influence of this most important of recent thinkers down to our own time. Kant, a genius both in science and philosophy, taught a natural system oi evolution as far as the inorganic world was con- cerned ; but, on the whole, adopted a supernaturalist system as regards the origin of living things. In his Genera/ History ami Theory of flic Heavens [translated in Kant's Cosmogony] Kant made a very happy effort to deal with the structure and mechanical origin o\ the universe on Newton's principles — in other words, to explain it on mechanical and monistic principles; and this effort to explain the origin o( the universe by natural, efficient causes is still the basis of cosmogony. But Kant affirmed that this "principle o\ natural mechanicism, without which there can be no real science," was quite incapable o( furnishing an explanation o( organic phenomena, and especially of the origin o( living things; and that we must turn to supernatural or final causes for the explanation of the origin o\ these designed structures. THE OLDER PHYLOGEXY He even went so far as to say: " It is quite certain that we cannot even satisfactorily understand, much less explain, the nature of an organism and its internal forces on purelv mechanical principles ; it is so certain, indeed, that we may confidently say: ' It is absurd for a man to imagine even that some dav a Newton will arise who will explain the origin of a single blade of grass by natural laws not controlled by design' — such a hope is entirely forbidden us." In these words Kant definitely adopts the dualistic and teleological point of view for biological science.1 Nevertheless, Kant deserted this point of view at times, particularly in several remarkable passages which I have dealt with at length in my Natural History of Creation (chap, v.), where he expresses himself in the opposite, or monistic, sense. In fact, these passages would justify one, as I showed, in claiming his support for the theory of evolu- tion. Several very significant passages which Fritz Schultze has brought to light in his interesting work, Kant unit Darwin, seem to give Kant the character of being the first Darwinian prophet. He quite clearly enunciates the great idea of an all-embracing and monistic evolution. He speaks of "a falling away from the primitive type of the genus by natural variations." In fact, he affirms that " man originally walked on four legs, and only gradually developed the erect attitude, and raised himself so proudly above his former animal comrades." However, these monistic passages are only stray gleams of light; as a rule, Kant adheres in biology to the obscure dualistic ideas, according to which the forces at work in inorganic nature are quite different from those of the organic world. This dualistic system prevails in academic philosophy to-day — most of our philosophers still regarding these two provinces as totally distinct. They put, on the one side, the inorganic or "lifeless" world, in which there are at work only mechanical laws, acting necessarily and without design; and, on the other, the province of organic nature, in which none of the phenomena can be properly understood, 1 Kritik tier teleologischen Urtheilskraft, %% 74 and 79. [I translate Haeckel's quotation. — Trans.] THE OLDER PHYLOGENY 67 either as regards their inner nature or their origin, except in the light of preconceived design, carried out by final or purposive causes. The prevalence of this unfortunate dualistic prejudice prevented the problem o\ the origin o( species, and the con- nected question of the origin of man, from being regarded by the bulk of people as a scientific question at all until [859. Nevertheless, a few distinguished students, free from the current prejudice, be^an, at the commencement Of the nine- teenth century, to make a serious attack on the problem. The merit of this attaches particularly to what is known as " the older school of natural philosophy," which has been so much misrepresented, and which included Jean Lamarck, Buffon, Geoffroy St. Hilaire, and Blainville in France; Wolfgang Goethe, Reinhold Treviranus, Schelling, and Lorentz Oken in Germany [and Erasmus Darwin in England]. The gifted natural philosopher who treated this difficult question with the greatest sagacity and comprehensiveness was Jean Lamarck. He was born at Bazentin, in Picardv, on August 1st, 1744; he was the son of a clergyman, and was destined for the Church. But he turned to seek glory in the army. In his sixteenth year he distinguished dimself by his bravery in the battle of Lippstadt, and was then in garrison in the south o\ France for several years. Here he be^an to study the interesting flora of the Mediterranean coast, and it inspired him with a love of botany. He resigned his com- mission, and in 1 778 published his important work, Flore Francaisc. for a long time he tailed to secure a place in science, and it was not until his fiftieth year ( 1 7<)4) that he was offered the chair of zoology at the museum o\ the Jardin des Plantes at Paris. He then went deeper into zoology, and he soon rendered as great a service in zoological classifi- cation as he had done in botany. In iSoj he published his Considerations sur les corps vivants, in which we find the i^erms o\ his theory o\ evolution. In 1809 appeared his chief work, the famous Philosophic Zoohgiqtte, in which he developed his theory. In [815 he published his comprehen- sive natural history o\ the vertebrates ( llistoirc naturelle des THE OLDER PHYLOGEXY animaux sans vertebres), in the introduction to which his theory is again touched upon. About this time he became totally blind. Fortune, in her jealousy, never favoured him. While his fortunate rival, Cuvier, rose to the highest point of scientific fame and prestige at Paris, the great Lamarck — far greater than Cuvier in the vastness of his speculations and his conception of Nature — had to struggle in solitude for the necessities of life. His laborious life ended, in circumstances of great poverty, in 1829. Lamarck's PhUosophie Zoologiquc1 was the first scientific attempt to sketch the real course of the origin of species, the first " natural history of creation " of plants, animals, and men. But, as in the case of Wolff's book, this remarkably able work had no influence whatever ; neither one nor the other could obtain any recognition from their prejudiced con- temporaries. No man of science was stimulated to take an interest in the work, and to develop the germs it contained of the most important biological truths. The most distinguished botanists and zoologists entirely rejected it, and did not even deign to reply to it. Cuvier, who lived and worked in the same city, has not thought lit to devote a single syllable to this great achievement in his memoir on progress in the sciences, in which the pettiest observations found a place. In short, Lamarck's Phi/osofihie Zoologique shared the fate of Wolff's theory of development, and was for half a century ignored and neglected. The German scientists, especially Oken and Goethe, who were occupied with similar specula- tions at the same time, seem ft) have known nothing about Lamarck's work. If they had known it, they would have been greatly helped by it, and might have carried the theory of evolution much farther than they found it possible to do. To give an idea of the great importance of the Philosophic Zoologique, I will briefly explain Lamarck's leading thought. He held that there was no essential difference between living and lifeless beings. Nature is one united and connected system of phenomena ; and the forces which fashion the 1 New edition, with biographical introduction by Charles Martin. (Paris, '873O r/lK OLDER PHYLOGENY (»> lifeless bodies arc the only ones at work in the kingdom of living things. We have, therefore) to use the same method of investigation and explanation in both provinces. Life is only a physical phenomenon. All the plants and animals, with man at their head, are to be explained, in structure and life, by mechanical or efficient causes, without any appeal to final causes, just as in the case of minerals and other inorganic bodies. This applies equally to the origin of the various species. We must not assume any original creation, or repeated creations (as in Cuvier's theory), to explain this, but a natural, continuous, and necessary evolution. The whole evolutionary process has been uninterrupted. All the different kinds of animals and plants which we see to-day, or that have ever lived, have descended in a natural way from earlier and different species ; all come from one common stock, or from a few common ancestors. These remote ancestors must have been quite simple organisms of the lowest type, arising by spontaneous generation from inorganic matter. The succeeding species have been constantly modified by adaptation to their varying environ- ment (especially by use and habit), and have transmitted their modifications to their successors by heredity. These are the chief outlines of Lamarck's theory, which we now call the theory of descent or " transformism," and which was unrecognised till Darwin took it up and gave it fresh support fifty years later. Lamarck is the real founder of the theory of evolution, and it is incorrect to speak of Darwin as its first champion. Lamarck was the first to formulate as a scientific theory the natural origin of living things, includ- ing man, and to push the theory to its extreme conclusions the rise of the earliest organisms by spontaneous generation (or abiogenesis) and the descent of man from the nearest related mammal, the ape. Lamarck sought to explain this last point, which is ol 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 THE OLDER PHYLOGEXY agencies. The most important modifications of the organs of plants and animals are due, in his opinion, to the function of these very organs, or to the use or disuse of them. To give a few examples, the woodpecker and the humming-bird have got their peculiarly long tongues from the habit of extracting their food with their tongues from deep and narrow folds or canals ; the frog has developed the web between his toes by his own swimming ; the giraffe has lengthened his neck by stretching up to the higher branches of trees, and so on. It is quite certain that this use or disuse of organs is a most important factor in organic development, but it is not sufficient to explain the origin of species. To adaptation we must add heredity as the second and not less important agency, as Lamarck perfectly recognised. He said that the modification of the organs in any one individual by use or disuse was slight, but that it was increased by accumulation in passing by heredity from generation to generation. But he missed altogether the principle which Darwin afterwards found to be the chief factor in the theory of transformation — namely, the principle of natural selection in the struggle for existence. It was partly owing to his failure to detect this supremely important element, and partly to the poor condition of all biological science at the time, that Lamarck did not succeed in establish- ing more firmly his theory of the common descent of man and the other animals. Lamarck tried to explain the descent of man from the ape chiefly by advance in the habits of the ape, and by a pro- gressive development and use of its organs and the trans- mission to posterity of the modifications thus produced. He considered the most important of these improvements to be man's erect attitude, the modification of the hands and feet, and the acquisition of speech and accompanying develop- ment of the brain. He believed that the man-like apes, which were man's ancestors, had taken the first step towards humanity when they ceased to climb trees and began to walk erect. This led to the distinctive human carriage, the modifi- cation of the vertebral column and the pelvis, and the THE Ol.ni-.K PHYLOGENY differentiation of the upper and lower limbs : the upper limbs became hands, and were used for grasping and touching things, while the lower were confined to locomotive purposes, and became feet pure and simple. As a result of this complete change of habits, and in virtue of the correlation of the various organs and their functions, a number of other modifications were caused. Thus the change in diet led to a modification of the jaws and teeth, and therefore of the whole face. The tail was no longer of any use, and it gradually disappeared. And as these apes lived in troops and had regular family relations (as is the case to-dav with the higher apes), the gregarious or social instincts were strongly developed. The simple sound-speech of the ape grew into the articulate speech of the man ; abstract ideas were formed from the groups of concrete impressions. Thus step by step the brain advanced, and with it the larynx — the organ of mind simultaneously with the organ of speech. In these most interesting speculations of Lamarck we have the germs of a sound theory of the evolution of man. (Cf. Packard). Independentlv of Lamarck, the older German school of natural philosophv, especially Reinhold Treviranus, in his Biologic (1802), and Lorentz Oken, in his NaturphUosophie (1809), turned its attention to the problem of evolution about the end of the eighteenth and beginning of the nineteenth century. I have described its work in my Natural History of Creation (chap. iv.). Here I can only deal with the brilliant genius whose evolutionary ideas are of special interest — the greatest of German poets, Wolfgang Goethe. With his keen eye for the beauties of nature, and his profound insight into its life, Goethe was early attracted to the study of various natural sciences. It was the favourite occupation of his leisure hours throughout life. He gave particular and protracted attention to the theory of colours. But the most valuable of his scientific studies are those which relate to that "living, glorious, precious thing," the organism. lie made profound research into the science of structures or morpho- logy (morphae forms). Here, with the aid of comparative THE OLDER PHYLOGEXY anatomy, he obtained the most brilliant results, and went far in advance of his time. I may mention, in particular, his vertebral theory of the skull, his discovery of the pineal gland in man, his system of the metamorphosis of plants, etc. These morphological studies led Goethe on to research into the formation and modification of organic structures which we must count as the first germ of the science of evolution. He approaches so near to the theory of descent that we must regard him, after Lamarck, as one of its earliest founders. It is true that he never formulated a complete scientific theory of evolution, but we find a number of remarkable suggestions of it in his splendid miscellaneous essays on morphology. Some of them are really among the very basic ideas of the science of evolution. I will quote here only one or two of the most remarkable passages : " We have got far enough, then, to say confidently that all the higher organic natures, in which we include the fishes, amphibia, birds, and mammals, with man at their head, are made after one primitive type, and this only oscillates a little to one side or other of its steady features, and daily advances and is modified by reproduction" (1796). This "primitive type," on which even man is modelled, corresponds to our common ancestral form of the vertebrate stem, from which all the different species of vertebrates have arisen by " incessant formation, modification, and reproduction." In another place Goethe says (1807) : "When we compare plants and animals in their most rudimentary forms, it is almost impossible to distinguish between them. But we may say that the plants and animals, beginning with an almost inseparable closeness, gradually advance along two divergent lines, until the plant at last grows in the solid, enduring tree and the animal attains in man to the highest degree of mobility and freedom." That Goethe was not merely speaking in a poetical, but in a literal genealogical, sense of this close affinity of organic forms is clear from other remarkable passages in which he treats of their variety in outward form and unity in internal structure. He believes that every living thing has arisen by THE OLDER PHYLOGENY the interaction of two opposing formative forces or impulses. The internal or "centripetal " force, the type or " impulse to specification," seeks to maintain the constancy of the specific forms in the succession of generations: this is heredity. The external or " centrifugal " force, the element of variation or " impulse to metamorphosis," is continually modifying the specie^ by changing their environment : this is adaptation. In these significant conceptions Goethe approaches very close to a recognition of the two great mechanical factors which we now assign as the chief causes of the formation of species. However, in order to appreciate Goethe's views on morphology, one must associate his decidedly monistic conception of nature with his pantheistic philosophy. The warm and keen interest with which he followed, in his last years, the controversies of contemporary French scientists, and especially the struggle between Cuvi^er and Geoffrey St. Ililaire (see chap. iv. of The Natural 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. Otherw ise, one is apt to make serious errors. In a lecture that I delivered in [882 at the Congress of German scientists and medical men at Eisenach I made a rather full comparison of the scientific ideas of Darwin, Goethe, and Lamarck, and showed their important bearing on the pantheistic philosophv. In my opinion, these three greatest figures in modern thought stand on the common ground of Monism, or the system which teaches the unity of the universe on scientific grounds. All held the belief in the unity of God and Xature which was defended by Giordano Bruno and Spinoza, and which Goethe expressed so nobly in his writings on God and the World. We can understand, therefore, the lively interest which Goethe maintained till his last days in the highest questions of biology. The passages which I have quoted on the title-pages of the chapters in my GenereUe Morphologie show how firm a grasp he had ot the intimate genetic relation of all organic forms. Me approached 74 THE OLDER PHYLOGEXY so close, at the end of the eighteenth century, to the principles of the science of evolution that he may well be described as the first forerunner of Darwin, although he did not go so far as to formulate evolution as a scientific system, as Lamarck did. THE MODERN SCIENCE OF EVOLUTION its influence, new structures, or alterations of structure, are produced ; and these are purposive in the sense that they serve the organism when formed, but they were produced without any pre-eonceived aim. This simple idea is the central thought of Darwinism, or the theory of selection. Darwin conceived this idea at an early date, and then, for more than twenty years, worked at the collection of empirical evidence in support of it before he published his theory. I have described the chief features of his method, his life, and his writings in my Natural History of Creation. The ample biography, in three volumes, pub- lished by his son, Francis Darwin, in 1S87, gives full information about him. Here 1 will only refer to some of the salient points. Charles Darwin was born on February 12th, 1S09, at Shrewsbury, where his father, Robert Darwin, had a medical practice. Mis grandfather, Erasmus Darwin, was an able scientist of the older school of natural philosophv, who published a number of natural-philosophic works about the end of the eighteenth century. The most important of them is his Zoonomta, published in 1704, in which he expounds views similar to those of Goethe and Lamarck, without, how- ever, knowing anything of the work of these contemporaries. By the law of latent heredity, or "atavism," Erasmus Darwin transmitted a part of his ability to his grandson Charles, though no trace of it is found in his son Robert. This is a very interesting case of atavism, a process which Charles Darwin himself treated so admirably. However, in the writings of the grandfather the plastic imagination rather outran the judgment, while in Charles Darwin the two were better balanced. As many narrow-minded scientists of our own day regard the imagination as superfluous in biology, and think their lack of it a great advantage in the way of " exactness," it is interesting to call attention to a striking saying of a gifted man of science who was himself one of the founders of the "exact" or strictly empirical school. Johannes Miiller, the German Cuvier, whose works will ever remain a model of accurate research, declared that a constant inter- action and harmonious adjustment of the imagination and 78 THE MODERX SCIEXCE OF EVOLUTIOX the intellect was an indispensable condition for making great discoveries. Charles Darwin was fortunate enough to take part in a scientific expedition at the close of his university career in his twenty-second year. This lasted five years, and greatly stimulated him and enriched his fund of knowledge. At the very beginning of it, as soon as he landed in America, he was attracted by a number of phenomena which suggested the chief problem of his life — the question of the origin of species. The instructive facts of the geographical distribu- tion of species, on the one hand, and the relation of living to dead species of the same locality on the other, prompted him to surmise that closely-related species must have descended from a common stem form. Then, at the close of his voyage, when he devoted himself for a year with great vigour to the systematic study of domestic animals and garden plants, he noticed the obvious analogies in structures between them and the corresponding species in the wild state. But he did not come to conceive the chief point of his theory, natural selection through the struggle for life, until he read Malthus's famous Essay on Population. He then saw clearly the analogy between the relations of population and over-popu- lation in civilised communities and the mutual relations of animals and plants in a natural state. For many years he collected material to give a massive support to his theory. At the same time, he made a number of experiments himself in artificial selection, and gave special attention to the action of selection on tame pigeons. The quietness of his life on his estate at Down, near Beckenham, gave him requisite leisure. He died there on April 19th, 18S2, working assidu- ously until death at the establishment of his epoch-making theory by new discoveries. Darwin did not publish any account of his theory until 1858, when Alfred Russel Wallace, who had independently reached the same theory of selection, published his own work. In the following year appeared the Origin of Species, in which he developes it at length and supports it with a mass of proof. As I have given my opinion on it fully in my Generelle THE MODERN SCIENCE OF EVOLUTION 79 Morphologic and Natural History of Creation, I need not stay to do so here, and will only add a word on the essence oi the Darwinian theory, on the understanding of which all the rest depends. This is the simple prineiple that the stru^le for lite modifies living things in the natural condition, and pro- duces new species, through the same agencies which man employs in artificially forming new varieties of animals and plants. These agencies virtually exercise a selection among the individuals brought into existence, heredity and adapta- tion acting together throughout as the chief plastic forces." Darwin's younger contemporary, Alfred Russel Wallace, the famous traveller, had reached the same conclusion. But he had not so clear a perception as Darwin of the effectiveness of natural selection in forming species, and did not develop the theory so fully. Nevertheless, Wallace's writings, espe- cially those on mimicry, etc., and an admirable work on The Geographical Distribution of Animals, contain many fine original contributions to the theory of selection. Unfortu- nately, this gifted scientist has since devoted himself to spiritism. Darwin's Origin of Species had an extraordinary influence, though not at first on the experts of the science. It took zoologists and botanists several years to recover from the astonishment into which they had been thrown through the revolutionary idea of the work. But its influence on the special sciences with which we zoologists and botanists arc concerned has increased from year to year; it has introduced a most healthy fermentation in every branch of biology, especially in comparative anatomy and ontogeny, and in zoological and botanical classification. In this way it has brought about almost a revolution in the prevailing views. However, the point which chiefly concerns us here — the extension of the theory to man — was not touched at all in Darwin's first work in 1859. It was believed for several years that he had no thought of applying his principles to man, ' Darwin and Wallace arrived al the theory quite independently. Vide Wallace's Contributions In the Theory of Natural Selection ( [870) and Darwinism (■891). THE MODERX SCIENCE OF EVOLVTIOX but that he shared the current idea of man holding a special position in the universe. Not only ignorant laymen (espe- cially several theologians), but also a number of men of science, said very naively that Darwinism in itself was not to be opposed ; that it was quite right to use it to explain the origin of the various species of plants and animals, but that it was totally inapplicable to man. In the meantime, however, it seemed to a good many thoughtful people, laymen as well as scientists, that this was wrong; that the descent of man from some other animal species, and immediately from some ape-like mammal, followed logically and necessarily from Darwin's reformed theory of evolution. Many of the acuter opponents of the theory saw at once the justice of this position, and, as this consequence was intolerable, they wanted to get rid of the whole theory. The first scientific application of the Darwinian theory to man was made by Huxley, the greatest zoologist in England. This able and learned scientist, to whom zoology owes much of its progress, published in 1S63 a small work entitled Evidence as to Man's Place in Mature. In the extremely important and interesting lectures which made up this work he proved clearly that the descent of man from the ape followed necessarily from the theory of descent. If that theory is true, we are bound to conceive the animals which most closely resemble man as those from which humanity has been gradually evolved. About the same time Carl Vogt published a larger work on the same subject — Vorle- sungen iiber den menschen seine Stellung in der Schcpjung and in der Geschichte der Erde. We must also mention Gustav Jaeger and Friedrich Rolle among the zoologists who accepted and taught the theory of evolution immediately after the publication of Darwin's book, and maintained that the descent of man from the lower animals logically followed from it. The latter published, in 1866, a work on the origin and position of man. About the same time I attempted, in the second volume of my Generelle Morphologie der Organismen (1866), to apply the THE MODERN SCIENCE OF EVOLUTION theory of evolution to the whole organic kingdom, including man.1 I endeavoured to sketch the probable ancestral trees of the various classes of the animal world, the protists, and the plants, as it seemed necessary to do on Darwinian principles, and as we can actually do now with a high degree of confidence. If the theory of descent which Lamarck first clearly formulated and Darwin thoroughly established is true, we seem to be able to draw up a natural classification of plants and animals in the light of their genealogy, and to conceive the large and small divisions of the system as the branches and twigs of an ancestral tree. The eight genealo- gical tables which I inserted in the second volume of the Generelle Morphologie are the first sketches of their kind. In the twenty-seventh chapter, particularly, I trace the chief stages in man's ancestry, as far as it is possible to follow it through the vertebrate stem. I tried especially to determine, as well as one could at that time, the position of man in the classification of the mammals and its genealogical signifi- cance. I have greatly improved this attempt, and treated it in a more popular form, in chaps, xxvi.-xxviii. of ray Natural History of Creation (1868).2 It was not until 1871, twelve years after the appearance of The Origin of Species, that Darwin published the famous work which made the much-contested application of his theory to man, and crowned the splendid structure of his system. This important work was The Descent of Man, and Selection in Relation to Sex. In this Darwin expressly drew the conclu- sion, with rigorous logic, that man also must have been developed out of lower species, and described the important part played by sexual selection in the elevation of man and the other higher animals. He showed that the careful selection which the sexes exercise on each other in regard to sexual relations and procreation, and the aesthetic feeling which the higher animals develop through this, are of the 1 Huxley spoke of this as "one of the greatest scientific works ever published." -Tkans. * Of which Darwin said that the Descent of Man would probably never have been written if he had seen it earlier. — TRANS. G THE MODERN SCIENCE OF EVOLUTION utmost importance in the progressive development of forms and the differentiation of the sexes. The males choosing the handsomest females in one class of animals, and the females choosing only the finest-looking males in another, the special features and the sexual characteristics are increasingly accentuated. In fact, some of the higher animals develop in this connection a finer taste and less prejudiced judgment than man himself. But, even as regards man, it is to this sexual selection that we owe the family-life, which is the chief foundation of civilisation. The rise of the human race is due for the most part to the advanced sexual selection which our ancestors exercised in choosing their mates. (Cf. the eleventh chapter of the Natural History of Creation and the second volume of the Generelle Morphologic.) Darwin accepted in the main the general outlines of man's ancestral tree, as I gave it in the Generelle Morpho- logie and the Natural History of Creation, and admitted that his studies led him to the same conclusion. That he did not at once apply the theory to man in his first work was a commendable piece of discretion ; such a sequel was bound to excite the strongest opposition to the whole theory. The first thing to do was to establish it as regards the animal and plant worlds. The subsequent extension to man was bound to be made sooner or later. It is important to understand this very clearly. If all living things come from a common root, man must be included in the general scheme of evolution. On the other hand, if the various species were separately created, man, too, must have been created, and not evolved. We have to choose between these two alternatives. This cannot be too frequently or too strongly emphasised. Either all the species of animals and plants are of supernatural origin — created, not evolved — and in that case man also is the outcome of a creative act, as religion teaches; or the different species have been evolved from a few common, simple ancestral forms, and in that case man is the highest fruit of the tree of evolution. We may state this briefly in the following principle: — The THE MODERN SCIENCE OF EVOLUTION 83 descent of man from the lower animals is a special deduction which inevitably follows from the general inductive lam of the whole theory of evolution. In this principle we have a clear and plain statement of the matter. Evolution is in reality nothing but a great induction, which we are compelled to make by the comparative study of the most important facts of morphology and physiology. But we must draw our con- clusion according to the laws of induction, and not attempt to determine scientific truths by direct measurement and mathematical calculation. In the study of living things we can scarcely ever directly and fully, and with mathematical accuracy, determine the nature of phenomena, as is done in the simpler study of the inorganic world — in chemistry, physics, mineralogy, and astronomy. In the latter, especially, we can always use the simplest and absolutely safest method — that of mathematical determination. But in biology this is quite impossible for various reasons ; one very obvious reason being that most o( the facts of the science are very complicated and much too intricate to allow a direct mathe- matical analysis. The greater part of the phenomena that biology deals with are complicated historical processes, which are related to a far-reaching past, and as a rule can only be approximate!}' estimated. Hence we have to proceed by induction — that is to say, to draw general conclusions, stage by Stage, and with proportionate confidence, from the accumulation of detailed observations. These inductive conclusions cannot command absolute confidence, like mathe- matical axioms; but they approach the truth, and gain increasing probability, in proportion as we extend the basis ot observed facts on which we build. The importance of these inductive laws is not diminished from the circumstance that they are looked upon merely as temporary acquisitions of science, and may be improved to any extent in the progress of scientific knowledge. The same may be said of the attainments o( many other sciences, such as geology or archeology. 1 lowever much they may be altered and im- proved in detail in the course of time, these inductive truths may retain their substance unchanged. 84 THE MODERN SCIENCE OF EVOLUTION Now, when we say that the theory of evolution in the sense of Lamarck and Darwin is an inductive law — -in fact, the greatest of all biological inductions — we rely, in the first place, on the facts of paleontology. This science gives us some direct acquaintance with the historical phenomena of the changes of species. From the situations in which we find the fossils in the various strata of the earth we gather confidently, in the first place, that the living population of the earth has been gradually developed, as clearly as the earth's crust itself; and that, in the second place, several different populations have succeeded each other in the various geological periods. Modern geology teaches that the formation of the earth has been gradual, and unbroken by any violent revolutions. And when we compare together the various kinds of animals and plants which succeed each other in the history of our planet, we find, in the first place, a constant and gradual increase in the number of species from the earliest times until the present day; and, in the second place, we notice that the forms in each great group of animals and plants also constantly improve as the ages advance. Thus, of the vertebrates there are at first only the lower fishes ; then come the higher fishes, and later the amphibia. Still later appear the three higher classes of vertebrates — the reptiles, birds, and mammals, for the first time; only the lowest and least perfect forms of the mammals are found at first; and it is only at a very late period that placental mammals appear, and man belongs to the latest and youngest branch of these. Thus perfection of form increases as well as variety from the earliest to the latest stage. That is a fact of the greatest importance. It can only be explained by the theory of evolution, with which it is in perfect harmony. If the different groups of plants and animals do really descend from each other, we must expect to find this increase in their number and perfection under the influence of natural selection, just as the succession of fossils actually discloses it to us. Comparative anatomy furnishes a second series of facts which are of great importance for the forming of our inductive law. This branch of morphology compares the adult THE MODERN SCIENCE OF EVOLUTION 85 Structures of living things, and seeks in the great variety of organic forms the stable and simple law of organisation, or the common type or structure. Since Cuvier founded this science at the beginning of the nineteenth century it has been a favourite study of the most distinguished scientists. Even before Cuvier's time Goethe had been greatly stimulated by it, and induced to take up the study of morphology. Compara- tive osteology, or the philosophic study and comparison of the bony skeleton of the vertebrates — one of its most interesting sections — especially fascinated him, and led him to form the theory of the skull which I mentioned before. Comparative anatomy shows that the internal structure of the animals of each stem and the plants of each class is the same in its essential features, however much they differ in external appearance. Thus man has so great a resemblance in the chief features of his internal organisation to the other mammals that no comparative anatomist has ever doubted that he belongs to this class. The whole internal structure of the human body, the arrangement of his various systems of organs, the distribution of the bones, muscles, blood-vessels, etc., and the whole structure of these organs in the larger and the finer scale, agree so closely with those of the other mammals (such as the apes, rodents, ungulates, cetacea, marsupials, etc.) that their external differences are of no account whatever. We learn further from comparative anatomy that the chief features of animal structure are so similar in the various classes (fifty to sixty in number alto- gether) that they may all be comprised in from eight to twelve great groups. But even in these groups, the stem- forms or animal types, certain organs (especially the alimen- tary canal) can be proved to have been originally the same for all. We can only explain by the theory of evolution this essential unity in internal structure of all these animal forms that differ so much in outward appearance. This wonderful fact can only be really understood and explained when \\l- regard the internal resemblance as an inheritance from common-stem forms, and the external differences as the effect of adaptation to different environments. THE MODERN SCIENCE OF EVOLUTION In recognising this, comparative anatomy has itself advanced to a higher stage. Gegenbaur, the most distin- guished of living 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 inconsistent with the theory of evolution ; indeed, they all lead to it. In this way the theory receives back from the science all the service it rendered to its method." Until then students had marvelled at the wonderful resemblance of living things in their inner structure without being able to explain it. We are now in a position to explain the causes of this, by showing that this remarkable agree- ment 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 rudimentary or useless organs ; I have given it the name of " dysteleology " in view of its philosophic consequences. Nearly every organism (apart from the very lowest), and especially every highly- developed animal or plant, including man, has one or more organs which are of no use to the body itself, and have no share in its functions or vital aims. Thus we all have, in various parts of our frame, muscles which we never use, as, for instance, in the shell of the ear and adjoining parts. In most of the mammals, especially those with pointed ears, these internal and external ear-muscles are of great service in altering the shell of the ear, so as to catch the waves of sound as much as possible. But in the case of man and other short-eared mammals these muscles are useless, though they are still present. Our ancestors having long abandoned the use of them, we cannot work them at all to-day. In the inner corner of the eye we have a small crescent-shaped fold of skin ; this is the last relic of a third inner eye-lid, called THE MODERN SCIENCE OF EVOLUTION 87 the nictitating (winking) membrane. This membrane is highly developed and o( 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 appendix. This small intestinal appendage is often the cause of a fatal illness. If a cherry-stone or other hard body is unfortunately squeezed through its narrow aperture during digestion, a violent inflammation is set up, and often proves fatal. This appendix has no use whatever now in our frame ; it is a dangerous relic of an organ that was much larger and was of great service in our vegetarian ancestors. It is still large and important in many vegetarian animals, such as the apes and the ungulates. There are similar rudimentary organs in all parts of our body, and in all the higher animals. They are among the most interesting phenomena to which comparative anatomy introduces us ; partly because they furnish one of the clearest proofs of evolution, and partly because they most strikingly refute the teleology of certain philosophers. The theory of evolution enables us to give a very simple explanation of these phenomena. We have to look on them as organs which have fallen into disuse in the course of many generations. With the decrease in the use of its function, the organ itself shrivels up gradually, and finally disappears. There is no other way of explaining rudimentary organs. Hence they arc also of great interest in philosophy; they show clearly that the monistic or mechanical view o( the organism is the only correct one, and that the dualistic or teleological conception is wrong. The ancient legend of the direct creation of man according to a pre-conceived plan and the empty phrases about "design " in the organism are completely shattered by them. It would lie difficult to conceive a more thorough refutation of teleology than is furnished by the fact that all the higher animals have these rudimentary organs. Moreover, in the light of these facts of dysteleology, we see THE MODERN SCIENCE OF EVOLUTION the hollowness of the phrases about a " moral government of the world." No one but a learned idealist or a well-meaning optimist who shuts his eyes to facts can speak to-day of such a "moral order." There is, unfortunately, no more trace of it in nature than in human life — no more in natural history than in the history of civilisation. A grim and ceaseless struggle for life is the real mainspring of the purposeless drama of the world's history. We can only see a " moral order" and "design" in it when we ignore the triumph of immoral force and the aimless features of the organism. Might goes before right as long as organic life exists. The theory of evolution finds its broadest inductive foundation in the natural classification of living things, which arranges all the various forms in larger and smaller groups, according to their degree of affinity. These groupings or categories of classification — the varieties, species, genera, families, orders, classes, etc. — show such constant features of co-ordination and subordination that we are bound to look on them as genealogical, and represent the whole system in the form of a branching tree. This is the genealogical tree of the variously related groups; their likeness in form is the expression of a real affinity. As it is impossible to explain in any other way the natural tree-like form of the system of organisms, we must regard it at once as a weighty proof of the truth of evolution. The careful construction of these genealogical trees is, therefore, not an amusement, but the chief task of modern classification. Among the chief phenomena that bear witness to the inductive law of evolution we have the geographical distri- bution of the various species of animals and plants over the surface of the earth, and their topographical distribution on the summits of mountains and in the depths of the ocean. The scientific study of these features — the " science of distri- bution," or chorology (chora = a place) — has been pursued with lively interest since the discoveries made by Alexander von Humboldt. Until Darwin's time the work was confined to the determination of the facts of the science, and chiefly aimed at settling the spheres of distribution of the existing THE MODERN SCIENCE OF EVOLUTION large and small groups of living things. It was impossible at that time to explain the causes of this remarkable distribu- tion, or the reasons why one group is found only in one locality and another in a different place, and why there is this manifold distribution at all. Here, again, the theory of evolution has given us the solution of the problem. It furnishes the only possible explanation when it teaches that the various species and groups of species descend from common stem-forms, whose ever-branching offspring have gradually spread themselves by migration over the earth. For each group of species we must admit a " centre of production," or common home ; this is the original habitat in which the ancestral form was developed, and from which its descendants spread out in every direction. Several of these descendants became in their turn the stem-forms for new groups of species, and these also scattered themselves by active and passive migration, and so on. As each migrating organism found a different environment in its new home, and adapted itself to it, it was modified, and gave rise to new for ins. This very important branch of science that deals with active and passive migration was founded by Darwin, with the aid of the theory of evolution ; and at the same time he advanced the true explanation of the remarkable chorological relation of the living population in any locality to the fossil forms found in it. Moritz Wagner very ably developed his idea under the title of "the theory of migration." In my opinion, this famous traveller has rather over-estimated the value of his theory of migration when he takes it to be an indispensable condition of the formation of new species and opposes the theory of selection. The two theories are not opposed in their main features. Migration (by which the stem-form of a new species is isolated) is really only a special case of selection. The striking and interesting facts of chorology can only be explained 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 THE MODERN SCIENCE OF EVOLUTION 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 (the oecology or ethology of organisms, from nomas, law or norm, and bios, life), the interesting facts of parasitism, domesticity, care of the young, social habits, etc., can only be explained by the action of heredity and adaptation. Formerly people saw only the guidance of a beneficent Providence in these phenomena ; to-day we discover in them admirable proofs of the theory of evolution. It is impossible to understand them except in the light of this theory and the struggle for life. Finally, we must, in my opinion, count among the chief inductive bases of the theory of evolution the fcetal develop- ment of the individual organism, the whole science of embryology or ontogeny. But as the later chapters will deal with this in detail, I need say nothing further here. I shall endeavour in the following pages to show, step by step, how the whole of the embryonic phenomena form a massive chain of proof for the theory of evolution ; for they can be explained in no other way. In thus appealing to the close causal connection between ontogenesis and phylogenesis, and taking our stand throughout on the biogenetic law, we shall be able to prove, stage by stage, from the facts of embryology, the evolution of man from the lower animals. The general adoption of the theory of evolution has definitely closed the controversy as to the nature or definition of the species. This question had received a great variety of answers during the last century, but no satisfactory result had been reached. Thousands of botanists and zoologists were engaged daily in the classification and description of species, but they made no progress. Many hundreds of thousands of animal and plant groups were declared to be " real species," without the authors being able to give any proof or logical justification of their divisions. There were endless controversies between the classifiers as to whether the group in question was a true or false species, a species or a variety, a sub-species or a race, though they had never THE MODERN SCIENCE OF EVOLUTION asked themselves the real meaning of these terms. If they had striven to be clear on this point, they would have seen long ago that the words have no absolute meaning whatever, but are only group-names, or categories of classification, with a purely relative value. In 1857, it is true, a famous and gifted, but inaccurate and dogmatic, scientist, Louis Agassiz, attempted to give an absolute value to these "categories of classification." He did this in his Essay on Classification, in which he turns upside down the phenomena of organic nature, and, instead of tracing them to their natural causes, examines them through a theological prism. The true species (bona species ) was, he said, an "incarnate idea of the Creator." Unfortu- nately, this pretty phrase has no more scientific value than all the other attempts to save the absolute or intrinsic value of the species. I believe I have shown this clearly enough in the exhaustive criticism of the morphological and physio- logical idea of the species and the categories of classification which I gave in my Generelle Morphologie (Band II., SS. 323- 402). Agassiz's "Creator" is an idealised man, an imagi- native architect, who is ever planning and producing new species. (See also the third chapter of the Natural History of Creation.) The dogma of the fixitv and creation of species lost its last great champion when Agassiz died in 1S73. 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 oi several species descending from a common ancestor, has been closed to-day by the removal of the sharp limits that had been set up between species and varieties on the one hand, and species and genera on the other. I gave an analytic proof o( this in my monograph on the sponges (1872), having made a very close study of variability in this small but highly instructive group, and shown the impossibility of making any dogmatic distinction of species. According as the classifier takes his ideas of genus, species, and variety in a broader or in a narrower sense, he will find in the small THE MODERN SCIENCE OF EVOLUTION group of the sponges either one genus with three species, or» three genera with 238 species, or 113 genera with 59I species. Moreover, all these forms are so connected by intermediate forms that we can convincingly prove the descent of all the sponges from a common stem-form, the olynthus. Here, I think, I have given an analytic solution of the problem of the origin of species, and so met the demand of certain opponents of evolution for an actual instance of descent from a stem-form. Those who are not satisfied with the synthetic proofs of the theory of evolution which are provided by comparative anatomy, embryology, paleontology, dysteleology, chorology, and classification, may try to refute the analytic proof given in my treatise on the sponge, the outcome of five years of assiduous study. I repeat: It is now impossible to oppose evolution on the ground that we have no convincing example of the descent of all the species of a group from a common ancestor. The monograph on the sponges furnishes such a proof, and, in my opinion, an indisputable proof. Any man of science who will follow the protracted steps of my inquiry and test my assertions will find that in the case of the sponges we can follow the actual evolution of species, in static nascenti. 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 longtime; one has only to open one's eyes to see them. It is a mistake to seek them in the discovery of intermediate forms between man and the ape, or the conversion of an ape into a human being by skilful education. The proofs lie in the great mass of empirical material we have already collected. They are furnished in the strongest form by the data of comparative anatomy and embryology, completed by paleontology. It is THE MODERN SCIENCE OF EVOLUTION 93 not a question now of detecting new proofs of the evolution of man, but of examining' and understanding the proofs we already have. It seems especially urgent to refer to-day to these various sources of phylogeny, and point out how they confirm each other, because the growth of specialism in every branch of biology and the enormous accumulation of fresh observations in detail have led to a certain amount of narrowness in appre- ciating them. Many modern embryologists occupy themselves with the application of their improved methods to the detailed study of minute sections of the embryo and the mechanical analysis of them, and fail to keep in view the entire organism and its important relations to others of the same stem, as shown in comparative anatomy and classification. Many of the misleading theories of this modern mechanical embryo- logy would never have been formulated if their authors had been acquainted with the relevant facts of paleontology. On the other hand, however, most of the paleontologists are ignorant of the most important results of comparative embryology, and so fail to appreciate the value of the bio- genetic law. However important it is to determine the facts of paleontology accurately, their evolutionary significance cannot be properly appraised without the aid of comparative anatomy and ontogeny. At the same time, workers in these latter sciences must never lose touch with the results of paleontology. Comparative anatomists will reach no satis- factory result if they seek to determine the homologies and affinities of animal forms merely by a comparison of living species, without any regard to their extinct ancestors. The distinguished New York paleontologist, Henry Osborn, has recently laid stress on the wisdom of basing the science of evolution on a comprehensive use of all the three sources of evidence. Our science requires these three supports as much as the stool needs its three legs. I was almost alone thirty-six years ago when I made the first attempt, in my Generellc Morpliologie, to put organic morphology on a mechanical foundation through Darwin's theory of descent. The association of ontogeny and 94 THE MODERN SCIENCE OF EVOLUTION phylogeny and the proof of the intimate causal connection between these two sections of the science of evolution, which I expounded in my work, met with the most spirited opposi- tion on nearly all sides. The next ten years were a terrible "struggle for life" for the new theory. But for the last twenty-five years the tables have been turned. The phyloge- netic method has met with so general a reception, and found so prolific a use in every branch of biology, that it seems superfluous to treat any further here of its validity and results. The proof of it lies in the whole morphological literature of the last three decades. But no other science has been so profoundly modified in its leading thoughts by this adoption, and been forced to yield such far-reaching consequences, as that science which I am now seeking to establish — monistic anthropogeny. This statement may seem to be rather audacious, since the very next branch of biology, anthropology in the stricter sense, makes very little use of these results of anthro- pogeny, and sometimes expressly opposes them. This applies especially to the attitude which has characterised the German Anthropological Society (the Deutsche Gesell- schaftfiir Autliropologie) for some thirty years. Its powerful president, the famous pathologist, Rudolph Virchow, is chiefly responsible for this. Until his death (September 5th, 1902) he never ceased to reject the theory of descent as unproven, and to ridicule its chief consequence — the descent of man from a series of mammal ancestors — as a fantastic dream. I need only recall his well-known expression at the Anthropological Congress at Vienna in 1894, that " it would be just as well to say man came from the sheep or the elephant as from the ape." Virchow's assistant, the secretary of the German Anthro- pological Society, Professor Johannes Ranke of Munich, has also indefatigably opposed transformism : he has succeeded in writing a work in two volumes ( Der Mensch), in which all the facts relating to his organisation are explained in a sense hostile to evolution. This work has had a wide circulation, owing- to its admirable illustrations and its able treatment of Till-: MODERN SCIENCE OF EVOLUTION 95 the most interesting facts of anatomy and physiology — exclusive o( the sexual organs! But, as it has done a great deal to spread erroneous views among the general public, I have included a criticism o( it in mv Natural History of Creation, as well as met Virchow's attacks on anthropogeny. Neither Virchow, nor Ranke, nor any other "exact" anthropologist, has attempted to give any other natural explanation o( the origin of man. Thev have either set completely aside this "question of questions" as a tran- scendental 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 ol man on the solid facts of his embryology. CHAPTER VI. THE OVUM AND THE AMCEBA1 The ovum of man and other animals is a simple cell. The fully-developed man is an organised community of cells. Independent cells and tissue- cells. Importance and chief features of the cell theory. Definition, form, and size of the cell. Consists of two parts : Nucleus (caryoplasm) and cell-body (cytosoma — cytoplasm). Active protoplasm and passive products of protoplasm. The cell as the elementary organism, or the unit-individual. Plastids, or constructive cells. Their vital phenomena. Vegetal functions (nutrition, reproduction). Animal functions (movement, sensation). The special features of the ovum. Yelk. Germinal vesicles. Germinal disc. Coverings of the ovum, ovolemma or chorion. Application of the biogenetic law to the ovum. Unicellular organisms. The amoeba. Structure and functions of the amoeba. Amoeboid movements. Amoeboid cells in the multicellular organism. Their movements and intussusception of ^olid matter. Blood-cells that eat. Comparison of the amoeba with the ovum. Amoeboid ova of the sponges and their movements. Evolutionary con- clusion from the unicellular ovum to the unicellular ancestor. In order to understand clearly the course of human embryo- logy, we must select the more important of its wonderful and manifold processes for fuller explanation, and then proceed from these to the innumerable features of less importance. The most important feature in this sense, and the best starting-point for ontogenetic study, is the fact that man is developed from an ovum, and that this ovum is a simple cell. The human ovum does not materially differ in form and composition from that of the other mammals, whereas there is a distinct difference between the fertilised ovum of the mammal and that of any other animal. This fact is so important that few should be unaware of its extreme significance ; yet it was quite unknown in the first quarter of the nineteenth century. As we have seen, the human and mammal ovum was not discovered until 1827, when Carl Ernst von Baer detected it. Up to that time the larger vesicles, in which the real and much smaller ovum is 1 Cf. Edmund Wilson, The Cell in Development and Inheritance. 96 the ovr.M A.xn the a M(E ha contained, had been wrongly regarded as ova. The important circumstance that this mammal ovum is a simple cell, like the ovum of other animals, could not, of course, be recognised until the cell theory was established. This was not done, by Schleiden for the plant and Schwann for the animal, until 1838. As we have seen, this cell theory is of the greatest service in explaining the human frame and its embryonic development. Hence we must say a few words about the actual condition of the theory and the significance of the \iews it has suggested. In order properly to appreciate the cellular theory, the most important ele- ment in our morphological and physio- logical science, it is necessary to under- stand in the first place that the cell is a unified organism, a self-contained living beinsj. When we anatomically dissect , „FlG- '"— Tne & - human ovum, mag- the fullv-formed animal or plant into its nified 100 times. The . globular mass of yelk various organs, and then examine the \/,) is enclosed by a finer structure of these organs with the ^am^o^lem™ microscope, we are surprised to find that or zona pellucida [a]), and contains a non- all these different parts are ultimately central nucleus (the , r , . , gferminal vesicle, c\. made up of the same structural element Cim plg_ t/. or unit. This common unit of structure is the cell. It does not matter whether we thus dissect a leaf, flower, or fruit, or a bone, muscle, gland, or bit of skin, etc.; we find in every case the same ultimate constituent, which has been called the cell since Schleiden's discovery. There are many opinions as to its real nature, but the essential point in our view of the cell is to look upon it as a self-contained or independent living unit. It is, in the words of Briicke, "an elementary organism," or, as Virchow puts it, "a vital focus," a "biomeron." We may define it most precisely as the ultimate organic unit, or "an individual of the first class"; and as the cells are the sole active principles in every vital function, we may call them the " plastids," or "forma- tive elements" (cf. the Gen. Morph., Band I., S 269). gS THE OVUM AXD THE AMCEBA 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 autonomous cell throughout life. But in the histonal (tissue-forming) animals and plants, which are the great majority, the organism begins its career as a simple cell, and then grows into a cell-community, or, more correctly, an organised cell- state. Our own body is not really the simple unity that it is generally supposed to be. On the contrary, it is a very elaborate social system of countless microscopic organisms, a colony or commonwealth, made up of innumerable independent units, or very different tissue-cells. In reality, the term "cell," which existed long before the cell theory was formulated, is not happily chosen. Schleiden, who first brought it into scientific use in the sense of the cell theory, gave this name to the elementary organisms because, when you find them in the dissected plant, they generally have the appearance of chambers, like the cells in a bee-hive, with firm walls and a fluid or pulpy content. This idea of a cell as a closed vesicle or little sac, with a fluid content and firm envelope or wall, was adopted, and came into general use ; but it is totally inapplicable to most of the cells in the body. The more we learned about the cells of the animal body, the more it became necessary to modify our concep- tion of the cell ; for some cells, especially young ones, are entirely without the enveloping membrane, or stiff wall. Hence we now generally describe the cell as a living, viscous particle of protoplasm, enclosing a firmer nucleus in its albuminoid body. There maybe an enclosing membrane, as there actually is in the case of most of the plants ; but it may be wholly lacking, as is the case with most of the animals. There is no membrane at all in the first stage. The young cells are usually round, but they vary much in shape later on. Illustrations of this will be found in the cells of 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 THE OVUM AND THE AM (EISA nucleus (or catyon, or cylob/astus, Fig. ic and Fig. 2k). The outer and larger part, which encloses the other, is the body of the cell (celleus, cytos, or cytosoma). The soft living sub- stance 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 the nuclei n (or caryoplasm) ; that of the cell body is called the plastin (or cytoplasm). In the most rudimentary cases both substances seem to be quite simple and homogeneous, without anv visible structure. But, as a rule, when we examine them under a high power of the micro- scope, we find a certain structure in the protoplasm. The chief and most common form of this is the fibrous or net-like "thread- 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 of adapting itself to the most diverse activities or environments. In its simplest form the cell is globular (Fig. 2). This normal globular form is especially found in cells of the simplest construction, and those that are developed in a free fluid without any external pressure. In such cases the nucleus also is not infrequently round, and located in the centre of the cell-body (Fig. 2k). In other cases, the cells have no definite shape ; they are constantly changing their form owing to their automatic movements. This is the case with the amcebce (Figs. 15 and 16) and the amoeboid travelling cells (Fig. n), and also with very young ova (Fig. 12). However, as a rule, the cell assumes a definite form in the course of its career. In the tissues of the multicellular organism, in which a number of similar cells are bound together in virtue of certain laws of Fig. 2. -Stem-cell of one of the eehinoderms (cytula, or ■first segmentation-cell" =ferti- lised ovum I, alter Herhoig. K is tlii* nucleus or carvon. THE OVUM AND THE AMCEBA heredity, the shape is determined partly by the form of their connection and partly by their special functions. Thus, for instance, we find in the mucous lining of our tongue very thin and delicate flat cells, or epithelial cells, of roundish shape (Fig. 3). In the outer skin we find similar, but harder, covering cells, joined together by saw-like edges (Fig. 4). In the liver and other glands there are thicker and softer cells, linked together in rows (Fig. 5). The last-named tissues (Figs. 3-5) belong to the simplest and most primitive type, the group of the " covering-tissues," or epithelia. In these "primary tissues" (to which the germinal layers belong) simple cells of the same kind are Fig. 3. Fig. 4. Fig. 5. Fig. 3. — Three epithelial cells from the mucous lining of the tongue. Fig. 4.— Five spiny OF grooved cells, with edges joined, from the outer skin (epidermis) : one of them (b) is isolated. Fig. 5. — Ten liver-cells : one of them (b) has two nuclei. arranged in layers. The arrangement and shape are more complicated in the " secondary tissues," which are gradually developed out of the primary, as in the tissues of the muscles," nerves, bones, etc. In the bones, for instance, which belong to the group of supporting or connecting organs, the cells (Fig. 6) are star-shaped, and are joined together by numbers of net-like interlacing processes ; so, also, in the tissues of the teeth (Fig. 7), and in other forms of supporting-tissue, in which a soft or hard substance (intercellular matter, or base) is inserted between the cells. The cells also differ very much in size. The great majority of them are invisible to the naked eye, and can be seen only through the microscope (being on an average between THE Orr.U AND THE AMCEBA o.oi and o.i millimetres in diameter). There are, however, many o( the smaller plastids — such as the famous bacteria — which only come into view with a very high magnifying power. On the other hand, many cells attain a considerable size, and run to several millimetres or centimetres in diameter, as do several kinds of rhizopods among the unicellular protists (such as the radiolaria and thalamophora). Among the tissue-cells of the animal body many of the muscular fibres and nerve fibres are more than a decimetre (4 inches), and some- times more than a metre (40 inches) in length. Among the Fig. 6.— Nine Star-Shaped bone-Cells, with interlaced branches. largest cells are the yelk-filled ova; as, for instance, the yellow "yelk-nucleus" in the hen's egg, which we shall describe later (Fig. 15). Cells also vary considerably in structure. In this con- nection we must first distinguish between the active and passive components of the cell. It is only the former, or active parts of the cell, that really live, and effect that marvel- lous world of phenomena to which we give the name of "organic life." The first of these is the inner nucleus (caryoplasmaji and the second the body of the cell (cytoplasma). The passive portions come third ; these are THE OVUM AND THE AMCEBA subsequently formed from the others, and I have in my Generelle Morphologie (chap, ix.) given them the name of " plasma-products." They are partly external (cell-membranes and intercellular matter) and partly internal (cell-sap and cell- contents). (See the table at the end of the next Chapter.) The nucleus (or car- yon), which is usually of a simple roundish form, is quite structure- less at first (especially in very young cells), and composed of homogene- ous nuclear matter or caryoplasm (Fig. zk). But, as a rule, it forms a Fig. 7.— Eleven star-shaped cells . ... from the enamel of a tooth, joined together SOrt Or vesicle later On, in by their branchlets. i • i_ j- <-• „ -„i, which wecandistinguisn a more solid nuclear base (caryobasis) and a softer or fluid nuclear sap ( caryolynnph ). The nuclear base forms the enve- loping membrane of globular nuclein and, as a rule, a skeleton or network of branching threads, which go out from the membrane, and pass through the cavity of the vesicle and its liquid contents. This nuclear skeleton (caryomitoma J con- sists of two different substances, one of which (the chromatin ) is strongly tinged with carmine and other colouring matter, and the other / 'achromia or lininj is not. In a mesh of the nuclear network (or it may be on the inner side of the nuclear envelope) there is, as a rule, a dark, very opaque, solid body, called the nucleolus. Many of the nuclei contain several of these nucleoli (as, for instance, the germinal vesicle of the ova of fishes and amphibia). Recently a very small, but particularly important, part of the nucleus has been distinguished as the central body (centrosoma) — a tiny particle that is originally found in the nucleus itself (as in the case of many spermacytes, carcinom- cells, etc.), 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 centrosoma with regard to the other THE OVru AND THE AMCEBA 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 (celleus or cytosoma) also consists origi- nally, and in its simplest form, o( a homogeneous viscid plasmic matter (cytoplasm ). But, as a rule, only the smaller part of it is formed of the living active cell-substance (proto- plasm) ; the greater part consists of dead, passive plasma- products (metaplasma). It is useful to distinguish between the inner and outer of these. External plasma-products (which are thrust out from the pro- toplasm as solid " structural matter ") are the cell-membranes and the in- tercellular matter. The internal plasma-products are either the fluid cell-sap ( cy/o/ymph ) or hard struc- tures (paraplasma). As a rule, in mature and differentiated cells these various parts are so arranged that the protoplasm (like the caryoplasm fig. 8. -Unfertilised ovum in the vesicular nucleus) forms a sort °f an eehinoderm (from HertwigJ. I he vesicular of skeleton or frame-work (cytonii- nucleus (or "germinal vesicle") ,-, ... is globular, halt' the size of toma, hlar matter or spongioplasm). the round ovum, and encloses The spaces of this network are filled a "uc,1e" f^i^i '" ?f t central knot ot which there is a partlv with the fluid cell-sap ( cv/a- dark nucleolus (the "germinal ' r l ' spot "). lymph) and partly by hard struc- tural products (paraplasma, or interfilar matter) ; among these there are small plasma-granules (granula or micro- somata), or fat-grains (liposomata), of great importance. Besides these, we can distinguish many other products in the cytoplasm, such as concrementa, crystals, gland-granules, etc. The simple globular ovum, which we take as the starting- point of our study (Figs, i and 2), has in many cases the vague, indifferent features of the typical primitive cell. As a contrast to it, and as an instance of a very highly differen- tiated plastid, we may consider for a moment a large nerve- cell, or ganglionic cell, from the brain. The ovum stands potentially for the entire organism — in other words, it has the THE O VUM AND THE AMCEBA faculty of building up out of itself the whole multicellular body. It is the common parent of all the countless generations of cells which form the different tissues of the body ; it unites all their powers in itself, though only potentially or in germ. In complete contrast to this, the neural cell in the brain (Fig. 9) developes along one rigid line. It cannot, like the ovum, beget endless generations of cells, of which some will become skin-cells, others muscle-cells, and others again bone-cells. But, on the other hand, the nerve-cell has become fitted to discharge the highest functions of life ; it has the powers of sensation, will, and thought. It is a real soul-cell, or an elementary organ of the psychic activity. It has, therefore, a most elaborate and delicate structure. Numbers of extremely fine threads, like the electric wires at a large telegraphic centre, cross and recross in the delicate protoplasm of the nerve-cell, and pass out in the branching processes which proceed from it and put it in communication with other nerve-cells or nerve-fibres (a, b). We can only partly follow their intricate paths in the fine nucleolar matter of the cytoplasmic body. \y Here we have a most elaborate apparatus, the delicate structure of which we are just beginning to appreciate through our most powerful microscopes, but whose significance is rather a matter of conjecture than knowledge. Its intricate structure corresponds to the very complicated functions of the mind. Nevertheless, this elementary organ of psychic activity — of which there are thousands in our brain — is nothing but a single cell. Our whole mental life is only the joint result of the combined activity of all these nerve-cells, or soul-cells. In the centre of each cell there is a large transparent nucleus, containing a small and dark nuclear body. Here, as elsewhere, it is the nucleus that determines the individuality of the cell ; it proves that the whole structure, in spite of its intricate composition, amounts to only a single cell. In contrast with this very elaborate and very strictly differentiated psychic cell (Fig. 9), we have our ovum (Figs. 1 and 2), which has hardly any structure at all. But even in the case of the ovum we must infer from its THE OVUM AND THE AMCEBA Fig. 9.— A large branching nerve-cell, or "soul-cell," from the brain of an electric fish (torpedo), magnified 600 times. In the middle of the cell is the large transparent round nucleus, one nucleolus, and, within the latter again, a nucleolinus. The protoplasm ol the cell is split into innumerable fine threads (or fibrils), which are embedded in nucleolar intercellular matter, and are prolonged into the branching processes of the cell (b). One branch (a) pas^e- into a nerve-fibre. (Krom Max Schultee.) THE OVUM AND THE AMCEBA properties that its protoplasmic body has a very complicated chemical composition and a fine molecular structure which escapes our observation. This hypothetical molecular struc- ture of the plasm is now generally admitted ; but it has never been seen, and, indeed, lies far beyond the range of micro- scopic vision. It must not be confused — as is often done — with the structure of the plasma (the fibrous net-work, groups of granules, honey-comb, etc.) which does come within the range of the microscope. But when we speak of the cells as the elementary organisms, or structural units, or " ultimate individualities," we must bear in mind a certain restriction of the phrases. I mean, that the cells are not, as is often supposed, the very lowest stage of organic individuality. There are yet more elementary organisms to which I must refer occasionally, and will return later on. These are what we call the " cytodes " (cytos = cell), certain living, independent beings, consisting only of a particle of plasson — an albuminoid substance, which is not yet differentiated into caryoplasm and cytoplasm, but combines the properties of both. Those remarkable beings called the monera — especially the chromacea and bacteria — are specimens of these simple cytodes. (Compare the nineteenth Chapter.) To be quite accurate, then, we must say : the elementary organism, or the ultimate individual, is found in two different stages. The first and lower stage is the cytode, which consists merely of a particle of plasson, or quite simple plasm. The second and higher stage is the cell, which is already divided or differentiated into nuclear matter and cellular matter. We comprise both kinds — the cytodes and the cells — under the name of plastids (" formative particles "), because they are the real builders of the organism. However, these cytodes are not found, as a rule, in the higher animals and plants; here we have only real cells with a nucleus. Hence, in these tissue-forming organisms (both plants and animal) the organic unit always consists of two chemically and anatomically different parts — the outer cell-body ( cytosoma) and the inner nucleus ( caryon ). In order to convince oneself that this cell is really an THE OVUM AXD THE AMCEBA independent organism, we have only to observe the develop- ment and vital phenomena of one of them. You see then that it performs all the essential functions of life — both vegetal and animal —which we find in the entire organism. Each of these tiny beings grows and nourishes itself indepen- dently. It takes its food from the surrounding fluid ; sometimes, even, the naked cells take in solid particles at certain points of their surface — in other words, " eat" them — without needing any special mouth and stomach for the purpose (cf. Fig. 19). Further, each cell is able to repro- duce itself. This multiplication, in most cases, takes the form of a simple cleavage, sometimes direct, sometimes indirect ; the simple direct (or " amitotic ") division is less common, and is found, for in- stance, in the blood cells (Fig. 10). In these the nucleus first divides into two equal parts by constriction. The indirect (or " mitotic ") cleavage is much more frequent ; in this the caryoplasm of the nucleus and the cytoplasm of the cell- body act upon each other in a peculiar way, with a partial dissolution 1 caryo- lysisj, the formation of knots and loops f mitosis), and a movement of the halved plasma-particles towards two mutually repulsive poles of attraction (caryokine- sis, Fig. 1 1). The intricate physiological processes which accompany this "mitosis" have been very closely studied of late years. The inquiry has led to the detection of certain laws of evolution which are of extreme importance in connection with heredity. As a rule, two very different parts of the nucleus play an important part in these changes. They are : the chromatin, or coloured nuclear substance, which has a peculiar property of tinging itself deeply with certain Fig. 10. — Blood- cells, multiplying by direct division, from the blood of the embryo of a goat. Originally, each blood-cell has a nucleus and is globular (a). When it is going to multiply, the nucleus divides into two (A, r, d). Then the protoplasmic body is constricted be- tween the two nuclei, and these move away from each other I el. Finally, the constriction is complete, and the cell splits into two daughter- cells (/). ( From Frey. \ THE OVUM AND THE AMCEBA colouring matters (carmine, haematoxylin, etc.), and the achromin (or linin, or achromatin), a colourless nuclear substance that lacks this property. The latter generally forms in the dividing cell a sort of spindle, at the poles of which there is a very small particle, also colourless, called the "central body " (centrosoma). This acts as the centre or _, Nuclear threads (chromo- somata) i (coloured nuclear matter, \ chromatin) I , A — • Nuclear membrane / ■ Nuclear sap A. Mother-cell cyto-- „ soma (,t (Knot, spirema) Protoplasm of the cell-body B. Mother-star, the loops beginning' to split lengthways (nuclear mem- brane gone). C. The two daughter- stars, produced by the breaking of the loops of the mother-star (moving away). D. The two daughter- cells, produced by the complete division of the two nuclear halves (cytosomata still con- nected at the equator) (Double-knot, Dispirema) — Star-like appearance in cytoplasm r — Centrosoma(sphere of attraction) A— Nuclear spindle (achromin, Hj colourless matter) '/ Nuclear loops (chromatin, coloured matter) — Upper daughter-crown ! _ Connecting threads of che two crowns (achromin) — ■ Lower daughter-crown Double-star (amphiaster) t ''/NVr''^ Upper daughter-nucleus Aequatorial constriction of the cell-body ' — - Lower daughter-nucleus Fig. ii.— Indirect Or mitotic cell-division (with caryolysis and caryo- kinesis) from the skin of the larva of a salamander. (From Rabl.) focus in a " sphere of attraction " for the granules of proto- plasm in the surrounding cell-body, and assumes a star-like appearance (the cell-star, or monaster). The two centroso- mata, standing opposed to each other at the poles of the nuclear spindle, form "the double-star" (or amphiaster,¥\g. n, THE OrC.U AND THE AMcE/l.l B, C). The chromatin often forms a long-, irregularly-wound thread — "the coil" (spiremay Fig. A). At the commence- ment of the cleavage it gathers at the equator of the cell, between the stellar poles, and forms a crown of U-shaped loops (generally four or eight, or some other definite number). The loops split lengthwise into two halves (B), and these back away from each other towards the poles of the spindle (C). Here each group forms a crown once more, and this, with the corresponding half of the divided spindle, forms a fresh nucleus (D). Then the protoplasm of the cell- body begins to contract in the middle, and gather about the new daughter-nuclei, and at last the two daughter-cells become independent beings. Between this common mitosis, or indirect cell-division — which is the normal cleavage-process in most cells of the higher animals and plants — -and the simple direct division (Fig. 10) we find every grade of segmentation; in some circumstances even one kind of division may be converted into another (as, for instance, in the segmentation of the yelk-cells in discoblastic ova). The plastid is also endowed with the functions of move- ment 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 again, and thus changes its shape (Fig. 12). Finally, the young cell is sensitive, or more or less responsive to stimuli; it makes certain movements on the application of chemical and mechanical irritation. Hence we can ascribe to the individual cell all the chief functions which we comprehend under the general heading of " life " — sensation, movement, nutrition, and reproduction. All these properties of the multicellular and highly developed animal are also found in the single animal-cell, at least in its younger stages. There is no longer any doubt about this, and so we may regard it as a solid and important base of our physiological conception of the elementary organism. Without going any further here into these very interesting THE OVUM AND THE AMCEBA phenomena of the life of the cell, we will pass on to consider the application of the cell theory to the ovum. Here com- parative research yields the important result that every ovum is at first a simple cell. I say this is very important, because our whole science of ontogeny now resolves itself into the problem : "How does the multicellular organism arise from the unicellular?" Every organic individual is at first a simple cell, and as such an elementary organism, or a unit of individuality. This cell produces a cluster of cells by segmentation, and from these developes the multicellular organism, or individual of higher rank. When we examine a little closer the original features of the ovum, we notice the ex- tremely significant fact that in its first stage the ovum is just the same simple and indefinite structure in the case of man and all the animals (Fig. 13). We are unable to detect any material difference between freely about, by (like the amceba or h jth j t gh Qr rhizopods) protruding fine processes ' r the uncovered protoplasmic internal constitution. Later, though the ova remain unicel- lular, they differ in size and shape, enclose various kinds of yelk-particles, have different envelopes, and so on. But when we examine them at their birth, in the ovary of the female animal, we find them to be always of the same form in the first stages of their life. In the beginning each ovum is a very simple, roundish, naked, mobile cell, without a membrane ; it consists merely of a particle of cytoplasm enclosing a nucleus (Fig. 13). Special names have been given to these parts of the ovum ; the cell- body is called the yelk fvitellusj, and the cell-nucleus the Fig. 12.— Mobile cells from the inflamed eye of a frog (from the watery fluid of the eye, the humor aqueus). The naked cells creep from the uncovered protoplasmic bodv. These bodies vary continually in number, shape, and size. The nucleus of these amoeboid lymph- cells (" travelling- cells," or plano- cytes) is invisible, because concealed by the numbers of fine granules which are scattered in the protoplasm. (From Frey.) THE OVCM AMI THE AMCEBA germinal vesicle r vesica la germinativa j. As a rule, the nucleus of the ovum is soft, and like a small pimple or vesicle. Inside it, as in many other cells, there is a nuclear skeleton or frame and a third, hard nuclear body (the nucleolus). In the ovum this is called the germinal spot ( macula germinal iva ). Finally, we find in many ova (but Fig. 13.— Ova of various animals, executing amoeboid movements, highly magnified. All the ova are naked cells of varying shape. In the dark led protoplasm (yelk) is a large vesicular nucleus (the germinal and m this is seen a nuclear body (the germinal spot), in which again we often rminal point. Figs. .1/ A4 represent the ovum of a sponge (leuculmis echinus) in lour successive movements. A'/ B8 are the ovum of a parasitic crab (chondracanthiu cornutus), in eight successive movements. (From Edward von Beneden.) Ci Cg show the ovum of the cat in various si movement (from Pfluger); Fig. I) the ovum of a trout; A' the ovum of a chicken ; Fa human ovum. THE OVUM AND THE AMCEBA not in all) a still further point within the germinal spot, a "nucleolin," which goes by the name of the germinal point (punctum germinativum). The latter parts (germinal spot and germinal point) have, apparently, a minor importance, in comparison with the other two (the yelk and germinal vesicle). In the yelk we must distinguish the active formative yelk (or protoplasm = first plasm) from the passive nutritive yelk (or deutoplasm = second plasm). Fig 14. — The human OVUm, taken from the female ovary, magnified 500 times. The whole ovum is a simple globular cell. The chief part of the globular mass is formed by the nuclear yelk (deutoplasm), which is easily dis- tributed in the active protoplasm, and consists of numbers of fine yelk-granules. In the upperpart of the yelk is the transparent globular germinal vesicle, which corresponds to the nucleus. This encloses a darker granule, the germinal spot, which shows a nucleolus. The globular yelk is surrounded by the thick transparent germinal membranes (ovolemma, or zona pellucida). This is traversed by numbers of lines as fine as hairs, which are directed radially towards the centre of the ovum. These are called the pore-canals ; it is through these that the moving spermatozoa penetrate into the yelk at impregnation. In many of the lower animals (such as sponges, polyps, and medusa?) the naked ova retain their original simple appearance until impregnation. But in most animals they at once begin to change ; the change consists partly in the formation of connections with the yelk, which serve to THE OYIM AND THE AMCEBA nourish the ovum, and partly of external membranes for their protection (the ovolemma, or prochorion). A membrane of this sort is formed in all the mammals in the course of the embryonic process. The little globule is surrounded by a thick capsule of glass-like transparency, the zona pellucida, or ovolemma pellucidum (Fig. 14). When we examine it closely under the microscope, we see very line radial streaks in it, piercing the zona, which are really very narrow canals. The human ovum, whether fertilised or not, cannot be distinguished from that of most of the other mammals. It is nearly the same everywhere in form, size, and composition. When it is fully formed, it has a diameter of (on an average) about i'o of an inch. When the mammal ovum has been carefully isolated, and held against the light on a glass-plate, it may be seen as a fine point even with the naked eye. The ova of most of the higher mammals are about the same size. The diameter of the ovum is almost always between ■.'.■■■ and iV of a line (0.1 — 0.2 millimetres). It has always the same globular shape; the same characteristic membrane; the same transparent germinal vesicle with its dark germinal spot. Even when we use the most powerful microscope with its highest power, we can detect no material difference between the ova of man, the ape, the dog, and so on. I do not mean to say that there are no differences between the ova of these different mammals. On the contrary, we are bound to assume that there are such, at least as regards chemical composition. Even the ova of different men must differ from each other ; otherwise we should not have a different indivi- dual from each ovum. In accordance with the law of the unlikeness of individuals, we must assume that "all organic individuals differ from the very beginning of their development, though they resemble each other so much " ( Gen. Morph., Band II., S 202). 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 morphological resemblance of their ova, so great as to seem to be a complete similarity, is a strong proof of the common parentage of man and the other mammals. THE OVUM AXD THE A MCE B A 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-ovum (Fig. 15) is notably different. It is true that in its earliest stage (Fig. 13 E) this ovum also is very like that of the mammal (Fig. 13 F). But afterwards, while still within the oviduct, it takes up a quantity of nourishment and works this into the familiar large yellow yelk. When we examine a very young ovum in the hen's oviduct, we find it to be a simple, small, naked, amoeboid cell, just like the young ova of other animals (Fig. 13). But it then grows to the size we are familiar with in the globular yelk of the egg. The nucleus of the ovum, or the germinal vesicle, is thus pressed right to the surface of the globular ovum, and is embedded there in a Fig. 15.— A fertilised small quantity of transparent matter, ovum from the oviduct M J F ' of a hen. The yellow yelk the so-called white yelk. This forms (c) consists of several con- , , . , . , . , round white spot, which is known as the egg-scar ( cicatricula) (Fig. 15 b). From the scar a thin column of the white yelk penetrates through centric layers (d), and is enclosed in a thin yelk- membrane [a). The nucleus or germinal vesicle is seen above in the cicatrix (b). From that point the white yelk penetrates to the cen- . trai yelk-cavity {d'). The the yellow yelk to the centre or the two kinds of yelk do not globular cell, where it swells into a diner very much. & small, central globule (wrongly called the yelk-cavity, or latebra, Fig. 15 d). The yellow yelk- matter which surrounds this white yelk has the appearance in the egg (when boiled hard) of concentric layers (c). The yellow yelk is also enclosed in a delicate structureless membrane (the membrana vitelline!, a). As the large yellow ovum of the bird attains a diameter of several inches in the bigger birds and encloses vesicular yelk- particles, there was formerly a reluctance to consider it as a simple cell. This, however, was an error from which His THE O 1 7 .1/ . I \7> THE . \M(EB. 1 and other embrvologists have even recently drawn wrong conclusions, though it was corrected by Gegenbaur forty years ago. The unfertilised and undivided ovum of the bird remains a real cell with its simple nucleus, however large it may grow by the production of yellow yelk. Every animal that lias only one cell-nucleus, every amoeba, every gregarina, every infusorium, is unicellular, and remains unicellular whatever variety of matter it feeds on. So the ovum remains a simple cell, however much yellow yelk it after- wards accumulates within its protoplasm. Gegenbaur and Van Beneden have clearly shown this in their admirable works on the ova of mammals. It is, of course, different with the bird's egg when it has been fertilised. Then its nucleus multiplies by repeated cleavage, and the protoplasm of the cicatrix which surrounds it is similarly divided. The ovum then consists of as many cells as there are nuclei in the cicatrix. Hence, in the fertilised egg which we eat daily, the yellow yelk is already a multicellular body. Its scar is composed of several cells, and is now commonly called the germinal disc (discus blasto- dermicus >. We shall return to this discogastrida in the ninth chapter. When the mature bird-ovum has left the ovary and been fertilised in the oviduct, 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 fine inner skin. All these gradually forming envelopes and processes are of no importance in the formation of the embryo; they serve merely for the protection of the original simple ovum. We sometimes find extraordinarily large eggs with strong envelopes in the case of other animals, such as fishes of the shark type. Hut here, also, the ovum is originally of the same character as it is in the mammal ; it is a perfectly simple and naked cell. But, as in the case of the bird, a considerable quantity of nutritive yelk is accumulated inside the original yelk as food for the developing embryo ; and various coverings are formed round the egg. The ovum THE OVUM AND THE AMCEBA of many other animals has the same internal and external features. They have, however, only a physiological, not a morphological, importance; they have no direct influence on the formation of the foetus. They are partly consumed as food by the embryo, and partly serve as protective envelopes. Hence we may leave them out of consideration altogether here, and restrict ourselves to material points — to the sub- stantial identity of the original ovum in man and the rest of the animals (Fig. 13). Now, let us for the first time make use of our biogenetic law, and directly apply this fundamental law of evolution to the human ovum. We reach a very simple, but very impor- tant, conclusion. From the fact that the human ovum and that of all other animals consists of a single cell, it follows immediately, according to the biogenetic laiv, that all the animals, including man, descend from a unicellular organism. If our biogenetic law is true, if the embryonic development is a summary or condensed recapitulation of the stem-history — and there can be no doubt about it — we are bound to conclude, from the fact that all the ova are at first simple cells, that all the multicellular organisms originally sprang from a unicel- lular being. And as the original ovum in man and all the other animals has the same simple and indefinite appearance, we may assume with some probability that this unicellular stem-form was the common ancestor of the whole animal world, including man. However, this last hypothesis does not seem to me as inevitable and as absolutely certain as our first conclusion. Thisjnference from the unicellular embryonic form to the unicellular ancestor is so simple, but so important, that we cannot sufficiently emphasise it. We must, therefore, turn next to the question whether there are to-day any unicellular organisms, from the features of which we may draw some approximate conclusion as to the unicellular ancestors of the multicellular organisms. The answer is : Most certainly there are. There are assuredly still unicellular organisms which are, in their whole nature, really nothing more than permanent ova. There are independent unicellular organisms THE OIT.]/ AND THE AMCEBA of the simplest character which develop no further, but repro- duce themselves as such, without any further growth. We know to-day of a great number o( 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 amoeba. For a long time now we have comprised under the general name of amoeba? a number of micro- scopic unicellular organisms, which are very widely distributed, especially in fresh water, but also in the ocean ; in fact, they have lately been discovered in damp soil. There are also parasitic amoebae which live inside other animals. When we place one of these amoeba? in a drop of water under the microscope and examine it with a high power, it generally appears as a roundish particle . F Fie. 16. —A creeping ot a very irregular and varying shape amoeba (highly magm- /i-- c , , , • r , • fied). The whole organism (bigs. 10 and 17). In Its Soft, slimy, is a'simpie naked clu, and semi-fluid substance, which consists of move? alHH" by meanflf the changing arms winch protoplasm, we see only the solid it thrusts out of and with- ... . . draws into its protoplasmic globular particle it contains, the nucleus, body. Inside it is the Thic ...-,:, -.11.. I u 1 u roundish nucleus with its inis unicellular body moves about nucieoiUSl continually, creeping in every direc- tion on the glass on which we are examining it. The movement is effected by the shapeless body thrusting out finger-like processes at various parts of its surface; and these are slowly but continually changing, and drawing the rest of the body after them. After a time, perhaps, the action changes. The amceba suddenly stands still, withdraws its projections, and assumes a globular shape. In a little while, however, the globular body begins to expand again, thrusts out arms in another direction, and moves on once more. These changeable processes are THE OVUM AND THE AMCEBA called "false feet," or pseudopodia, because they behave physiologically as feet, yet are not special organs in the anatomic sense. They disappear as quickly as they come, and are nothing more than temporary projections of the semi- fluid, homogeneous, 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 in consequence of this mechanical or physical stimulus. As a rule, the body then resumes its globular shape. In certain circumstances — for instance, if the impurity of the water lasts some time — the amcebas begins to develop a covering. It exudes a homogeneous membrane or capsule, which immediately hardens, and assumes the appearance of a globular cell with a protective membrane. The amoeba either takes its food directly by imbibition of matter floating in the water, or by pressing into its protoplasmic body solid particles with which it comes in contact. The latter process may be observed at any moment by forcing it to eat. If finely ground colouring matter, such as carmine or indigo, is put into the water, you can see the soft body of the amoeba pressing these coloured particles into itself, the substance of the cell closing round them. The amoeba can take in food in this way at any point on its surface, without having any special organs for intussusception and digestion, or a real mouth or gut. The amoeba grows by thus taking in food and dissolving the particles eaten in its protoplasm. When it reaches a certain size by this continual feeding, it begins to reproduce. This is done by the simple process of cleavage (Fig. 17). First, the nucleus divides into two parts. Then the proto- plasm is separated between the two new nuclei, and the whole cell splits into two daughter-cells, the protoplasm gathering about each of the nuclei. The thin bridge of protoplasm which at first connects the daughter-cells soon breaks. Here we have the simple form of direct cleavage of the nuclei. Without mitosis, or formation of threads, the homogeneous nucleus divides into two halves. These move away from each other, and become centres of attraction for the enveloping THE off.U AND THE AMCEBA matter, the protoplasm. The same direct cleavage o( the nuclei is also witnessed in the reproduction oi' many other protists, while other unicellular organisms show the indirect division of the cell. Hence, although the amoeba is nothing but a simple cell, it is evidently able to accomplish all the functions of the multicellular organism. It moves, feels, nourishes itself, and reproduces. Some kinds of these amoebae can be seen with the naked eye, but most of them are microscopically small. Fig. 17. —Division of a unicellular amoeba (amoeba polypodia) in six stages. (From F. E. Stluiltze.) The dark spot is the nucleus, the lighter spot a contractile vacuole in the protoplasm. The latter re-forms in one of the daughter-cells. It is for the following reasons that we regard the amoeba; as the unicellular organisms which have special phylogenetic (or evolutionary) relations to the ovum. In many of the lower animals the ovum retains its original naked form until fertilisation, developes no membranes, and is then often indistinguishable from the ordinary amceba. Like the amoeba?, these naked ova may thrust out processes, and move about as travelling cells. In the sponges these mobile ova THE OVUM AXD THE A MCE B A move about freely in the maternal body like independent amoeba; (Fig. 17). They had been observed by earlier scientists, but described as foreign bodies — namely, parasitic amoeba;, living parasitically on the body of the sponge. Later, however, it was discovered that they were not para- sites, but the ova of the sponge. We also find this remark- able phenomenon among other animals, such as the graceful, bell-shaped zoophyta, which we call 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 repeated segmentation. It is, therefore, no audacious hypo- thesis, but a perfectly sound conclusion, to regard the amoeba as the particular unicellular organism which offers us an approximate illustration of the ancient common unicellular ancestor of all the metazoa, or multicellular animals. The simple naked amoeba has a less definite Fig. 18. -Ovum of a and more original character than any sponge(o/y>it/ntsj. The other cell. Moreover, there is the fact ovum creeps about in ,. j i_ the bodv of the sponge that recent research has discovered such ftaKJp^Tu amoeba-like cells everywhere in the is indistinguishable from mature body of the multicellular animals. the common amoeba. ' 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 verte- brates. 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 amoeba;, take up solid particles, or "eat" (whence they are called phagocytes = "eating-cells," Fig. 19). Lately, it has been discovered that many different cells may, if they have room enough, execute the same movements, creeping about and eating. They behave just like amoeba; (Fig. 12). It has also been shown that these " travelling-cells," or planocy tes, play an important part in man's physiology and pathology THE OVCM AND THE AMCEBA (as moans of transport for food, infectious matter, bacteria, etc.). The power of the naked cell to execute these characteristic amoeba-like movements comes from the contractility (or auto- matic mobility) of its protoplasm. This seems to be a universal property of young cells. When they are not enclosed by a firm membrane, or confined in a "cellular prison," they can always accomplish these amceboid move- ments. This is true of the naked ova as well as of any other naked cells, of the "travelling-cells " of various kinds in connective tissue, ot the mesenchymic cells, lymph -cells, mucus-cells, etc. We have now, by our study of the ovum and the com- parison of it with the amoeba, pro- vided a perfectly sound and most valuable foundation for both the embrvo- logy and the evolu- tion of man. We nave learned that the human ovum is a simple cell, that this ovum is not materiallv different from that of other mammals, and that we may conclude from it to the existence of a primitive unicellular ancestral form, witli a substantial resemblance to the amoeba. The statement that the earliest progenitors 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, hut also been violently censured in theological journals as " shameful and immoral." But, as I observed in my essay On (tie Origin and Ancestral Fig. iq.— Blood-cells that eat, op phago- cytes, from a naked sea-snail ( thetisj, greatly magnified. I was the first to observe ill the blood- cells of tliis snail tin- important fact that "the blood-cells of the invertebrates are unprotected pieces of plasm, and take in food, by means of their peculiar movements, like the amoeba?." I had (in Naples, on May ioth, 1859) injected into the blood-vessles of one of these snails an infusion oi water and ground indigo, and was greatly astonished to find the blood-cells themselves more or less filled with the particles of indigo after a few hours. After repeated injections I succeeded in "observing the very entrance o\~ the coloured particles in 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 oil the Radiolaria. THE OVUM AXD THE A MCE B A Tree of the Human Race in 1870, this offended piety must equally protest against the " shameful and immoral " fact that each human individual is developed from a simple ovum, and that this human ovum is indistinguishable from those of the other mammals, and in its earliest stage is like a naked amoeba. We can show this to be a fact any day with the microscope, and it is little use to close one's eyes to " immoral " facts of this kind. It is as indisputable as the momentous conclusions we draw from it and as the vertebrate character of man (see Chapter XL). We now see very clearly how extremely important the cell theory has been for our whole conception of organic nature. " Man's place in nature " is settled beyond question by it. Apart from the cell theory, man is an insoluble enigma to us. Hence philosophers, and especially physiologists, should be thoroughly conversant with it. The soul of man can only be really understood in the light of the cell-soul, and we have the simplest form of this in the 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 of man, was gradually evolved from them. The academic psychologists who lack this zoological equipment are unable to do so. This naturalistic and realistic conception is a stumbling- block to our modern idealistic metaphysicians and their theological colleagues. Fenced about with their transcendental and dualistic prejudices, they attack not only the monistic system we establish on our scientific knowledge, but even the plainest facts which go to form its foundation. An instructive instance of this was seen three years ago, in the academic discourse delivered by a distinguished theologian, Willibald Beyschlag, at Halle, January 12th, 1900, on the occasion of the centenary festival. The theologian protested violently against the " materialistic dustmen of the scientific world who offer our people the diploma of a descent from the ape, and would prove to them that the genius of a Shakespeare or a Goethe is merely a distillation from a drop of primitive THE OVCM AND THE AMCEBA mucus." Another well-known theologian protested against the horrible idea that the greatest of men, Luther and Christ, were descended from a mere globule of protoplasm." Nevertheless, not a single informed and impartial scientist doubts the fact that these greatest men were, like all other men — and all other vertebrates — developed from an impreg- nated ovum, and that this simple nucleated globule of protoplasm has the same chemical constitution in all the mammals. The actual amoeba? and other unicellular organisms (arcella, radiolaria, etc.) are of great importance for our conclusion, because they exhibit these single cells to us in permanent independence, as autonomous cells. The human organism and that of the other higher animals are only one- celled in the earliest stage of existence. As soon as the ovum is fertilised, it increases by segmentation, and forms a group or colony of social cells, a cell-community or a ccenobium. These take on different forms, and, by a division of labour among the cells and their development along different lines, the multifarious tissues that make up the animal body are produced. Thus the mature multicellular organism of man and the other higher animals and plants is a his ton (or " tissue-body "), a social community of the various kinds of tissue-cells. The innumerable organic units in this " histon " may vary considerably when their development is complete, but they were originally simple cells of the same type, the equal citizens of the cell-state. CHAPTER VII. CONCEPTION The meaning- of sexual reproduction. Nature of conception ; fusion of the female ovum and male spermatozoon. Various forms of the sperm-cells (usually cone-shaped ciliary cells). Theory of the spermatozoa. Inheri- tance from both parent-cells. The new stem-cell or cytula. Its herma- phroditic character. Process of fertilisation of ovum : release of the germinal vesicle and protrusions of the directing- body. Penetration of a spermatozoon in the body of the ovum : movement and blending of the two pronuclei. Formation of the stem-nucleus ( archicaryon ), the vehicle of ir'ieritance. Older theories of conception. Importance and equal share the two sexual cells. Male microspores and female macrospores. yspermism of the chloroformed ovum. Importance of this fact in chology, the theory of the cell-soul and personal immortality. Imperma- ice of all that is personal and individual. Th: recognition of the fact that every man begins his individual existence as a simple cell is the solid foundation of all research into the genesis of man. From this fact we are forced, in virtue of our biogenetic law, to draw the weighty phylogenetic conclusion that the earliest ancestors of the human race were also unicellular organisms ; and among these protozoa we may single out the vague form of the amoeba as particularly important (cf. Chapter VI.). That these unicellular ancestral forms did once exist follows directly from the phenomena which we perceive every day in the fertilised ovum. The development of the multicellular organism from the ovum, and the formation of the germinal layers and the tissues, follow the same laws in man and all the higher animals. It will, therefore, be our next task to consider more closely the impregnated ovum and the process of conception which produces it. The process of impregnation or sexual conception is one of those phenomena that people love to conceal behind the mystic veil of supernatural power. We shall soon see, however, that it is a purely mechanical process, and can be reduced to familiar physiological functions. Moreover, this amphigony (or conception) is of the same type, and is effected 124 C OXt F.l'TlOX 1 25 bv the same organs, in man as in all the other mammals. The pairing of the male and female has in both cases for its main purpose the introduction of the ripe matter o( the male seed or sperm into the female body, in the sexual canals of which it encounters the ovum. Conception then ensues by the blending of the two. We must observe, first, that this important process is by no moans 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, and especially the sexless monera (chromacea, bacteria, etc), but also many other protists, such as the amoeba;, foraminifera, radiolaria, myxomycetae, etc. In these there is no fertilisation whatever; the multiplicatir F individuals and propagation of the species take plae< y unsexual reproduction, which takes the form of cleav ;e, budding, or spore-formation. The copulation of two coal sc- ing cells, which in these cases often precedes the reproduc- tion, cannot be regarded as a sexual act when the two copulating plastids differ in size or structure (microspores and macrospores). 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 reproduc- tion among them. There are, in particular, no cases of parthenogenesis (virginal conception) among the vertebrates. Sexual reproduction offers an infinite variety of interesting forms in the different classes of animals and plants, especiallv as regards the mode of conception, and the conveyance of the spermatozoon to the ovum. These features are of great importance not only as regards conception itself, but for the development of the organic form and especially for the differ- entiation of the sexes. There is a particularly curious cor- relation of plants and animals in this respect. The splendid studies o( Charles Darwin and Hermann Miiller on the fertili- sation of flowers by insects have given us very interesting particulars of this.1 This reciprocal service has given rise to ' See Darwin's work. On t/if Various Contrivances by which Orchids are Fertilised (1862). CONCEPTION a most intricate sexual apparatus. Equally elaborate struc- tures have been developed in man and the higher animals, serving partly for the isolation of the sexual products on each side, partly for bringing them together in conception. But, however interesting these phenomena are in themselves, we cannot go into them here, as they have only a minor importance — if any at all — in the real process of conception. We must, however, try to get a very clear idea of this pro- cess and the meaning of sexual reproduction. In every act of conception we have, as I said, to consider two different kinds of cells — a female and a male cell. The female cell of the animal organism is always called the ovum (or ovulum, egg, or egg-cell) ; the male cells are known as the sperm or seed-cells, or the spermatozoa (also spermium and zoospermium). The female ovum, the form and com- position of which we have already considered, is of the same simple nature in the early stages in all the animals. It is at first merely a globular naked cell, consisting of protoplasm and a nucleus (Fig. 13). When it has freedom to move, it often makes slow amoeboid movements, as we have seen in the case of the ovum of the sponge (Fig. 18). But, as a rule, it is enclosed subsequently by a number of very different, and often very complicated, shells or membranes. The ripe ovum is, on the whole, one of the largest cells we know. It attains colossal dimensions when it absorbs great quantities of nutritive yelk, as is the case with birds and reptiles, and many of the fishes. In the great majority of the animals the ripe ovum is rich in yelk and much larger than the other cells. On the other hand, the next cell which we have to con- sider in the process of conception, the male sperm-cell or spermatazoon, 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 CO.XCEPTIOX quantity of fluid. It is not the fluid, but the independent male cells that swim in it, that cause conception. The spermatozoa of the great majority of animals have two characteristic features. Firstly, they are extraordinarily small, being usually the smallest cells in the body ; and, secondly, they have, as a rule, a peculiarly lively motion, which is known as spermatozoic motion. The shape of the cell has a good deal to do with this motion. In most of the animals, and also in many of the lower plants (but not the higher), each of these spermatozoa has a very small, naked Fig. 20.- Spermia or spermatozoa from the male sperm of various mammals. The pear-shaped flattened nucleus of the seed-cell (the so-called •■ head of the spermatozoon ") is seen from the front in /. , and sideways in //. k i-. the nucleus, "/ its middle pari (protoplasm), s the mobile, serpent-like tail (or whip) ; .1/ four human spermatozoa, .i four spermatozoa from the ape; A' from the hare ; H from the house-mouse ; C from the dog- ; S from the pig. cell-body, enclosing an elongated nucleus, and a long thread hanging from it (Fig. 20). It was long before we could recognise that this structure is a simple cell. Thev were formerly held to be special organisms, and were called "seed- animals" (spermato-zoa, or spermato-zoidia) ; they are now scientifically known as spermia or spermidia, or as sperma- tosomata (seed-bodies) or spermatofila (seed threads). It took a good deal of comparative research to convince us that each of these spermatozoa is really a simple cell. They have the same shape as in many other vertebrates and most of the invertebrates. However, in many of the lower animals they CO.XCEPTIOX have quite a different shape. Thus, for instance, in the river crab thev are large round cells, without any movement, equipped with stiff outgrowths like bristles (Fig. 21/). They have also a peculiar form in some of the worms, such as the thread-worms (filarial; in this case, they are sometimes amoeboid and like very small ova (Fig. 21 c-e). But in most of the lower animals (such as the sponges and polyps) they have the same pine-cone shape as in man and the other mammals (Fig. 21 a, h). When the Dutch naturalist Leeuwenhoek discovered these thread-like lively particles in 1677 in the male sperm, it was generally believed that they were special, independent, tiny animal- cules, like the infusoria, and so were called " seed-animals " or spermatozoa. I have already mentioned that they played an important part in the pre-forma- tion theory, as it was believed that the whole mature organism existed already, with all its parts, Fig. 21. -Spermatozoa or but veiT sma11 and packed spermidia of various animals, together, in each spermatozoon (rrom Lang.) a oi a fish, b of a ° ^ turbeiiaria (with two side-lashes), (see p. 27). The spermatozoa had c-e of a nematode (amoeboid sper- , , r ., matozoa), f from a river crab only to penetrate into the fertile mSldt^wil'h i\,nHT,-the sa'a" soil of the female ovum, and then mancler (with undulating; mem- ' brane), // of a ring-worm ('« and /; the pre-formed body would ex- are the usual shape). pand and grow in all its parts. This erroneous view is now wholly abandoned ; we know by the most accurate investigation that the mobile spermatozoa are nothing but simple and real cells, of the kind that we call " ciliated " (equipped with lashes, or cilia). In the previous illustrations we have distinguished in the spermatozoon a head, trunk, and tail. The " head " (Fig. 20 k) is merely the oval nucleus of the cell; the body or middle-part (m) is an accumulation of cell-matter ; and the tail (s) is a thread-like prolongation of the same. Moreover, we now know that these spermatozoa are not at CONCEPTION all a peculiar form of cell ; precisely similar cells are found in various other parts of the body. If they have many short threads projecting, they are called ciliated; if only one long, whip-shaped process (or, more rarely, two or four), caudate (tailed) cells. Caudate cells, like those of the spermatozoa, are found in the gastric cells of the sponges and the cnidaria. Very careful recent examination of the spermia, under a very high microscopic power (Fig. 22 a, b), has detected some further details in the liner structure of the ciliated cell, and these are common to man and the anthropoid ape. The head (k) encloses the elliptic nucleus in a thin envelope of cytoplasm; it is a little flattened on one side, and thus looks rather pear-shaped from the front (b). In the central piece (w) we can distinguish a short neck and a longer connective piece (with centro- soma). The tail consists of a long main section (//) and a short, very fine tail (e). The process of fertilisation by sexual conception consists, therefore, essentially in the coalescence and blending together of two different cells. The most curious opinions prevailed about this act formerly. People always saw something mystic about it, and framed the most marvellous hypo- theses on it. It is only in the last ten years that we have learned that the process of conception is reallv very simple and has sicl°- * head (with nucleus), m middle- no element of the mvsterious. The essence stem, h long-stem, r -t ■ .. . , , • and r tail. (From ot it is that a male spermatozoon combines Retsius.) with a female ovum. The lively sperma- tozoon travels towards the ovum by its serpentine movements, and bores its way into the female cell (Fig. 23). The nuclei ot both sexual cells, attracted by a certain "affinity," approach each other and melt into one. This would be an admirable place for poetic description in the most glowing colours of the wonderful mystery of concep- tion and the struggle of the living spermatozoa, which hover K Fig. 22.— A single human spermato- zoon magnified 2,000 times : a shows it from the broader and b from the narrower CONCEPTION anxiously about the ovum, seeking to penetrate nto the fine porous canals of the ovolemma and plunge " consciously " into the protoplasmic yelk, where they die away to find their higher selves. The supporters of teleology, too, might pause here to admire the wisdom of the Creator in providing these porous canals in the membrane of the ovum for the sperma- tozoa to enter through. However, the scientist coldly describes this process — this " crowning of love " — as a blend- ing of two cells and the combination of their nuclei. The new cell that arises from the process is the simple product of the copulation of the two blending sexual cells. Hence the fertilised cell is quite another thing from the un- fertilised cell. For if we must regard the spermia as real cells no less than the ova, and the process of conception £s a coa- lescence of the two, we must consider the resultant cell as a quite new and independent organism. It bears in the cell Fig. 23.— The fertilisation of and nuclear matter of the pene- the ovum by the spermatozoon , (of a mammal). Oik- of the many tratlllg SpermatOZOOn a part OI thread-like, lively spermidia pierces , father's hnrivr and in the through a fine pore-canal into the tlle 'atlier s DOdy, ana in tne nuclear yelk. The nucleus of the protoplasm and caryoplasm of ovum is invisible. the ovum a part of the mother s body. This is clear from the fact that the child inherits many features from both parents. It inherits from the father by means of the spermatozoon and from the mother by means of the ovum. The actual blending of the two cells produces a third cell, which is the germ of the child, or the new organism conceived. One may also say of this sexual ■coalescence that the stem-cell is a simple hermaphrodite ; it unites both sexual substances in itself. I think it necessary to emphasise the fundamental impor- tance of this simple, but often unappreciated, feature in order to have a correct and clear idea of conception. With that end, I have given a special name to the new cell from which CONCEPTION the child developes, and which is generally loosely called " the fertilised ovum " or " the first segmentation sphere." I call ii "the stem-cell" (cytula or archicytos ,, its cell-matter "the stem-plasm" (archiplasma or cytuloplasma ) , and its nucleus "the stem-nucleus" ( archicaryon or cytulocaryonj. The name " stem-cell " seems to me the simplest and most suitable because all the other cells o( the body are derived from it, and because it is, in the strictest sense, the stem- father and stem-mother ol~ all the countless generations of cells o( which the multicellular organism is to be composed. That complicated molecular movement o( the protoplasm which we call "life" is, naturally, something quite different in this stem-cell from what we find in the two parent-cells, from the coalescence o\ which it has issued. The life of the stem-cell or cytula is the product or resultant of the paternal life-movement that is conveyed in the spermatozoon and the maternal life-movement that is contributed by the ovum. On the principle of the parallelogram o\ forces, it may be said that the potential energy of the stem-cell is the diagonal of the parallelogram, while its two sides represent the potential energy ot the paternal spermatozoa and that of the maternal bvum. The combined potential energy of the two, or the hereditary potentiality, is converted into living force as soon as the individual development of the stem-cell begins after the coalescence. 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 " k^' this character, and that this then passes, by repeated segmentation (or fission), into a cluster oi cells, known as " the segmentation sphere " or " segmentation cells" (segmentella or blastomeraj. L'ntil 1875 there was a spirited controversy as to the origin of the stem-cell, and as to the real behaviour oi the spermatozoon and the ovum in its formation or at conception. It had been generally assumed that the original nucleus o( the ovum, called the germinal vesicle, remained unchanged at concep- tion, and passed over directly to the stem-nucleus (or nucleus COXCEPTIOX of "the first segmentation sphere"). However, most modern observers are convinced that the germinal vesicle sooner or later disappears, and that the stem-nucleus is a new forma- tion. But there were different opinions as to the mode of formation of this new nucleus of the stem-cell. Some thought that the germinal vesicle disappeared before impreg- nation and some after. Some said that it was thrust out of the ovum, and others that it melted away in the velk. Some believed that it was wholly, and others that it was only partially, lost. All these contradictory opinions and diffi- culties about these important processes have now been happilv settled. The solution began in 1875, when a number of very careful microscopic studies of them were published about the same time, especially those of Oscar Hertwig and Edward Strasburger (both then at Jena), Edward Van Beneden, O. Biitschli, etc. By the work of these many succeeding observers we have gradually come to a happy agreement as to the essential features of conception, and are convinced that it has the same physiological features in the whole animal and plant worlds. This is most clearly observed in the ova of the echinoderma (star-fishes, sea urchins, sea-gherkins, etc.). The investigations of Oscar and Richard Hertwig were chiefly directed to these. The main results may be summed up as follows : — Conception is preceded by certain preliminary changes, which are very necessary — in fact, usually indispensable — for its occurrence. They are comprised under the general heading of "Changes prior to impregnation." In these the original nucleus of the ovum, the germinal vesicle, is lost. Part of it is extruded, and part dissolved in the cell contents ; only a very small part of it is left to form the basis of a fresh nucleus, the pronucleus femininus. It is the latter alone that combines in conception with the invading nucleus of the fertilising spermatozoon (the pronucleus masculinusj. The impregnation of the ovum commences with a decay of the germinal vesicle, or the original nucleus of the ovum (Fig. 24). We have seen that this is in most unripe ova a large, transparent, globular vesicle. This germinal vesicle CONCEPTION LIBRARY. contains a viscous fluid (the caryofympm). j^ffie) I'M1) »ihoM^jJuIJ.E frame (caryobasis) is formed oi the enveloping membrane and a mesh-work of nuclear threads running across the interior, which is filled with the nuclear sap. In a knot o\ the network is contained the dark, stiff, opaque nuclear corpuscle or nucleolus. When the impregnation of the ovum sets in, (he greater part of the germinal vesicle is dissolved in the cell; the nuclear membrane and mesh-work disappear; tile nuclear sap is distributed in the protoplasm ; a small portion of the nuclear base is extruded ; another small portion is left, and is converted into the secondary nucleus, or the female pro-nucleus (Fig. 25 rk). Vic. 24. An unfertilised ovum of an echinoderm, with nuclear net- work and dark nucleolus in the large globular germinal vesiole. | From Herttrig. ) Fig. J5. — An impregnated echinoderm ovum, with small homogeneous nucleus (« i). 1 From Hertmig. < The small portion of the nuclear base which is extruded from the impregnated ovum is known as the " directive bodies" or "polar cells"; there are many disputes as to their origin and significance, but we are as yet imperfectl) acquainted with them. As a rule, they are two small round granules, o( the same size and appearance as the remaining pro-nucleus. The polar cells arise successively by the con- striction or cleavage of that part oi the nuclear base (probably, as a rule, the germinal spot) which also forms the female pro-nucleus. We may, therefore, regard this cleavage- process, in which the surrounding protoplasm shares, as a twice-repeated cell division, or, rather, as a gemmation (budding) of cells ; because the two parts into which the COXCEPTIOX impregnated ovum divides each time are not of the same size and appearance. The two small polar cells are detached cell- buds; their separation from the large mother-cell takes place in the same way as in ordinary " indirect cell-division," with the formation of nuclear spindle, plasma stars, polar radia- tion, halving of the nuclear spindle, mitosis, etc. Hence, the polar cells are probably to be conceived as " abortive ova," or " rudimentary ova," which proceed from a simple original ovum by cleavage in the same way that several sperm-cells arise from one spermatoblast, or one "sperm-mother-cell," in spermatogenesis. 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 (spermatoblasts or spermatogonia J divides by double segmentation into four daughter-cells, each furnished with a fourth of the original nuclear matter (the hereditary chromatin); and each of these four descendant cells becomes a spermium or spermatozoon, ready for impregnation. Thus is prevented the doubling of the chromosomata and the hereditative chromatin in the coales- cence of the two nuclei at conception. As the two polar cells are extruded and lost, and have no further part in the fertili- sation of the ovum, we need not discuss them any further. But we must give more attention to the female pro-nucleus which alone remains after the extrusion of the polar cells and the dissolving of the germinal vesicle (Fig. 23 ek). This tiny round corpuscle of chromatin now acts as a centre of attraction for the invading spermatozoon in the large ripe ovum, and coalesces with its " head," the male pro-nucleus. The product of this blending, which is the most important part of the act of impregnation, is the stem-nucleus, or the first segmentation nucleus ( arcliicaryon 1 — that is to say, the nucleus of the new-born embryonic stem-cell or "first segmentation cell " (archicytos or cytulaj. This stem-cell is the starting-point of the subsequent embryonic processes. Hertwig has shown that the tiny transparent ova of the echinoderms are the most convenient for following the details of this important process of impregnation. We can, in this CONCEPTION case, easily and successfully accomplish artificial impregna- tion, and follow the formation o( the stem-cell step by step within the space k^\ ten minutes. If we put ripe ova o( the star-fish or sea-urchin in a watch-glass with sea-water and add a drop o( ripe sperm-fluid, we find each ovum impregnated within five minutes. Thousands of the fine, mobile ciliated cells, which we have described as " sperm-threads" (Fig. 20), make their way to the ova, owing to a sort o( chemical sensitive action which may he called " smell." But only one o( these innumerable spermatozoa is chosen — namely, the one that first reaches the ovum by the serpentine motions of its tail, and touches the ovum with its head. At the spot .1 \>Js4.fi Fig. _o. Impregnation of the ovum of a star-fish. (From Hertwig.) Only ;i small part of the surface of the ovum is shown. One of the numerous spermatozoa approaches the "impregnation rise" (A), touches it (BJ, and then penetrates into the protoplasm of the ovum ( Cj. where the point o( its head touches the surface of the ovum the protoplasm of the latter is raised in the form of a small wart, the "impregnation rise" (Fi,^r. 26 A). The spermatozoon then bores its way into this with its head, the tail outside wriggling about all the time (Fig. 26 />, C). Presently the tail also disappears within the ovum. At the same time the ovum secretes a thin external yelk-membrane (Fig. 26 C), Starting from the point o\ impregnation; and this prevents any more spermatozoa from entering. Inside the impregnated ovum we now see a rapid series o\ most important changes. The pear-shaped head o( the sperm-cell, or the "head o\ the spermatozoon," grows larger and rounder, and is converted into the male pro-nucleus 136 COXCEPTIOX (Fig. 27 $ k). This has an attractive influence on the fine granules or microsomata which are distributed in the proto- plasm of the ovum ; they arrange themselves in lines in the figure of a star (cytulaster). But the attraction or the " affinity " between the two nuclei is even stronger. They move towards each other inside the yelk with increasing speed, the male (Fig. 28 j k) going more quickly than the female nucleus (e k). The tiny male nucleus takes with it the radiating mantle which spreads like a star about it. At last the two sexual nuclei touch (usually in the centre of the globular ovum), lie close together, are flattened at the points of contact, and coalesce into a common mass. The small ^-rv. Impregnation of the ovum of the sea-urchin. ( From Hertwig. ) In Fig-. 27 the little sperm-nucleus (sh) moves towards the larger nucleus of the ovum (ck). In Fig. 28 they nearly touch, and arc surrounded by the radiating mantle of protoplasm. central particle of nuclein which is formed from this combina- tion of the nuclei is the stem-nucleus, or the first segmenta- tion nucleus (archicaryon or eytulocaryon J ; the new-formed cell, the product of the impregnation, is our stem-cell, or "first segmentation sphere" (cytttla or archicytos, Fig. 29). Hence the one essential point in the process of sexual reproduction or impregnation is the formation of a new cell, the stem-cell. This cytula is always the resultant of the com- bination of two originally different cells, the female ovum and the male spermatozoon. This process is of the highest impor- tance and merits our closest attention ; all that happens in the later development of this first cell and in the life of the organism that comes of it is determined from the first by the chemical ( < > VI EPTIOX ■37 and morphological composition of the stem-cell, Its nucleus and its body. We must, therefore, make a very careful study of the rise and structure of the stem-cell. The first question that arises is as to the 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 theor) of conception in the principle : "Conception consists in the copulation of two cell-nuclei, which come from a male and a female cell." And as the phenomenon of heredity is inseparably connected with the reproductive process, we may further conclude that these two copulating nuclei "convey the characteristics which are trans- mitted from parents to offspring." In this sense I had in 1866 (in the ninth chapter of the Generelle Morphologie) ascribed to the re- productive nucleus the function ol generation and heredity, and to the nutritive protoplasm the duties of nutrition and adaptation. As, moreover, there is a complete coalescence of the mutually attrac- ted nuclear substances in concep- tion, and the new nucleus formed (the stem-nucleus) is the real starting-point for the development of the fresh organism, the further conclusion may be drawn that the male nucleus conveys to the child the qualities of the father, and the female nucleus the features oi the mother. We must not forget, however, that the protoplasmic bodies of the copulating cells also fuse together in the act of impreg- nation ; the cell-body of the invading spermatozoon (the trunk and tail oi the male ciliated cell) is dissolved in the yelk oi the female ovum. This coalescence is not so important as that oi 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 Fig. -a Stem-cell of ey- tula of a sea-urchin (first- segmentation-cell, or impreg- nated ovum). (From Herhuig.) In the centre of the globular cell is the small globular stem-nucleus or segmentation-nucleus (fk). CONCEPTION figure (the radial arrangement of the microsomata in the plasma) in it (Figs. 27-29). Mention must also be made of the reciprocal action o\~ the cell-constituents on both sides. The formation of the proto- plasmic star around the invading male nucleus, and after- wards round the copulated stem-nucleus, suggests the idea that this alone has an active influence on the arrangement of the granules and threads in the protoplasm. However, the reproductive nucleus itself changes its size, shape, and con- sistency, and is on its side influenced, from the conditions under which it is nourished, by the nutritive protoplasm. How close the interaction of the two elements is can be seen at once from the above-mentioned preliminary processes of the maturing of the ovum before impregnation, and from the segmentation processes that follow it. In both cases we observe the complete phenomena of caryokinesis and mitosis, which are found always in indirect cleavage, and which reveal to us the significant interaction of cell-nucleus and cell-body. These phenomena have also been called caryolysis, or the " dissolving of the nucleus in the protoplasm." This may be granted up to a certain point, and used in support of our monera theory — for the belief that the oldest and simplest organisms were innucleated plastids, and that the real unicel- lular forms of life were subsequently developed from these by the cleavage of nucleus and cell-body. (Cf. the nineteenth Chapter.) The older theories of impregnation generally went astray in regarding the large ovum as the sole base of the new organism, and only ascribed to the spermatozoon the role of stimulating and originating its development. The stimulus which it gave to the ovum was sometimes thought to be purely chemical (a catalytic process), at other times rather physical (on the principle of transferred movement), or again quite dualistic (that is, a mystic and transcendental process). This error was partly due to the imperfect know- ledge at that time of the facts of impregnation, and partly to the striking difference in the sizes of the two sexual cells. Most of the earlier observers thought that the spermatozoon CONCEPTION did not penetrate into the ovum. And even when this had been demonstrated, the spermatozoon was believed to dis- appear in the ovum without leaving a trace. However, the splendid research made in the last three decades with the finer technical methods of our time has completely exposed the error o( this. It lias been shown that the tiny sperm-cell is not subordinated to, but co-ordinated with, the large ovum. The nuclei of the two cells, as the vehicles o\ the hereditary features of the parents, are of equal physiological importance. In some cases we have succeeded in proving that the mass of the active nuclear substance which combines in the copula- tion o( the two sexual nuclei is orginally the same for both. Edward Van Beneden has shown that in the ovum of the horse maw-worm f ascaria megalocephcUa) the union of the two sexual nuclei is delayed until the stem-cell created begins to divide. The characteristic nuclear spindle which is then formed, and which falls into the nuclei of the two first segmentation daughter-cells, is formed half of the nucleus of the ovum and half of the sperm-nucleus ; of the four " daughter-loops " of the segmentation spindle two are of male and two of female origin. These morphological facts are in perfect harmony with the familiar physiological truth that the child inherits from both parents, and that on the average they are equally distributed. I say " on the average," because it is well known that a child may have a greater likeness to the father or to the mother ; that goes without saying, as far as the primary sexual characters (the sexual glands) are concerned. But it is also possible that the determination of the latter — the weighty determination whether the child is to be a boy or a girl — depends on a slight qualitative or quantitative difference in the nuclein or the chromatic nuclear matter which comes from both parents in the act of conception. The striking differences of the respective sexual cells in si/.e and shape, which occasioned the erroneous views *>t earlier scientists, are easily explained on the principle ol division of labour, or ergonomy. The inert, motionless ovum grows in size according to the quantity o\~ provision it COXCEPTIOX stores up in the form of nutritive yelk for the development of the germ. The active swimming sperm-cell is reduced in size in proportion to its need to seek the ovum and bore its way into its yelk. These differences are very conspicuous in the higher animals, but they are much less in the lower animals. In those protists (unicellular plants and animals) which have the first rudiments of sexual reproduction the two copulating cells are at first quite equal. In these cases the act of impregnation is nothing more than a sudden growth, in which the originally simple cell doubles its volume, and is thus prepared for reproduction (cell-division). Afterwards slight differences are seen in the size of the copulating cells ; though the smaller microspores (or microgonidia) still have the same shape as the larger macrospores (or macrogonidia). It is only when the difference in size is very pronounced tli£ a notable difference in shape is found : the sprightly sperr cell changes more in shape and the ovum in size. Quite in harmony with this new conception of /the equivalence of the two gonidia, or the equal physiological importance of the male and female sex-cells and their equal share in the process of heredity, is the important fact established by Hertwig (1875), that in normal impregnation only one single spermatozoon copulates with one ovum ; the membrane which is raised on the surface of the yelk imme- diately after one sperm-cell has penetrated (Fig. 26 C) prevents any others from entering. All the rivals of the fortunate penetrator are excluded, and die without. But if the ovum passes into a morbid state, if it is made stiff by a lowering of its temperature or stupefied with narcotics (chloroform, morphia, nicotine, etc.), two or more sperma- tozoa may penetrate into its yelk-bcdy. We then witness polyspermism. The more Hertwig chloroformed the ovum, ie more spermatozoa were able to bore their way into its unconscious body. These remarkable facts of impregnation are also of the greatest interest in psychology, especially as regards the theory of the cell-soul, which I consider to be its chief foundation. All the phenomena we have described can only CONCEPTION be understood and explained by ascribing a certain lower degree of psychic activity to the sexual principles. They feel each other's proximity, and are drawn together by a sensitive impulse (probablj related to smell) ; they move towards each other, and do not rest until they fuse together. Physiologists may say that it is only a question of a peculiar physico- chemical phenomenon, and not a psychic action ; but the two cannot be separated. Even the psychic functions, in the strict sense of the word, are only complex physical processes, or "psycho-physical" phenomena, which are determined in all cases exclusively by the chemical composition of their material substratum. The monistic view of the matter becomes clear enough when we remember the radical importance of impregnation as regards heredity. It is well known that not only the most delicate bodily structures, but also the subtlest traits of mind, are transmitted from the parents to the children. In this the chromatic matter of the male nucleus is just as important a vehicle as the large caryoplasmic substance of the female nucleus ; the one transmits the mental features of the father, and the other those of the mother. The blending of the two parental nuclei determines the individual psychic character of the child. But there is another important psychological question — the most important of all — that has been definitely answered by the recent discoveries in connection with conception. This is the question of personal immortality. This dogma, which we meet in the most varied forms among uncivilised peoples, occupies an important place also in the higher conceptions of civilised nations. But the fact that it is untenable has been growing clearer and clearer during the last fifty years, chiefly through the vast progress we have made in comparative morphology, experimental physiology, empirical psychology, psychiatry, monistic anthropology, and ethnography. However, no fact throws more light on it and refutes it more convincingly than the elementary process of conception that we have described. For this copulation ot the two sexual nuclei (Figs. 27-29) indicates the precise CO.XCEPTIOX moment at which the individual begins to exist. All the bodily and mental features of the new-born child are the sum-total of the hereditary qualities which it has received in reproduction from parents and ancestors. All that man acquires afterwards in life by the exercise of his organs, the influence of his environment, and education — in a word, by adaptation — cannot obliterate that general outline of his being which he inherited from his parents. But this heredi- tary disposition, the essence of every human soul, is not "eternal," but "temporal"; it comes into being only at the moment when the sperm-nucleus of the father and the nucleus of the maternal ovum meet and fuse together. It is clearly irrational to assume an " eternal life without md " for an individual phenomenon, the commencement of' which we can indicate to a moment by direct visual observa- tion. But the unbroken chain of plasma-movements which we comprise under the title of a man's " soul " is just such an individual phenomenon. This chain of molecular move- ments begins at the moment when the paternal nucleus fuses with the maternal. From the stem-nucleus thus produced it is transmitted, in the repeated segmentation, to all the similar cells of the germinal layer. When these blastodermic cells grow into the two primary germinal layers of the gastrula, the first division of labour in the cells takes place ; and this continues when the various tissues arise from them. Later, in man and the higher animals, it is only the central nerve- cells which are the primary organs of psychic life. At their death the mental life is extinguished, just as the faculty of vision perishes with the eye. We often hear it said that the belief in immortality is an indispensable foundation of religion and morality, like the belief in a personal God. This opinion is totally opposed to the facts of history. In any case it is clear that all that is "personal " must be transitory, a mere passing phenomenal form in the course of the evolutionary process. Hence it is a curious error to speak, as Weismann does, of the immortality of the unicellular beings. The unicellular protists are transitory individuals just as truly as the multicellular CONCEPTION organisms, to which man belongs. Ii is true that our human soul is often regarded as something unique, and credited with peculiar powers that are not found in the other vertebrates. But an impartial study of comparative psychology completely disposes o\ this illusion. We shall see that the special Organs o( man's mental life are evolved in just the same way as those of other vertebrates. The great importance of the process of impregnation in answering these and other cardinal questions is quite clear. It is true that conception has never been studied micro- scopically in all its details in the human case — notwith- standing its occurrence A. at every moment — for reasons that are obvious enough. However, the two cells which need con- sideration, the female ovum and the male sper- matozoon, proceed in the case of man in just the same way as in all the other mammals ; the human foetus or embryo which results from copula- tion has the same form as with the other animals. Hence, no scientist who is acquainted with the facts doubts that the processes of impregnation are just the same in man as in the other animals. The stem-cell which is produced, and with which every man begins his career, cannot be distinguished in appearance from those of other mammals, such as the hare (Fig. p,o). In the case of man, also, this stem-cell differs materially from the original ovum, both in regard to form (morphologically), in regard to material composition (chemically), and in regard to vital properties (physiologically). It comes partly from the father and partly from the mother. Hence it is nol Fig. 30.— Stem-cell of a hare, mag- nified joo times. In the centre of the granular protoplasm of the fertilised ovum ( ) Nucleolus, nuclear cor- puscles. el Caryomita, nuclearth reads. d) Caryotheka, nuclear mem- brane. / i. Cytomitoma, i. Filar matter, or i. Protoplasma. Cell-skeleton, spongroplasm , Active (living) made up of cyto- Mesh-work or cell-matter. mita or proto- { plasmic threads. honeycomb 2. Metaplasma. a) Paraplasma, II. Cell-body Passive (dead) Definite inter- [cetteus or cyto- cell-matter filar matter. plasm). (plasma-products) 2. A. Internal h) Aficrosomata,or Originally In very young plasma-products granula, granules of composed of cells of primary (stored within plasma. homogeneous composition there the protoplasm |. c) Lipsomata, cellular matter is nometaplasm ; granules of tat. ( cytoplasm ). the whole cell- d) Cytolymph, body consist s cell-sap. solely of homo- a) Cytotheta, geneous proto- 2. B. External membrane ol plasm. plasma products (extruded from cell. b) Intercellular 1 the protoplasm). matter. CHAPTER VIII. THE GASTR^EA THEORY1 First changes after the impregnation of the ovum. The original or palingenetic form of segmentation. Nature of the segmentation-process. Repeated cleavage of the stem-cell. Formation of several segmentation spheres or blastomeres. Mulberry-like structure, or morula. Blastula. Germinal membrane or blastoderm. Folding of the blastula. Formation of the gastrula. Depula, transition from the blastula to the gastrula. Primitive gut and primitive mouth. The two primary germinal layers : ectoderm (epiblast) and entoderm (hypoblast). Differences between their cells. Similarity of the original gastrulation in the most distant groups of the animal world. The gastrulation of the amphioxus ; transition from the primary (uni-axial) to the secondary (bi-lateral or tri-axial) form of the gastrula. Bending of the chief axis. Flattening of the hinder side, large growth of the fore-side. The secondary, modified, or cenogenctic forms of gastrulation. Significance and unequal distribution of the yelk. Total and partial cleavage. Holoblastie and meroblastie ova. Disc-like cleavage and disc-gastrula : fishes, reptiles, birds. Superficial cleavage and globular gastrula : articulata. Permanent two-layered structure of the lower animals. The two-layered primitive stem-form : gastrffia. Homology of the two primary germinal layers. There is a substantial agreement throughout the animal world in the first changes which follow the impregnation of the ovum and the formation of the stem-cell ; they begin in all cases with the segmentation of the ovum and the formation of the germinal layers. The only exception is found in the protozoa, the very lowest and simplest forms of animal life ; these remain unicellular throughout life. To this group belong the amoebae, gregarina^, rhizopods, infusoria, etc. As their whole organism consists of a single cell, they can never form germinal layers, or definite strata of cells. But all the other animals — all the tissue-forming animals, or 1 Cf. E. Ray-Lankester's essays " On the Primitive Cell-layers 01 the Embryo as the Basis of Genealogical Classification of the Animals " (Ann. Mag. Nat. Hist., vol. xi., 1873) and '• Notes on the Embryology and Classifi- cation of the Animal Kingdom " ( Quarterly Journal of Microscopic Science, vol. xvii. , 1877), and Francis Balfour's Manual of Comparative Embryology, and " On the Structure and Homology of the Germinal Layers of the Embryo " ( Quart. Journal of Micros. Science. 1SS0). 146 THE GAST8AVA THEORY metazoa, as we call them, in contradistinction to the protozoa — construct real germinal layers by the repeated cleavage of the impregnated ovum. This we find in the lower cnidaria and worms, as well as in the more highly-developed molluscs, echinoderms, articulates, and vertebrates. In all these metazoa, or multicellular animals, the chief embryonic processes are substantially alike, although they often seem to a superficial observer to differ considerably. The stem-cell that proceeds from the impregnated ovum always passes by repeated fission 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 segmenta- tion-cells, or segmentation-spheres (blaslomera or segmen- tella). The repeated cleavage of the stem-cell, which gives rise to these segmentation-spheres, has long been known as •• segmentation." Sooner or later the segmentation-cells join together to form a round (at first, globular) embryonal sphere (bias tula J; they then form into two very different groups, and arrange themselves in two separate strata — the two primary germinal layers. These enclose a digestive cavity, the primitive gut, with an opening, the primitive mouth. We give the name of the gastrula to the important embryonic form that has these primitive organs, and the name of gastru- lation to the formation o( it. This ontogenetic process has a very great significance, and is the real starting-point of the construction of the multicellular animal body. The fundamental embryonic processes of the cleavage of the ovum and the formation of the germinal lavers 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 gastraja theory in 1S72, 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. THE GASTR.EA THEORY Man is like all the other higher animals, especially the apes, in regard to these earliest and most important pro- cesses. As the human embryo does not essentially differ, even at a much later stage of development — when we already perceive the cerebral lobes, the eyes, ears, gill-arches, etc. — from the similar forms of the other higher mammals (cf. Plate XIII., first row), we may confidently assume that they agree in the earliest embryonic processes, segmentation and formation of germinal layers. This has not yet, it is true, been established by observation. We have never yet had occasion to dissect a woman immediately after impregna- tion and examine the stem-cell or the segmentation-cells in her oviduct. However, as the earliest human embryos (in the form of embryonal spheres) we have examined, and the later and more developed forms, agree with those of the hare, dog, and other higher mammals, no reasonable man will doubt but that the segmentation and formation of layers are the same in both cases, as Figs. 12-17 on Plate H- represent. But the special form of segmentation and layer-formation which we find in the mammal is by no means the original, simple, palingenetic form. It has been much modified and cenogenetically altered by a very complex adaptation to embryonic conditions. We cannot, therefore, understand it altogether in itself. In order to do this, we have to make a comparative study of segmentation and layer-formation in the animal world ; and we have especially to seek the original, palingenetic form from which the modified cenogenetic form has gradually been developed. This original palingenetic form ot segmentation and layer-formation is found to-day in only one case in the vertebrate-stem to which man belongs — the lowest and oldest member of the stem, the wonderful lancelet or ampbioxus (cf. Chapters XVI. and XVII., and Plates XVIII. and XIX.). But we find a precisely similar palingenetic form of embryonic development in the case of many of the invertebrate animals, as, for instance, the remarkable ascidia, the pond-snail (limnceusj, the arrow-worm (sagit/a), and many of the echinoderms and cnidaria, such as the ordinary star-fish and muSSEST lIBMk, /'///•; GASTRjEA theory sea-urchin, many of the medusae and corals, and the simpler sponges ' olyiithus ,. We may take as an illustration the palingenetic segmentation and germinal layer-formation in an eight-fold insular coral, which I discovered in the Red Sea, and described in my Arabische Korallen as monoxenta Darwinti. The impregnated ovum of this coral (Fig. 31 A, B) first splits into two equal cells (C). First, the nucleus of the Stem-cell and the dependent centrosoma divide into two halves. These recede from and repel each other, and act as centres of attraction on the surrounding protoplasm ; in consequence of this, the protoplasm is constricted by a circular furrow, and, in turn, divides into two halves. Each of the two segmentation-cells thus produced splits in the same way into two equal cells, and, in fact, the plane of cleavage of the latter two lies vertically on that of the first (Fig. D). The four familiar 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- lour, 128, and soon.1 The final result of this repeated cleavage is the formation of a globular cluster of similar segmentation- cells, which we call the mulberry-formation or morula. The cells are thickly pressed together like the parts of a mulberry or blackberry, and this gives a lumpy appearance to the surface of the sphere (Fig. E). [Cf. also Fig. 3 on Plate II.]2 When the cleavage is thus ended, the mulberry-like mass changes into a hollow globular sphere. Watery fluid or jelly gathers inside the globule ; the segmentation cells are ' Tin' number of blastomeres or segmentation-cells increases geometrically in the original gastrulation, or the purest palingenetic form of cleavage. However, in different archiblastic 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 ol 128, or more, rolls. •' The segmentation-cells which make up the morula after tin- close oi the palingenetic cleavage seem usually to in- quite similar, ami to present no morphological differences as to size, form, and composition. That, however, does not prevent them from differentiating into animal and vegetative rolls even during the cleavage, as Figs. 2 and ,', on Plate II. indicate. THE GASTR.EA THEORY Fig. 31.— Gastrulation Of a . Gastrula of an eehinoderm (star-fish, uraster), not com- pletely Folded in (depula). (From Alexander Agassis. 1 Fig. 35 ( D ).— Gastrula of an arthropod (primitive crab, nauplius) (as 34). Ftg. 36 (E).— Gastrula of a mollusc (pond-snail, limtueus). (From Karl RabL ) Fig. si(F). Gastrula of a vertebrate (lancelet, amphioxus). (From Kowalevsky.) (Front view.) In each figure ', C three stages in the formation of the blastula ; />, £ curving of the blastula ; F complete gastrula. /; segmen- tation-cavity, g primitive gfut-cavity. former constitute the animal, and the latter the vegetal, hemisphere. Hatschek rightly observes that the segmenta- tion ot 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 o\ the upper hemisphere divide more quickly than the larger vegetal cells of the lower (Fig. 40 A, />). Hence the blastoderm, which forms the single- layer wall o\ the globular blastula at the end of the cleavage- process, does not consist ot homogeneous cells of equal size, THE GASTRMA THEORY 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. 40 C, h) is not quite globular, but forms a flattened spheroid with unequal poles of its verticle axis. While the blastula is being folded into a cup at the vegetal pole of its axis, the difference in the size of the blastodermic cells increases (Fig. 40 D, E); it is most conspicuous when the invagination is complete and the segmentation- cavity has disappeared (Fig. 40 F). The larger vegetal cells of the ento- derm 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. 31), and the olyntluts (Fig. 38), not only by this early (or cenogenetically premature) differen- tiation of the blastodermic cells, but also in another important particular. The pure archigastrula of the latter forms is uni-axial, and it is round in its whole length in transverse section. The vegetal pole of the vertical axis is just in the centre of the primitive mouth. This is not the case in the gastrula of the amphioxus. During the folding of the blastula the ideal axis is already bent on one side, the growth of the blastoderm (or the increase of its cells) being brisker on one side than on the other; the side that grows more quickly, and so is more curved (Fig. 41 v), will be the anterior or belly-side, the opposite, flatter side will form the back (d). The primitive mouth, which at first, in the typical archigastrula, lay at the vegetal pole of the main axis, is forced away to the dorsal side ; and whereas its two lips lay at first in a plane at right Fig. 41. — Gastrula of the amphioxus, seen from left Side (diagram- matic median section). (From Hatschek.) g primi- tive gut, u primitive mouth, p peristomal pole-cells, i entoderm, e ectoderm, d dorsal side, v ventral side. THE GASTRJEA THEORY 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. 41 p) : these arc the important polar cells of the primitive mouth, or "the primitive cells of the mesoderm." In consequence of these considerable variations arising in the course of the gastrulation, the primitive uni-axial form of the archigastrula in the amphioxus has already become tri-axial, and thus the two-sidedness, or bilateral symmetry, of the vertebrate body has already been determined. The vertical middle plane (or arrow-plane) passes between the two polar cells of the prostoma, and goes the whole length of the body, dividing it into two equal halves or "antimera," right and left. The primitive mouth lies at the further and hinder end, a little above the anti-oral pole of the long axis. The arrow-axis, or dorso-ventral axis, lies verti- cally to this chief axis on the middle plane, joining the central lines of the flat dorsal side and the convex ventral side. The horizontal transverse axis, or lateral axis, vertical to the two (unequally polar) axes, is equi-polar, and crosses diagonally from right to left. Thus, the gastrula of the amphioxus alreadv exhibits the characteristic two-sidedness of the verte- brate body, and 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 o( the amphioxus resembles the typical archigastrula of the lower animals (Figs. 32 38) in developing the two primary germinal layers from a single layer of cells. This is clearly the oldest and original form of the metazoic embryo. Although the animals I have mentioned belong to the most diverse classes, they nevertheless agree with each other, and many more animal forms, in having retained to the present day, by a conservative heredity, this palingenetic form of gastrulation which they have from their earliest common ancestors. Hut this is not the case with the great majority of the animals. M 162 THE GASTR.EA THEORY With these the original embryonic process has been gradually more or less altered in the course of millions of years by adaptation to new conditions of development. Both the segmentation of the ovum and the subsequent gastrulation have in this way been considerably changed. In fact, these variations have become so great in the course of time that the segmentation was not rightly understood in most animals, and the gastrula was unrecognised. It was not until I had made an extensive comparative study, lasting a considerable time (in the years 1866-75), in animals of the most diverse classes, that I succeeded in showing the same common typical process in these apparently very different forms of gastrulation, and tracing them all to one original form. I regard all those that diverge from the primary palingenetic gastrulation as secondary, modified, and cenogenetic. The more or less divergent form of gastrula that is produced may be called a secondary, modified gastrula, or a metagastrula. Among the many and varied cenogenetic forms of segmentation and gastrulation I distinguish three chief types: 1, unequal segmentation (Plate II., Figs. 7-17); 2, discoid segmentation (Plate III., Figs. 18-24); an^ 3i super- ficial segmentation (Plate III., Figs. 25-30). From the unequal cleavage we have the tufted foetus ( ainphigaxtru/a, Plate II., Figs. 11 and 17); the discoid cleavage produces the disk-shaped gastrula ( discogastntta, Plate III., Fig. 24) ; and the superficial produces the globular gastrula ( perigas- trula, Plate III., Fig. 29). In the vertebrates, with which we are chief!)' concerned, the last-named form is not found at all ; on the other hand, it is the commonest form among the articulates (crabs, spiders, insects, etc.). Mammals and amphibia have the unequal segmentation and the tufted foetus ; so also the ganoid (scaley) and the round-mouthed fishes (the lamprey and myxine). On the other hand, most fishes, and all reptiles and birds, have the discoid segmenta- tion and gastrula. (Cf. Table II., p. 171.) By far the most important process that determines the various cenogenetic forms of gastrulation is the change in the nutrition of the ovum and the accumulation in it of nutritive THE GASTRASA THEORY 163 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 or vitcllus format 'ivi/s 1 and the passive food-yelk (deutoplasm, or vitcllus nutritious, wrongly spoken oi as " the yelk," lecithus). 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 oi 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, there is then a polar differentiation of the ovum, in the sense that a chief axis can be discerned in it, and the formative yelk (with the germinal vesicle) gathers at one pole and food-yelk at the other. The first is the animal, and the second the vegetal, pole of the vertical axis of the ovum. In these " telolecithal " ova (for instance, in the cyclos- toma and amphibia, Plate II., Figs. 7-11) the gastrulation then usually takes place in such a way that in the cleavage of the impregnated ovum the animal (usually the upper) half splits up more quickly than the vegetal (lower). The contractions oi the active protoplasm, which effect this continual cleavage oi the cells, meet a greater resistance in the lower vegetal half from the passive deutoplasm than in the upper animal half. Hence we find in the latter more but smaller, and in the former fewer but larger, cells. The animal cells produce the external, and the vegetal cells the internal, germinal layer. Although this unequal segmentation of the cyclostoma, ganoids, and amphibia seems at first sight to differ from the original equal segmentation (for instance, in the monoxenia, 1 64 THE GASTK.EA THEORY Fig. 31), they both have this in common, that the cleavage process throughout affects the whole cell ; hence Remak called it total segmentation, and the ova in question holoblastic. It is otherwise with the second chief group of ova, which he distinguished from these as meroblastic : to this class belong the familiar large eggs of birds and reptiles, and of most fishes. The inert mass of the passive food-yelk is so large in these cases that the protoplasmic contractions of the active yelk cannot effect any further cleavage. In consequence, there is only a partial segmentation. While the protoplasm in the animal section of the ovum continues briskly to divide, multiplying the nuclei, the deutoplasm in the vegetal section remains more or less undivided ; it is merely consumed as food by the forming cells. The larger the accumulation of food, the more restricted is the process of segmentation. It may, however, continue for some time (even after the gastru- lation is more or less complete) in the sense that the vegetal cell-nuclei distributed in the deutoplasm slowly increase by cleavage ; as each of them is surrounded by a small quantity of protoplasm, it may afterwards appropriate a portion of the food-yelk, and thus form a real " yelk-cell " ( merocyte I. When this vegetal cell-formation continues for a long time, after the two primary germinal layers have been formed, it takes the name of the "after-segmentation " (Waldeyer). The meroblastic ova (Plate III.) are only found in the larger and more highly developed animals, and only in those whose embryo needs a longer time and richer nourishment within the foetal membranes. According as the yelk-food accumulates at the centre or the side of the ovum, we dis- tinguish two groups of dividing ova, periblastic and dis- coblastic. In the periblastic the food-yelk is in the centre, enclosed inside the ovum (hence they are also called "centro- lecithal " 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., Plate III., Figs. 25-30). In the discoblastic ova the food-yelk gathers at one side, at the vegetal or lower pole of the vertical axis, while the nucleus of the ovum and the great bulk of the THE CASTh'.EA THEOR\ 165 formative yolk lie at the upper or animal pole (hence these ova are also called " tclolethical "). 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 (monotrema). The gastrulation of the discoblastic ova, which chiefly concerns us, otters serious difficulties to microscopic investi- gation and philosophic consideration. These, however, have been mastered by the comparative embryological research which has been conducted by a number of distinguished observers during the last few decades — especially the brothers Hertwig, Rabl, Kupffer, Selenka, Riickert, 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 /lie Gastrula and the Segmentation of the Animal Ovum |not translated], in 1875. As it is very important to understand these views and their phylogenetic foundation clearly, not only as regards evolution in general, but particularly in connection with the genesis of man, I will give here a brief statement o( 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. Consequentlv, the embryonic features in their indi- vidual development must also hang together phylogenetically. 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 eenogenetic modifications of the latter are more appreciable the more food-yelk is stored up in the ovum. 5. Although the mass of the food-yelk may be very large in the ova of the discoblastic vertebrates, nevertheless in every case a blastula is developed from the morula, as in the holoblastic ova. 166 THE GASTRASA THEORY 6. Also, in every case, the gastrula developes from the blastula by folding, or invagination. 7. The cavity which is produced in the foetus by this folding is, in each case, the primitive gut (progaster), and its opening the primitive mouth (prostoma). 8. The food-yelk, whether large or small, is always stored in the ventral wall of the primitive gut ; the cells (called "merocvtes") which may be formed in it subsequently (bv " after-segmentation ") also belong to the inner germinal layer or endoblast, like the cells which immediately enclose the primitive gut-cavity. 9. The primitive mouth, which at first lies below at the basic pole of the vertical axis, is forced, by the growth of the yelk, backwards and then upwards, towards the dorsal side of the embryo ; the vertical axis of the primitive gut is thus gradually converted into horizontal. 10. The primitive mouth is closed sooner or later in all the vertebrates, and does not pass into the permanent mouth- aperture; it rather corresponds to the "properistoma," or region of the anus. From this important point the formation of the middle germinal layer proceeds, between the two primary layers. The wide comparative studies of the scientists I have named have further shown that in the case of the discoblastic higher vertebrates (the three classes of amniotes) the primi- tive mouth of the embryonic disc, which was long looked for in vain, is found always, and is nothing else than the familiar " primitive groove." This is a groove that lies in the hinder dorsal surface of the discoid gastrula, and was formerly con- fused with the hinder part of the medullary tube. It is true that it is directly connected with this for some time (bv the canalis neurentericus, which we shall discuss later), but originally it is a totally different thing, both in structure and purport. The two parallel longitudinal swellings which enclose this slender " primitive groove" (lying on the middle line) are the right and left primitive lips. The primitive mouth, which is at first (in the holoblastic vertebrates) a small round opening, is thus altered (in consequence of the increasing THE GASTRJEA THEORY 167 accumulation of food-yelk and the resulting extension of the ventral wall oi the primitive gut) not only in position and direction, but also in shape and size. It changes first into a sickle-shaped tranverse told (the " sickle-groove "), in which we distinguish a ventral (lower) and a dorsal (upper) primitive lip. However, the broad transverse fold soon narrows, and changes into a longitudinal fold (something like a hare-slit), the right and left halves of the sickle-groove (called the "sickle-horns") being shortened, the middle part and the two halves of the dorsal upper lip being drawn forward. The latter meet subsequently in the middle line, and form the important " primitive streak." Thus gastrulation may be reduced to one and the same process in all the vertebrates. Moreover, the various forms it takes in the invertebrate metazoa can always be reduced to one of the four types of segmentation described above. In relation to the distinction between total and partial segmenta- tion, the grouping of the various forms is as follows : — I. Palinerenetic ( r- , -, 1. Equal segmentation 1 , t . , primitive , ',, I, , 1 , A. tola segmenta- 1 beu-erastrula). segmentation. t I tion il« ithout indepen- dent food-yelk), ( enogenetic v ""SSI " ' 3- Discoid segmentation , , p ^ (modified by (discoid gastrula). | ^.^ adaptation). c c ■ , , ,• I (with indepen- c .- . . with indepen- 4. Super lii-ial segmentation , , , ' .. , ^ ,11.1, tli.-iit lood-yelkl. (spherical gastrula). J The lowest metazoa we know — namely, the lower zoophyta (sponges, simple polyps, etc.) — remain throughout life at a stage o\ development which differs little from the gastrula ; their whole body consists of two layers of cells. This is a fact o( extreme importance. We see that man, and even other vertebrates, pass quickly through a stage o( develop- ment in which they consist oi two layers, just as these lower zoophyta do throughout life. If we apply our biogenetic law to the matter, we at once reach this important conclusion : " Man and all the other animals which pass through the two- layer stage, or gastrula-form, in the course of their embryonic THE GASTR.EA THEORY development, must descend from a primitive simple stem-form, the whole body of which consisted throughout life (as is the case with the lower zoophyta to-dav) merely of two cell-strata or germinal lavers." We will call this primitive stem-form, with which we shall deal more fully later on, the gastrcea — that is to sav, " primitive-gut animal." According to this gastraea theory, one organ was originally of the same morphological and physiological significance in all multicellular animals — the primitive gut ; and the two primarv germinal layers which form its wall must also be regarded as similar or homologous in all. This important homology 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 folding of the blastula; and, on the other hand, by the fact that in everv case the same fundamental organs arise from the germinal layers. The outer or animal layer, or ectoderm, always forms the chief organs of animal life — the skin, nervous system, sense-organs, etc. ; the inner or vegetal layer, or entoderm, gives rise to the chief organs of vegetative life — the organs of nourishment, digestion, blood-forma- tion, etc. In the lower zoophvta, whose bodv remains at the two- layer stage throughout life, the gastrajada, the simplest sponges i olynthus j, and polyps ( hydra J, these two groups of functions, animal and vegetative, are strictly divided between the two simple primary layers. Throughout life the outer or animal blastodermic layer acts simply as a covering for the body, and accomplishes its movement and sensation. The inner or vegetative laver of cells acts throughout life as a gut-epithelium, or nutritive laver of enteric cells, and often also releases the reproductive cells. The best known of these " gastrasada," or " gastrula-like animals," is the common fresh-water polyp (hydra). This simplest of all the cnidaria has, it is true, a crown of tentacles round its mouth. Also its outer germinal layer is slightly differentiated histologically. But these are secondary addi- tions, and the inner germinal laver is a simple stratum of THE GASTRJEA THEORY 169 colls. On the whole, the hydra lias preserved to our day by heredity the simple structure o( our primitive ancestor, the gastrcea (cf. Chapter XIX.). In all other animals, particularly the vertebrates, the gastrula is merely a brief transitional stage. Here the two- layer Stage of the embryonic development is quickly succeeded by a three-layer, and then a four-layer, stage. With the appearance of the four superimposed germinal layers we reach again a firm and steady standing-ground, from which we may follow the further, and much more difficult and complicated, course of embryonic development. EXPLANATION OF PLATES II. AND III. Segmentation and Gastrulation. Plates II. and III. illustrate the chief differences in the ovum-segmentation and gastrulation of animals by diagrammatic sections. Plate II. shows holoblastic ova (with total segmentation) ; Plate III., meroblastic ova (or with partial segmentation). The animal half of the ova (ectoderm) is tinted grey, and the vegetal half (entoderm with food-yelk) red. The food-yelk is vertically grained. All sections are vertical and median (through the axis of the primi- tive gut). The letters have the same meaning throughout: c Stem-cell ( ' cytula ). f Segmentation-cells ( segmentella or blastomeres ). m Mulberry- stage (morula), b Blastula. g Cup-structure (gastrula). s Segmentation- cavity (blastoccelum), d Primitive gut-cavity (progasier). u Primitive mouth ( prostoma ). n food-yelk (lecithus). i gut-layer ( entodcrma ). e skin-layer ( ectoderma ). Figs. 1-6. Equal Segmentation of a lower metazoon ^///ii, ascidia). Fig. r. Stem-cell (cytula). Fig. 2. Cleavage-stage with four segmentation- cells. Fig. 3. Mulberry-stage (morula). Fig. 4. Blastula. Fig. 5. The same in process of folding or invagination (depula). Fig. 6. Bell-gastrula (archi- gastrula). Cf. Figs. 31-40. Figs. 7-1 1. Unequal Segmentation of an amphibian (frog). Fig. 7. Stem-cell. Fig. 8. Cleavage stage with four segmentation-cells. Fig. 9. Morula. Fig. 10. Blastula. Fig. 11. Tuft-gastrula (amphigastrula). Cf. Figs. -+2-5.3- Figs. 12-17. Unequal segmentation of a mammal (hare). Fig. 12. Cytula. Fig. 13. Cleavage with two segmentation-cells (e mother-cell of the ectoderm, i mother-cell of the entoderm). Fig. 14. Cleavage-stage with four segmentation-cells. Fig. 15. Beginning of the folding of the blastula. Fig. 16. Progress of the invagination. Fig. 17. Tufted gastrula (amphigastrula), Cf. Figs. 6b-75. Figs. 18-24. Discoid segmentation of a bony fish (labrus? cottus?). Most of the food-yelk ( n ) is left out (cf. Figs. 60-65). Fi»- lS- Cytula. Fig. 19. Cleavage-stage with two cells. Fig. 20. Cleavage-stage with thirty-two cells. Fig. 21. Mulberry-stage (morula). Fig. 22. Blastula. Fig. 23. The same in process of invagination (depula). Fig. 24. Discoid gastrula (disco- gastrula). Figs. 25-30. Superficial segmentation of a crab (peneus). Fig. 25. Cytula. Fig. 26. Cleavage-stage with eight cells (only four visible). Fig. 27. Cleavage stage with thirty-two cells. Fig. 28. Morula and blastula. Fig, 29, Spherical gastrula (perigastrula). Fig. 30. Passage of the gastrula into the nauplius embryo : the gullet-cavity has been formed in front of the primitive gut by folding from without. (Cf. the following Tables II.-III.) Gastrulation. PI R VI. A if). '•' $5& 0 &r 11. 1 0 p g 1 Won h roo Mammal Gastrulation. Pi. 18 <&¥ Fish SECOND TABLE SUMMARY OF THE CHIEF DIFFERENCES IX THE OVUM-SEGMENTATION AND GASTRU- LATION OF ANIMALS Tin' animal stems arc indicated by the letters a g: n Zoophyta. b Annelida. c Mollusca. d Echinoderma. e Articulata. f Tunicata. g Vertebrata. I. Total Segmentation. Holoblastic Gastrula without separate food-yelk. I lologastrula. I. Primitive segmentation. Archiblastic ova. Bell-gastrula (archigastrula). Plate II., Fijfs. 1-6. - II. Unequal Segmentation. Amphiblastic ova. Tufted-gastrula (amphigastrula i. Plate II.. Fies. 7 17. a. Many lower zoophyta (sponges, hydrapolyps, medusae, simpler corals). b. Many lower annelids (sagitta, phoronis, many nematoda, etc. , terebratula, argiope, pisidium). r. Some lower molluscs. (/. Many echinoderms. e. A few lower articulata (some branchiopods, copepods: Tar- digrades, pteromalina ). f. Many tunicata. g. The acrania (amphioxus). 11. Many zoophyta (sponges, medusae, corals, siphonophora, ctenophora). />. Most worms. r. Most molluscs. (/. Many echinoderms (viviparous species and some others). e. Some of the lower articulata (both Crustacea and tracheal.! |. f. Many tunicata. g. Cyclostoma, the oldest fishes, amphibia, mammals 1 not includ- ing man). II. Partial segmentation. Meroblastic ova. Gastrula with separate food-yelk. Merogastrula. III. Discoid Segmentation. Discoblastic ova. Discoid gastrula. Plate III., Figs, is 2+ IV. Superficial Segmentation. Periblast ic ova. Spherical-gas- trula. Plate III., Figs. 2S 30. Cephalopods or cuttle-fish. Many articulata, wood-lie scorpions, etc. Primitive lishes. bony lishe reptiles, birds, monot i ernes. Tlu- great majority of the arli- t usi iceans.iin riapods, arachnids, insi THIRD TABLE SUMMARY OF THE FIRST FOUR EMBRYONAL STAGES IN ANIMALS IN RELATION TO THE FOUR CHIEF FORMS OF SEGMENTATION A. Total Segmentation. a. Original or Primordial Segmentation. b. Unequal Segmentation. B. Partial Segmentation. d. Superficial Segmentation. Examples : Monoxenia, Sagitta, Amphioxus. la. Arehieytula, Archiblastic stem-cell (Plate II., Fit;-, i). A single cell, in which formative yelk and food-yelk are not separated. I la. Arehimorula (Plate II., Fig. 3). A solid, gene- rally globular, clus- ter of homogeneous cells. Ilia. Arehi- blastula (Plate II., Fig. 4). A hollow (gene- rail y g lobular) sphere, the wall consisting of a sin- gle layer of homo- geneous cells. IVa. Arehi- gastrula, Bell-gastrula (Plate II., Fig. 6). F'gs. 32-3S. P r i 111 i t i v e g u t empty, " wit hout food-yelk. Primary germinal layers of one stratum. Examples : Cyclostoma, Amphibia, Mammals. lb. Amphieytula, Amphiblastie stem-cell (Plate II., Figs. 7, iz). A uni-axial cell, containing- forma- tive yelk at the animal pole and food-yelk at the vegetal pole, not clearly separated. llb.Amphimorula (Plate II., Fig. 9). A roundish clus- ter of two kinds of cells, the smaller at the animal and the larger at the vegetal pole. nib. Amphi- blastula (Plate II., Fig. 10). A roundish sphere, the wall consisting of small cells at the animal and large cells at the vegetal pole. I\'b. Amphi- gastrula, Tufted-gastrula (Plate II., Figs, n, 17)- Fig- So- Primitive gut partly filled with divided food-yelk. Germinal layers of several strata. Examples : Fishes, Reptiles, Birds. Ic. Diseoeytula, Discoblastic stem-cell (Plate III., Fig. lS). A very large uni- axial cell, contain- ing formative yelk at the animal pole and food-yelk at the vegetal, the two clearly sepa- rated. lie. Diseomorula (Plate III., Fig. zi). A flat disc, con- sisting of homo- geneous cells at the animal pole of the food-velk. Examples : Crustacea, Arachnida, Insects. id. Perieytula, Periblast ic stem-cell (Plate III., Fig. 25). A large cell, con- taining formative yelk at the peri- phery, and food- velk in the centre. 1 id. Perimorula (Plate III., Fig. 27). A closed sphere ; one layer of cells encloses the whole of the central food- yelk, which con- tains dividing nu- clei. nic. Diseo- Hid. Peri- blastula blastula (Plate III., Fig. 22).(Plate III., Fig. jS). Aroundishsphere, A closed sphere ; the smaller liemi- one layer of cells sphere consisting encloses the whole of segmentation- of the central food- cells and the larger yelk ; all the nuclei of food-velk. have been driven to the surface. IVc. Diseo- gastrula, IVd. Peri- gastrula, Disc-gastrula Spherical-gastrula (Plate III., Fig. J4 ). (Plate III., Fig. 29). Figs. 62-65. Segmentation- Primitive gut cavity full of undi- filled with undi- vided food - yelk, vided food -yelk. Primitive gut su- Flat germinal disc, perficial. FOURTH TABLE SUMMARY OF THE CHIEF VARIATIONS l\ llll". RHYTHM OF THE OVUM-SEGMENTATION (Only the tir>t row [Sagitta] shows the original palingenetic rhythm of the segmentation in regular geometrical progression. All the other rows show dary, cenogenetic modifications, c— Stem-cell, s = Segmentation- cells, e Ectoderm-cells, i : Entoderm-cells.) I. ,,. III. IV. Y. VI. Arrow- Amphibian Mammal Snail Worm Worm worm (Sagitta) (Frog) (Hare) (Troehus) (Fabricia) (Cyglo- gena) 1. \< If If If If 2s 2 s 2 s (1 e 1 «) 2 s 2s (1 e 4 it) 2 s (IM If) 4rS 4s 4s 4 s 3 s 3 s (2,-+ 2f) (2 « if) (2*— 1 f) 8 s as 8s 8 s 5 s 4s (4« + 4») •4'' 4'") (4« + 4«) (4«+i 0 (3« . 1 12 x 12 s 12 s 6 s 5 s (8« 4.-' (8« 4'l (8e + 4«) i4.- 21) (4<^+ "') 16 s 16 s 16 s 20 s 10 s 6 s IS,- - S/l (8« 8») (8«4-2f) (S< if) 24 s 24 s 24 s lis 7 s (i6e + 8«) (16 e +8/) (i6 4- if) 32 s 32 v 32 s 40 s 19 s 8 s (i64 16 i) (32 e+ 8i) (i6« 3O (7.' if) 48 5 48 s 44 21s 9 s (32 '+ '6') (32 t6«) (3»+ '-'l 5«) (8c if) 64 s 64 s 64 s 76 s 37 s 10 s {32 e + 32 i) (32 t 32 f) (32 1 32 1) (64 e - 12 1) 1.5- <■ s«) ige + 1 / ) 96 s 96 s 84 s 38 s (64 e 32.) (64 ( + 20 «) 128^ 160 $ 148 s 70s (64 « 61) CHAPTER IX. THE GASTRULATION OF THE VERTEBRATE' Phylogenetic unity of the vertebrate-stem. Ontogenetic unity of its gastrula- tion. Historical relations of holoblastic and meroblastic vertebrates. Unequal segmentation of the ovum and amphigastrula of the amphibia (tailless frogs and tailed salamanders). Their segmentation-cavity (blas- tocoel) and primitive-gut cavity (Rusconic gastric cavity). Derivation of partial from total segmentation. Discoblastic vertebrates, with germinal disc (discoid gastrula). Deep-sea bony fishes with small and shark with large food-yelk. Epigastrula (or narrow-mouthed discoid gastrula) of the amniota. The hen's <^^g and its large food-yelk. Discoid gastrula- tion of the sauropsida (reptiles and birds) and monotrema. The primitive groove of the amniote-embryo is the primitive mouth of their discoid gastrula. Phylogenetic disappearance of the food-yelk in the mammal. Oviparous and viviparous mammals. Gastrulation of the opossum and the hare. Superficial segmentation of the articulata. The remarkable processes of gastrulation, ovum-segmenta- tion, and formation of germinal layers present a most con- spicuous variety. There is to-day only the lowest of the vertebrates, the amphioxus, that exhibits the original form of those processes, or the palingenetic gastrulation which we have considered in the preceding Chapter, and which culmi- nates in the formation of the archigastrula (Fig. 40). In all other extant vertebrates these fundamental processes have been more or less modified by adaptation to the conditions of embryonic development (especially by changes in the food- yelk) ; they exhibit various cenogenetic forms of the formation of germinal layers, and thus develop by means of a meta- gastrula. However, the different classes vary considerably from each other. In order to grasp the unity that underlies the manifold differences in these phenomena and their his- torical connection, it is necessary to bear in mind always the unity of the vertebrate-stem. This "phylogenetic unity," which I systematically developed in my Generelle Morphologie 1 Cf. Balfour's Manual of Comparative Embryology, vol. ii.; Theodore Morgan's The Development of the Frog's Egg. '74 THE GASTRULATION OF THE VERTEBRATE 175 in 1866, is now generally admitted. All impartial zoologists agree to-day that all the vertebrates, from the amphioxus and the fishes to the ape and man, descend from a common ancestor, "the primitive vertebrate." Hence the ontoge- netic processes, by which each individual vertebrate is developed, must also be capable of being reduced to one common type ol embryonic development ; and this primitive type is most certainly exhibited to-day by the amphioxus. It must, therefore, be our next task to make a comparative study of the various forms of vertebrate gastrulation, and trace them phylogenetically to that of the lancelet. Broadly speaking, they fall first into two groups : the older cyclos- toma, the earliest fishes, most of the amphibia, and the viviparous mammals, have holoblastic ova with total, unequal segmentation ; while the younger cyclostoma, most of the fishes, ccecilia, reptiles, birds, and monotrema, have mcro- blastic ova, with partial discoid segmentation. A closer study oi them shows, however, that these two groups do not present a natural unity, and that the historical relations between their several divisions are very complicated. In order to understand them properly, we must first consider the various modifications of gastrulation in these classes. We may begin with that of the amphibia. The most suitable and most available object of study in this class are the eggs of our indigenous amphibia, the tailless frogs and toads, and the tailed salamander. In spring they are to be found in clusters in every pond, and careful examination of the ova with a lens is sufficient to show at least the external features of the segmentation. In order to understand the whole process rightly and follow the forma- tion of the germinal layers and the gastrula, the ova of the frog and salamander must be carefully hardened ; then the thinnest possible sections must be made of the hardened ova with the microtome, and the tinted sections must be very closely compared under a powerful microscope. The ova of the frog or toad are globular in shape, about two millimetres in diameter, and are clustered in jelly- like masses, which are lumped together in the case of 176 THE GASTRULATION OF THE VERTEBRATE 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.1 In this way we get a definite axis of the ovum with two poles. To give a clear Fig. 42.— The Cleavage Of the frog's OVUm (magnified ten times), -i stem-cell. B the first two segmentation-cells. C four cells. D eight cells (4 animal and 4 vegetative). E twelve cells (8 animal and 4 vegetative). /"sixteen cells (8 animal and 8 vegetative). G twenty-tour cells (10 animal and 8 vegetative). H thirty-two cells. / forty-eight cells. A' sixty-tour cells. L ninety-six cells. .1/ 160 cells ( 128 animal and ,}2 vegetative). idea of the segmentation of this ovum, it is best to compare it with a globe on the surface of which are marked the various parallels of longitude and latitude. The superficial dividing 1 The colouring of the eggs of the amphibia is caused by the accumulation of dark-colouring matter at the animal pole of the ovum. In consequence of this, the animal cells of the ectoderm are darker than the vegetable cells of the entoderm. We find the reverse of this in the case of most animals, the proto- plasm of the entoderm cells being usually darker and coarser-grained. THE GASTRULATION OF THE VERTEBRATE lines between the different cells, which come from the repeated segmentation of the ovum, look like deep furrows on the surface, and hence the whole process has been given the name of furcation. In reality, however, this "segmentation," which was formerly regarded as a very mysterious process, is nothing but the familiar, repeated cell-segmentation. Hence also the segmentation-cells which result from it (the segmen- tella or blastomeres) 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 advances towards the lower and brighter pole (the south pole). Also the upper and darker hemisphere remains in this position throughout the course oi 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. 42 A) begins with the formation of a complete meridian furrow, which starts from the north pole and reaches to the south ( B ). An hour later a second meridian furrow arises in the same way, and this cuts the first at a right angle (Fig. 42 C ). The ovum is thus divided into four equal parts. Each of these four " segmentation-cells " has an upper and darker and a lower brighter half. A few hours later a third furrow appears, vertically to the first two 1 Fig. 42 D). This circular furrow is usually, but improperly, called the "equatorial furrow"; it lies to the north of the equator, and is more like the tropic of cancer. 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 meridian cleavage beginning from the north pole, so that we now have eight above and four below (Fig. 42 E). Later, the four new meridian divisions extend gradually to the lower cells, and the number rises from twelve to sixteen (F). Then a second circular furrow appears, parallel to the first, and nearer to the north pole, so that we may compare it to the north polar circle. In this way we get twenty-four segmentation-cells — 178 THE GASTRULATION OF THE VERTEBRATE sixteen upper, smaller, and darker ones, and eight smaller and brighter ones below ( G). Soon, however, the latter also sub-divide into sixteen, a third or "meridian of latitude " appearing, this time in the southern hemisphere: this makes thirty-two cells altogether ( ' H ). Then eight new meridian lines are formed at the north pole, and these proceed to divide, first the darker cells above and afterwards the lighter southern cells, and finally reach the south pole. In this way we get in succession forty, forty-eight, fifty-six, and at last sixty-four cells (I, K). In the meantime, the two hemi- spheres differ more and more from each other. Whereas the sluggish lower hemisphere long remains at thirty-two cells, the lively northern hemisphere briskly sub-divides twice, pro- ducing first sixty-four and then 128 cells ( L, M ). Thus we reach a stage in which we count on the surface of the ovum 128 small cells in the upper half and thirty-two large ones in the lower half, or 160 altogether. The dissimilarity of the two halves increases : while the northern breaks up into a great number of small cells, the southern consists of a much smaller number of larger cells. Finally, the dark cells of the upper half grow almost over the surface of the ovum, leaving only a small circular spot at the south pole, where the large and clear cells of the lower half are visible. This white region at the south pole corresponds, as we shall see after- wards, to the primitive mouth of the gastrula. The whole mass of the inner and larger and clearer cells (including the white polar region) belongs to the entoderm or ventral layer. The outer envelope of dark smaller cells forms the ectoderm or skin layer. The repeated segmentation which can thus easily be followed on the surface of the ovum is not confined to the surface, but extends to the whole interior. Thus, the cells divide in planes which correspond pretty closely to concen- tric planes of the spherical body : more quickly in the upper and more slowly in the lower half. In the meantime, a large cavity, full of fluid, has been formed within the globular body — the segmentation-cavity or embryonic-cavity (blastocosl, Figs. 43-46 F, and also J in the transverse sections on THE GASTRULATION OF THE VERTEBRATE ■79 Plate II., Figs. 8-1 1). The first trace of this cavity is found in the middle of the upper hemisphere, where the first three successive planes of cleavage cut each other (Plate II., Fig. 8 s ). It extends considerably by progressive cleavage, and afterwards assumes an almost semi-circular form 1 P fl Fig. 44- Fig. 45. Fig. 4''. Figs. 43-46.— Four vertical sections of the fertilised ovum of the toad, in four successive stages 01 development. The Letters have the same - throughout: — F segmentation-cavity. D covering of same ( D dorsal half of the embryo, P ventral half). P yelk-stopper (white round field at the lower poll-). Z yelk-cells of the entoderm (Remak's " glandular embryo"). X primitive gul cavity (progaster or Rusconian alimentary cavity). The primitive mouth (prostoma) is closed by the yelk-stopper, P. s partition between the primitive gut cavity ( X ). and the segmentation cavity ( F ). k k' il the large circular lip-border of the primitive mouth (the Rusconian anus). The line of dots between k and U indicates the earlier connection of the yelk-stopper (P) with the central mass of tin- yelk-cells (Z). In V\^. 4<> tho ovum has turned 90°, so that the back of the embryo is uppermost and the ventral side down. ( From Strieker. ) (Fig. 4.; F; Plate II., Figs. 9 s, \os). The vaulted roof of this hemispherical segmentation-cavity is formed by the smaller and dark-coloured cells of the ectoderm (Fig. 43 D ); Oil the other hand, its level floor is composed of the larger and lighter cells of the entoderm (Fig. 43 z). The globular THE GASTRULATION OF THE VERTEBRATE frog-embryo now represents a modified germinal vesicle or blastula, with hollow animal half and solid vegetal half. Now a second, narrower but longer, cavity arises by bending from the lower pole, and by the falling away from each other of the white entoderm-cells (Figs. 43-46 N). This is the primitive gut-cavity or the gastric cavity of the gastrula, progaster or archenteron. It was first observed in the ovum of the amphibia by Rusconi, and so called the Rusconian alimentary cavity. In vertical section (Fig. 44) it seems to be bent in the form of a sickle, and reaches almost from the south pole to the north, forcing upwards a part of the gut- cells (between the segmentation-cavity F and the dorsal covering D). The reason of the peculiar narrowness of the primitive gut-cavity here is that it is, for the most part, full of yelk-cells of the entoderm. These also stop up the whole of the wide opening of the primitive mouth, and form what is known as the "yelk-stopper," which is seen freely at the white round spot at the south pole ( P). Around it the ectoderm is much thicker, and forms the border of the primitive mouth (the properistoma ), the most important part of the embryo (Fig. 46 k, k). Soon the primitive gut-cavity stretches further and further at the expense of the segmenta- tion-cavity ( F), until at last the latter disappears altogether. The two cavities are only separated by a thin partition (Fig. 45 s). The part of the embryo under which the primitive gut-cavity developes is the later dorsal-surface ( D ). The segmentation-cavity lies to the front and the yelk-stopper at the hinder part of the body; the thick hemispherical mass of the yelk-cells forms the ventral wall of the primitive gut. With the formation of the primitive gut our frog-embryo has reached the gastrula stage (Plate II., Fig. 11). But it is clear that this cenogenetic amphibian gastrula is very different from the real palingenetic gastrula we have considered (Figs. 32-38). In the latter, the be\\-ga.str\i\a.(archigastruhi )y the body has only one axis. The primitive gut-cavity is empty and its mouth wide open. Both the ectoderm and the entoderm consist of a single layer of cells. They lie close together, the segmentation-cavity having wholly THE GASTRVLATION OF THE VERTEBRATE disappeared in the process of folding. It is quite otherwise with the tufted gastrula (amphigastrula) of our amphibia (Figs. 4,1-46 ; Plate II., Fig. 1 1). In this case the segmenta- tion-cavity ( F) remains for a long time beside the primitive gut-cavitv ( X ). The latter is, for the most part, filled with yelk-cells, and the primitive mouth almost stopped up with them (yelk-stopper, P). Both entoderm and ectoderm consist o( several layers of cells. Finally the typical form of the whole gastrula is no longer uni-axial, but tri-axial ; owing to the eccentric development of the primitive gut-cavity, the three straight axes are determined which characterise the bilateral body of the higher animals. In the growth of this tufted gastrula we cannot sharply mark off the various stages which we distinguish successively in the bell- gastrula as mulberry-form and vesicular embryo. The morula-stage (Plate II., Fig. 9) is no more clearly distinct from that of the blastula (Fig. 10) than this is from the gastrula (Fig. 11). Nevertheless, it is not difficult to reduce the Fig. 47. Embryonic vesicle of the water-salamander ( triton). fh seg- mentation-cavity, cte yelk-cells, ra border- zone. ( From Hertwig. | whole cenogenetic or disturbed development of this amphi- gastrula to the true palingenetic formation of the archi- gastrula of 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 particularly j^ood subject for observation. Its nutritive yelk is much smaller and its 182 THE GASTRULATION OF THE VERTEBRATE formative yelk less troubled with black pigment-cells than in the case of the frog ; and its gastrulation has better retained the original palingenetic character. • It was first described by- Scott and Osborn (1879), and Oscar Hertwig especially made a careful study of it (1881), and rightly pointed out its great importance in helping us to understand the vertebrate development. The globular embryonic vesicle of iriton (Fig. 47) consists of loosely-aggregated, yelk-filled entodermic cells or yelk- cells (dz) in the lower vegetal half; the upper, animal half encloses the hemispherical segmentation-cavity (fh), the curved roof of which is formed of two or three strata of small ectodermic cells. At the point where the latter pass into the former (at the equator of the globular vesicle) we have the border zone (rz). The folding which leads to the formation of the gastrula takes place at a spot in this border zone. This invagination- opening, the primitive mouth (Fig. 48 it), is a horizontal transverse fold „ _ . with a dorsal upper lip and ventral Fig. 48.— Embryonic . rr r vesicle of triton (bias- under lip. While the primitive gut tula J, outer view, with ,„. , , . , , . . the transverse fold of (Fig. 49 ud) is being bent in, a part the primitive mouth ( „ ). f th secrmentation-cavity (fh) remains \Yrom Hertivtg.) & j \j s at first. But it grows smaller (Fig. 49), and finally disappears. In the complete gastrula (Fig. 50) the external germinal layer ( ak ) consists of a single laver of high cylindrical cells. The internal germinal layer ( ' ik ) is, in the upper and dorsal half, also composed of a single stratum of cells ; these form the covering of the primitive gut- cavity. But the floor of the latter, or the lower and ventral half, consists of several layers of large yelk-cells (dz). This part of the entoderm, which is also known as the yelk- embryo ( lecithoblastus ), is much smaller in the water- salamander than in the frog. Here, again, a projection of it reaches into the primitive mouth as "yelk-stopper" (Fig. 50 p). At the thick borders of the latter begins the formation of the middle germinal layer (ink ). THE GASTRULATION OF THE VERTEBRATE '83 Fig. 4" Sagittal section of a hooded- embryo (depula) of triton 1 \ esicular em- bryo at the commencement of gastrulation 1. til- outer germinal layer, ik inner terminal layer, fh segmentation-cavity, ud primi- tive gut. 11 primitive mouth, dl and vl dorsal and ventral lips of the mouth, dz yelk-cells. 1 From Hertmig. ) The unequal segmen- tation takes place in some of the cyclostoma and in the oldest fishes in just the same way as in most of the amphibia. Among the cyclostoma (" round- mouthed ") the familiar lamprey s(petromyzontes ) are particularly interest- ing. In respect of organi- sation and development they are half-way between the acrania and the lowest real fishes ( scla- c/ui); hence I divided the group of the cyclo- stoma in 1S66 from the real fishes with which they were formerly associated, and formed of them a special class of vertebrates. The ovum-segmentation in our common river-lampreys (petromyzon fiwviatUis) was described by Max Schultze in 1856, and afterwards by Scott (1882) and Goette (1890). Unequal total segmentation follows the same lines in the oldest fishes, the selachii and ganoids, which are directly connected phy- logenetically with the cyclostoma. The primi- tive fishes (selachii), which we must regard as the ancestral group o( the true fishes, were generally considered until a short time ago to be discoblastic. It was not until the begin- ning of the twentieth l'n,. 50. Sagittal section of the gas- trula of the water-salamander (triton). (From Hertmig.) Letters as in Fig. 40; except — p yelk-stopper, mk beginning of the middle germinal layer. 1 84 THE GASTRULATIO.Y OF THE VERTEBRATE century that Bashford Dean made the important discovery in Japan that one of the oldest living fishes of the shark type (cestracion japonicus ) has the same total unequal Fig-. 51.— Ovum-segmentation in the lamprey (petromyeonfluviatilis) in four successive stages. The small cells of the upper (animal) hemisphere divide much more quickly than the cells of the lower (vegetal) hemisphere. segmentation as the amphiblastic plated fishes (ganoides).z This is particularly interesting in connection with our subject, because the few remaining survivors of this Fig. 52. — Gastrulation of the lamprey (petromyson fiuviatilis). A blastula, with wide embryonic cavity (blastocoel, bl), g incipient invagination. B depula, with advanced invagination, from the primitive mouth ( g). C gas- trula. with complete primitive gut : the embryonic cavity has almost disappeared in consequence of invagination. ' Bashford Dean, Holoblastic Cleavage in the Egg of a Shark, cestracion japonicus Macleay. Annotationes soologicae japonenses, vol. iv., Tokio, 1901. THE GASTRULATION OF THE VERTEBRATE 185 division, which was so numerous in paleozoic times, exhibit three different types oi' gastrulation. The oldest and most conservative forms of the modern ganoids are the scaley sturgeons (sfurtones), plated fishes of great phyletic impor- tance, ilie eggs of which are eaten as caviare ; their cleavage is not essentially different from that of the petromyzontes and amphibia. On the other hand, the most modern of the plated fishes, the beautifully scaled bony pike of the North American rivers ( lepidosteus ), approaches the osseous fishes, and is discoblastic like them. A third genus (amia) is midway between the sturgeons and the latter. The group of the lung-fishes (dipneusta or dipnoi ) is closely connected with the older ganoids. In respect of their whole organisation they are midway between the gill- breathing fishes and the lung-breathing amphibia ; they share with the former the shape of the body and limbs, and with the latter the form of the heart and lungs. Of the older dipnoi (paladipneusta) we have now only one specimen, the remarkable ceratodus of East Australia ; its amphiblastic gastrulation has been recently explained by Richard Semon (cf. Chapter XXI). That of the two modern dipneusta, of which protopterus is found in Africa and lepidosiren in America, is not materially different. (Cf. Fig. 53.) All these amphiblastic vertebrates, petromyzon and cestracion, accipenser and ceratodus, and also the salamanders and batrachia, belong to the old, conservative groups of our stem. Their unequal ovum-segmentation and gastrulation have many peculiarities in detail, but can always be reduced with comparative ease to the original cleavage and gastrula- tion of the lowest vertebrate, the amphioxus ; and this is little removed, as we have seen, from the very simple archigastrula of the sagitta and monoxenia (see p. 152, Figs. 31-38). All these and many other classes of animals generally agree in the circumstance that in segmentation their ovum divides into a large number of cells by repeated cleavage. All such ova have been called, after Remak, "whole-cleaving" f/10/0- blasta), because their division into cells is complete or total (Plate II.). i86 THE GASTRULATION OF THE VERTEBRATE In a great many other classes of animals this is not the case, as we find (in the vertebrate stem) among the birds, reptiles, and most of the fishes ; among the insects and most of the spiders and crabs (of the articulates) ; and the cephalo- pods (of the molluscs). In all these animals the mature ovum, and the stem-cell that arises from it in fertilisation, fh gh Fig. 53.— Gastrulation of ceratodus (from Semon). A and C stage with four cells, B and D with sixteen cells. A and B are seen from above, C and D sideways. £ stage with thirty- two cells ; .Fblastula; G gastrula in longitudinal section, fh segmentation cavity, gh primitive gut or gastric cavity. consists of two different and separate parts, which we have called formative yelk and nutritive yelk. The formative yelk (vitclhis formativus or morpholecithus ) alone consists of living protoplasm, and is the active, evolutionary, and nucleated part of the ovum ; this alone divides in segmentation, THE GASTRULATION OF THE VERTEBRATE 187 and produces the numerous cells which make up the embryo. On the other hand, the nutritive yelk (vitellus nutritivus or tropholecithus ) is merely a passive part of the contents of the ovum, a subordinate element which contains nutritive material or deutoplasm (albumin, fat, etc.). and SO represents in a sense the provision-store of the developing embryo. The latter takes a quantity of food out oi this store, and finally consumes it all. Hence the nutritive yelk is of great indirect importance in embryonic development, though it has no direct share in it. It either does not divide at all, or only later on, and does not generally consist of cells. It is some- times large and sometimes small, but generally many times larger than the formative yelk ; and hence it is that it was formerly thought the more important of the two. As the respective significance of these two parts of the ovum is often wrongly described, it must be borne in mind that the nutritive yelk is only a secondary addition to the primary cell ; it is an inner enclosure, not an external appendage. All ova that have this independent nutritive yelk are called, after Remak, "partially-cleaving" ( meroblasta). Their segmentation is incomplete or partial (Plate III.). There are many difficulties in the way of understanding this partial segmentation and the gastrula that arises from it. We have only recently succeeded, by means of comparative research, in overcoming these difficulties, and reducing this cenogenetic form of gastrulation to the original palingenetic type. This is comparatively easy in the small meroblastic ova which contain little nutritive yelk — for instance, in the pelagic ova of a bony fish, the development of which I observed in i.S;5at Ajaccio in Corsica (Plate III., Figs. 1.S-24). I found them joined together in lumps of jelly, floating on the surface of the sea ; and as the little ovula were com- pletely transparent, I could easily follow the development of the germ step by step. These ovula are glossy and colourless globules of little more than half a millimetre in diameter (0.64-0.66 mm). Inside a structureless, thin, but firm mem- brane ((/oolemma. Fig. 54 c ) we find a large, quite clear, and transparent globule of albumin (d). At both poles o\ its THE GASTRULATION OF THE VERTEBRATE axis this globule has a pit-like depression. In the pit at the upper, animal pole (which is turned downwards in the floating ovum) there is a bi-convex lens composed of protoplasm, and this encloses the nucleus (k); this is the formative yelk of the stem-cell, or the germinal disk (b). From the neighbour- hood of this lense-shaped nutritive yelk a very thin proto- plasmic skin spreads around, and this protects the nutritive yelk, the " border-layer." At the opposite or vegetal pole of the ovum, in the lower pit, there is a clear simple globule of fat (f). The small fat-globule and the large albumin-globule together form the nutritive yelk. Only the formative yelk under- goes cleavage, the nutritive yelk not dividing at all at first. The segmentation of the lens- shaped formative yelk (b) pro- ceeds quite independently of the nutritive yelk, and in perfect geometrical order (cf. Plate III., Figs. 18-24; or|ly tne formative yelk with the nearest part of the nutritive yelk (n) is given in section [through a meridian plane] in this illustration, the greater part of the latter and the The stem-cell (Fig. 18) first divides into two equal segmentation-cells (Fig. 19). From these we get by repeated sub-division first four, then eight, then sixteen cells (Fig. 20). By continued cleavage we then get thirty-two cells, sixty-four, and so on. All these segmentation-cells are at first of the same size and character. Closely joined together, they form a lens-shaped mass (Plate III., Fig. 21), something like the globular mulberry - embryo of the primordial cleavage (morula, Plate II., Fig. 5). But afterwards the border cells of the lens separate from the rest, and travel into the yelk and the border-layer; they form the " embryonic border" (periblast, Fig- 55 C, p). From this lens-shaped mulberry-form there Fig. 54.— Ovum of a pelagic bony fish, b protoplasm of the stem-cell. /■ nucleus of same. d clear globule of albumin, the nutritive yelk, f fat-g-lobule of same, c outer membrane of the ovum, or ovolemma. ovolemma being left out). THE GASTRULATION OF THE VERTEBRATE |S) then developes a vesicular embryo (blastula), the cells of the periblast making their way centripetally underneath the lens (Plate III., Fig. 22). The regular bi-convex lens is converted into a disk like a watch-glass with thick borders. This convex cell-disk lies on the upper and less curved polar surface of the nutritive yelk like the watch-glass 011 the watch. As fluid gathers in the space between the blastoderm and the periblast, a round low cavity is formed (Fig. 22 s). This is the segmentation-cavity, corresponding to the central segmentation-cavity of the palingenetic blastula (Plate II., Fig. 4). The slightly curved floor of the lower segmentation cavity is formed by the periblast and nutritive yelk (11 J, and Fig. 5.v— Ovum-segmentation of a bony fish. (Cf. Plate III., Figs. 1S-24.) .1 first cleavage of the stem-cell (cytula). J! division of same into tour segmentation-cells (only two visible). C the germinal disk divides into the blastoderm (b) and the periblast CpJ- bor- der-swelling or primitive mouth. n albuminous globule of the nutri- tive velk. f fat-globule of same. c external membrane (ovolemma). d partition between entoderm and ectoderm I earlier the segmenta- tion cavity). ovum), we find that it is THE GASTRULATION OF THE VERTEBRATE 191 the primitive-mouth edge (border-swelling or properistoma), the entoderm and ectoderm pass into each other without definite limit (Fig. 5010). Ot late years this discoid gastrulation of the bony fishes has been very carefully described by Kupffer, Van Bambeke, Whitman, Wilson, Kopsch, H. E. Ziegler, and others. In most of the teleostei it is more complicated and changed cenogenetically, because the food-yelk is very large and forms an extensive globular body, an emulsion of albumin and fat- particles. During the growth of the lens-shaped germinal disk a part of the nucleus at the border of it travels into the yelk, and forms what is called a periblast, which surrounds the blastoderm like a ring. The incompletely divided yelk- cells of the periblast that are thus formed are also called '• yclk-syncytium "; they are used upas food by the embryo with the rest of the yelk, and have no part in the building-up of the body. The same applies to the covering-layer, a .simple thin stratum of flat epithelial cells, which, in many fishes, forms the uppermost layer of the blastoderm, and at its border connects with the contiguous part of the periblast, the germinal wall.1 Very similar to the discoid gastrulation of the osseous fishes is that of the myxinoida, the remarkable cyclostoma that live parasitically in the body-cavity of fishes, and are distinguished by several notable peculiarities from their nearest relatives, the lampreys (petromyzon). While the amphiblastic ova of the latter are small and develop like those of the amphibia, the cucumber-shaped ova of the myxinoida are several centimetres long, and form a discoid gastrula. Up to the present it has only been observed in one species (bdellostoma Stouti), by Dean and Doflein (1898). It is clear that the important features which distinguish the discoid gastrula from the other chief forms we have con- sidered are determined by the large food-yelk. This takes no direct part in the building o( the germinal layers, and 1 Cf. Kingsley and Conn, Embryology of the Teleosts (1883); A. Agassiz anil C. O. Whitman, The Development 0/ Osseous Fishes (1885); M'Intosh, Development and Life-histories of Fishes ( 1890). 192 THE GASTRVLATIOX OF THE VERTEBRATE completely fills the primitive gut-cavity of the gastrula, even protruding at the mouth-opening. If we imagine the original bell-gastrula (Figs. 32-38) trying to swallow a ball of food which is much bigger than itself, it would spread out round it in discoid shape in the attempt, just as we find to be the case here (Fig. 56). Hence we may derive the discoid gas- trula from the original bell-gastrula, through the inter- mediate stage of the tufted gastrula. It has arisen phylo- genetically by the accumulation of a store of food-stuff at the vegetal pole, a "nutritive yelk" being thus formed in contrast to the "formative yelk." Nevertheless, the gastrula is formed here, as in the previous cases, by the folding or invagination of the blastula. We can, therefore, reduce this cenogenetic form of the discoid segmentation ( gastrulatio discoidalis J to the palingenetic form of the primitive cleavage. This reduction is tolerably easy and confident in the case of the small ovum of our pelagic bony fish, but it becomes difficult and uncertain in the case of the large ova that we find in the majority of the other fishes and in all the reptiles and birds. In these cases the food-yelk is, in the first place, comparatively colossal, the formative yelk being almost invisible beside it ; and, in the second place, the food-yelk contains a quantity of different elements, which are known as "yelk-granules, yelk-globules, yelk-plates, yelk-flakes, yelk- vesicles," and so on. Frequently these definite elements in the yelk have been described as real cells, and it has been wrongly stated that a portion of the embryonic body is built up from these cells.1 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 blastoderm-border, and form a periblast — the nutritive yelk remains a dead accumulation of food, which is taken into the gut during embryonic development and consumed by the embryo. The latter developes solely from the living formative yelk of the ' The coll-like matter fruit we find in the undivided food-yelk of birds, reptiles, and fishes is anything but true cells, as His and others affirm. The true cells which we find in the food-yelk of these meroblastic ova after cleavage are migrated segmentation-cells (merocyta, Fig. 447.) THE GASTRULATION OF TIIF. VERTEBRATE 193 sti.-ni-ji.-ll. This is equally true o( the ova of our small bony fishes and of the colossal ova of the primitive fishes, reptiles, and birds. The gastrulation of the primitive fishes or selachii (sharks and rays) has been carefully studied of late years by Riickert, Rabl, and H. E. Ziegler in particular, and is very important in the sense that this group is the oldest among living fishes, and their gastrulation can be derived directly from that of the cyclostoma by the accumulation of a large quantity of food- yelk. The oldest sharks (cestracion) still have the unequal segmentation inherited from the cyclostoma. But while in this case, as in the case of the amphibia, the small ovum Fig. 57.— Longitudinal section through the blastula of a shark (pristiuris). (From Riickert.) (Looked at from the left ; to the right is the hinder end, //. to the lefl the fore end, V.) />' segmentation-cavity, he cells of the germinal membrane, dk yelk-nuclei. completely divides into cells in segmentation, this is no longer so in the great majority of the selachii (or elasmo- branchii). In these the contractility of the active protoplasm no longer suffices to break up the huge mass of the passive deutoplasm completely into cells ; this is only possible in the upper or dorsal part, but not in the lower or ventral section. Hence we find in the primitive fishes a blastula with a small eccentric segmentation-cavity (Fig. 57 b ), the wall of which varies greatly in composition. Only the roof (or upper wall) of it consists of real blastodermic cells, and forms the germinal disk (kz); the floor or lower wall is formed of undivided yelk-stuff, in which the presence of "elementary organisms" is only indicated by scattered yelk-granules (dk). THE GASTRULATION OF THE VERTEBRATE The circular border of the germinal disk or the thin "transition zone," which connects the roof and floor of the segmentation-cavity, corresponds to the border-zone at the equator of the amphibian ovum. In the middle of its hinder border we have the beginning of the invagination of the primitive gut (Fig. 58 ud); it extends gradually from this spot (which corresponds to the Rusconian anus of the amphibia) forward and around, so that the primitive mouth becomes first crescent-shaped and then circular, and, as it opens wider, surrounds the ball of the larger food-yelk ( disco- gastrula eurvstoma ). Not only the obviously divided cylindrical cells of the roof (the blastocytes), but also the contiguous parts of the yelk that contain the yelk-nuclei ( dk j v dk Fig. 58.— Longitudinal section of the blastula of a shark (pristiurus) at the beginning' of gastrulation. (From Ruder/. ) (Seen from the left. ) Kfore end, H hind end, B segmentation-cavity or blastocoel, ud first trace of the primitive gut, dk yelk-nuclei, fd fine-grained yelk, gd coarse-grained yolk. or the nuclei of the still undivided merocytes, take part in the invagination. As these gradually divide and become inde- pendent round entodermic cells, they form the ventral wall of the primitive gut ; its dorsal wall is made up of the cylindrical cells which are formed, in a continuous simple layer, at the inner side of the roof of the segmentation-cavity during the advancing invagination. The cavity is thus pressed in on this side also, and displaced by the cavity of the primitive gut (ud). But only the back wall of this wide- mouthed discoid gastrula continues for some time to consist of two distinct strata of cells (the primary germinal layers), its ventral wall being composed of the yelk-stuff. As .this gradually disappears, the wide primitive mouth becomes I HE C. I S TS ULA TION OF THE l '/•;/,• TF.IiR. I TE 195 smaller. In this discoid gastrula the ventral lip of the primitive mouth is in front, the dorsal lip behind. Essentially different from this wide-mouthed discogastrula of most of the selachii is the epigastrula (of Rabl), the narrow -mouthed discoid gastrula of the amniotes, the reptiles, birds, and monotremes ; between the two — as a phylogenetic intermediate stage — we have the holoblastic amphigastrula of the amphibia. The latter has developed from the amphi- gastrula of the ganoids and dipneusts, whereas the discoid amniote gastrula has, in turn, evolved from the amphibian gastrula by the addition of food-yelk. This phylogenetic change of gastrulation is still found in the remarkable ophidia ( gymnophionayc(Bcilia, orperomelaj, serpent-like amphibia that live in moist soil in the tropics, and in many respects repre- sent the transition from the gill-breathing amphibia to the lung-breathing reptiles. Their embryonic development has been explained by the fine studies of the brothers Sarasin of ichthyophis glutmosa at Ceylon (1887), and those of August Brauer of the hypogeopliis rostrata in the Seychelles (1897). It is only by the historical and comparative study of these that we can understand the difficult and obscure gastrulation of the amniotes. The bird's egg is particularly important for our purpose, because most of the chief studies of the development of the vertebrates are based on observations of the hen's egg during hatching. The mammal ovum is much more difficult to obtain and study, and for this practical and obvious reason very rarely thoroughly investigated. But we can get hens' eggs in any quantity at any time, and, by means of artificial incubation, follow the development of the embryo step by step. The bird's egg differs considerably from the tiny mammal ovum in size, a large quantity of food-yelk accumu- ating within the original yelk or the protoplasm of the ovum. This is the yellow ball which we commonly call the yelk 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 ig6 THE GASTRULATIOX OF THE VERTEBRATE the original ovum is a quite small, naked, and simple cell with a nucleus, not differing in either size or shape from the original ovum of themammalsand otheranimals (cf. Fig. 13-fi). As in the case of all the craniota, the original or primitive ovum ( protovum ) is covered with a continuous layer of small cells, like an epithelium. This epithelial membrane is the follicle, from which the ovum afterwards issues. Immediately underneath it the structureless yelk-membrane is secreted from the yelk. The small primitive ovum of the bird begins very early to take up into itself a quantity of food-stuff through the yelk-membrane, and work it up into the "yellow yelk." In this way the ovum enters on its second stage (the metovum j, which is many times larger than the first, but still only a single enlarged cell. Through the accumulation of the store of yellow yelk within the ball of proto- plasm the nucleus it contains (the germinal vesicle) is forced to the surface of the ball. Here it is sur- rounded by a small quantity of proto- plasm, and with this forms the lens- shaped formative yelk (Fig. 59 b). This is seen on the yellow yelk- ball, at a certain point of the surface, as a small round white spot — the " scar " f cicatriculaj. From this scar a thread-like column of white nutritive yelk (dj, which contains no yellow yelk-granules, and is softer than the yellow food-yelk, proceeds radially to the middle of the yellow yelk-ball, and forms there a small central globule of white yelk (Fig. 59 d). The whole of this white yelk is not sharply separated from the yellow yelk, which shows a slight trace of concentric layers in the hard-boiled egg (Fig. 59 c). We also find in the hen's egg, when we break the shell and take out the yelk, a round small white disk at its Fig. 59.— A ripe ovum from the ovary of a hen (in section). The yellow food-yelk is composed of concentric layers (c), and surrounded by a thin yelk- membranef'rt^. The nucleus or germinal vesicle forms, with the protoplasm of the ovum, the formative yelk (b) or the "scar." From this the white yelk (here dark) goes into the yelk- cavity (d ). But the two kinds of yelk are not sharply distinct. THE GASTRULATION OF THE VERTEBRATE 197 surface which corresponds to the scar. 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, Fig. "i. Diagram ol discoid segmentation in the bird's ovum (magnified about ton times). Only the formative yolk (the scar) is shown in these -i\ figures ( A-F ), because cleavage only takes place in this. The much larger food-yelk, which does not share in the cleavage, is left out and merely indicated by the dark ring- without. .1 By the first division the ovinia splits into two cells. B These two first segmentation-cells divide by a second cleavage (vertical to the firsii into four cells. C From these tour cells sixteen arc formed, two other radial divisions taking place between the first two transverse divisions, and the inner ends of these eight-rayed segments being cut off by a central ring-cleavage. /> A stage with sixteen peripheral and some four concentric radial clefts. E A stage with sixty-four peripheral and six circular clefts. F By continuous repetition of radial and circular divisions tin' whole soar breaks into a heap of small cells, and now forms the lens-shaped mulberry-type (morula). The division of the nuclei .always precedes the formation o\' clefts. glarous mass of albumin that surrounds the yellow yelk of the bird's egg, and also the hard calcareous 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 {gastrula discoidalis, Fig. 60). First two equal segmentation-cells (A ) are formed 198 THE GASTRULATION OF THE VERTEBRATE from the cytula. These divide into four ( B J, then into eight, sixteen (C), thirty-two, sixty-four, and so on. The cleavage of the cells is always preceded by a division of their nuclei. The cleavage surfaces between the segmentation-cells appear at the free surface of the scar as clefts. The first two divisions are vertical to each other, in the form of a cross ( B J. Then there are two more divisions, which cut the former at an angle of forty-five degrees. The scar, which thus becomes the germinal disk, now has the appearance of an eight-rayed star. A circular cleavage next taking place round the middle, the eight triangular cells divide into sixteen, of which eight are in the middle and eight distributed around ( C). Afterwards circular clefts and radial clefts, directed towards the centre, alternate more or less irregularly ( D, E). In most of the amniotes the formation of concentric and radial clefts is irregular from the very first ; and so also in the hen's egg. But the final outcome of the cleavage-process is once more the formation of a large number of small cells of a similar nature. As in the case of the fish-ovum, these segmentation- cells form a round, lens-shaped disk, which corresponds to the mulberry-embryo, and is embedded in a small depression of the white yelk. Between the lens-shaped disk of the morula-cells and the underlying white yelk a small cavity is now formed by the accumulation of fluid, as in the fishes. Thus we get the peculiar and not easily recognisable blastula of the bird (Fig. 61). The small segmentation-cavity (fh) of this notably cenogenetic blastula is very fiat and much com- pressed. The upper or dorsal wall (dw) is formed of a single layer of clear, distinctly separated epithelial cells ; this corre- sponds to the upper or animal hemisphere of the triton- blastula (Fig. 47). The lower or ventral wall of the flat dividing space (vw) is made up of larger and darker segmentation-cells, which are in part not yet separated, and pass directly into the substance of the underlying white yelk (wd); it corresponds to the lower or vegetal hemisphere of the blastula of the water-salamander (Fig. 47 da J. The nuclei of the yelk-cells, which are in this case especially numerous at the edge of the lens-shaped blastula, travel THE GASTRULATION OF THE VERTEBRATE (as merocytes) into the white yelk, increase by cleavage, and contribute oven to the further growth of the germinal disk by furnishing it with food-stuff. The invagination or the typical folding of the bird- blastula takes place in this case also at the hinder (aboral) pole of the subsequent chief axis, in the middle of the hind Fig. '.i. hi vl i. dk Fig. 63. Fu;. 62. Fig. 61.— Vertical section of the blastula of a hen (discoblastula). fh segmentation-cavity, ' in the first hour of incubation. (From Koller.) ks germinal disk. V it-, lore and // its hind border; cs embryonic shield ; s sickle-groove ; sk sickle knob ; d yelk. Fig. 63. Longitudinal section of the germinal disk of a siskin (discogastrula). (From DuvaL) ud primitive gut, vl, M fore and hind lips oi the primitive mouth (or sickle-edge) ; ok outer germinal layer, ik inner germinal layer, ilk yelk-nuclei, wd white yelk. border of the round germinal disk (Fig. 62 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 oi' the disk is less clear but whiteF behind, and is more sharply separated from contiguous parts. In the middle of its hind border there is a white, crescent-shaped groove — Roller's 200 THE GASTRULATIOX OF THE VERTEBRATE sickle-groove (Fig. 62 s) ; a small projecting process in the centre of it is called the sickle-knob ( sk). This important cleft is the primitive mouth, which was described for a long time as the "primitive groove." If we make a vertical section through this part (in the middle or sagittal plane), we see that a flat and broad cleft stretches under the germinal disk forwards from the primitive mouth ; this is the primitive gut (Fig. 63 ltd). Its roof or dorsal wall is formed by the folded upper part of the blastula, the segmentation-cavity of which is now only visible as an insignificant channel, bordered above by the simple cell-layer of the outer germinal layer (ak), and below by the inner germinal layer with its several strata (ik). The floor or the ventral wall of the flat primitive gut is formed by the white yelk fwd), in which a Fig. 64.— Longitudinal section of the diseoid gastrula of the nightingale. (From Duval.) ml primitive gut, -■/, hi fore and hind lips of the primitive mouth ; . To this category belong all true vermalia (excepting the platodes), and also the higher typical animal stems that have been evolved from them — molluscs, echinoderms, articulates, tunicates, and vertebrates. The body-cavity (caeloma) is therefore a new acquisition oi the animal body, much younger phylogenetically than the alimentary system, and of great importance both morphologi- cally and physiologically. I first pointed out this funda- mental significance of the ccelom in my monograph on the .sponges (1872), in the section which draws a distinction THE CCELOM THEORY between the body-cavity and the gut-cavity, and which follows immediately on the germ-layer theory and the ancestral tree of the animal kingdom (the first sketch of the gastraja theory). Up to that time these two principal cavities of the animal body had been confused, or very imperfectly distinguished ; chiefly because Leuckart, the founder of the ccelenterata group (1848), 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 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 only at a much later stage that the body-cavity, which is entirely wanting in the ccelenterata, is developed in some of the metazoa between the ventral and the bodv wall. The two cavities are entirely different in content and purport. The alimentary cavity (enteron) serves the purpose of diges- tion ; it contains water and food taken from without, as 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 itself and its glandular appendages, and also contains the sexual products and a certain amount of blood or lymph, a fluid that is transuded through the ventral wall. As soon as the body-cavity appears, the ventral wall is found to be separated from the enclosing body-wall, and the two continue to be directly connected at various points. We can also then always distinguish a number of different lavers of tissue in both walls — at least two in each. These tissue- layers are formed originally from four different simple cell- layers, which are the much-discussed four secondary germinal layers. The outermost of these, the skin-sense-layer (Figs. 77, 78 /is), and the innermost, the gut-gland-layer (del), remain at first simple epithelia or covering-layers. The one limits the outer surface of the body, the other the inner surface of the ventral wall ; hence thev are the ccelom theory called limiting-layers, or methoria. Between them arc the two middle layers, or mesoblasts, which enclose the body- cavity. The four secondary germinal layers are so distributed in the structure of the body in all the coslomaria (or all metazoa that ha\c 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 epithelia, 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 lu . . ,<■ Fig. So. Fig. 7.). Ccelomula of sagitta (ic.-isirula with a couple of coelom-pouches). (From kowalevsky.) bUp primitive mouth, al primitive gut, frv ccelom-folds, m permanent mouth. '. -Ccelomulaof sagitta. in soil ion. CFroxaHertiuig.) D dorsal side, V ventral sido, ik inner germinal layer, mv visceral mesoblast, //; body-cavity, mp parietal mesoblast, ak outer germinal layer. by wide apertures; they also communicate for a short time with the dorsal side (Fig. 80 and 83.— Transverse section of amphioxus-larvae. (From Haischek.) Fig. 82 at the commencement of ccelom-formation (siill without segments), Fig. 83 at the stage with lour primitive segments, til-, ik, »ik outer, inner, and middle germinal layer, lip horn plate, mp medullary plate, ch chorda, * ana disposition of the ccelom-pouches, //; body-cavity. other and ijrow underneath the horn-plate, a cylindrical tube is formed, the medullary tube (Fig. 85 n); this quickly detaches itself altogether from the horn-plate. At each side of the medullary tube, between it and the alimentary tube (l;ii,rs. S2 S5 lies on the dorsal side of the bilateral, worm-like body, the visceral tube fdj on the ventral side, the chorda fchj between the two, on the long axis, and the ccelom- pouches (c) at each side. In every case these 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 heredity of the theory of descent, that all these chordonia or chordata (tunicates and vertebrates) descend from an ancient common ancestral form, which we may call chordcea. We should regard this long-extinct chordasa, if it were still in existence, as a special class of unarticulated worm (chordariaj. It is especially noteworthy that neither the dorsal nerve-tube nor the ventral gut-tube, nor even the chorda that lies between them, shows any trace of articulation or metamera-formation ; even the two ccelom-sacs are not segmented at first (though in the amphioxus they quickly divide into a series of somites by transverse folding). These ontogenetic facts are of the greatest importance for the THE CCELOM THEORY purpose of learning those ancestral forms of the vertebrates which we have to seek in the group of the unarticulated vermalia. The ccelom-pouches were originally sexual glands in these ancient chordonia. From the phylogenetic point of view the ccelom-pouches Fig. 87. o h b d z dd ch ti" m n' h ch Fig. 88. Fig. 89. Figs. 86 and 87.— Chordula Of the amphioxus. Fig. 86 median longi- tudinal section (seen from the left). Fig. 87 transverse section. 1 From Hatschek.) In F"ig. 86 the ccelom-pouches are omitted, in order to show the chordula more clearly. F~ig. 87 is rather diagrammatic. /; horn-plate, m medullary tube, // wall of same (»' dorsal n" ventral), ch chorda, np neuroporus, tie canalis neurentericus, d gut-cavity, r gut dorsal wall, b gut ventral wall, s yelk-cells in the latter, 11 primitive mouth, 0 mouth-pit, p promesoblasts (primitive or polar cells of the mesoderm), ti' parietal layer, ?' visceral layer oi the mesoderm, c ccelom, f rest of the segmentation-cavity. Figs. S8 and 89.— Chordula of the amphibia (the ringed snake). (From Goette.) Fig. 88 median longitudinal section (seen from the left), Fig. 89 transverse section (slightly diagrammatic). Lettering as in Figs. 86 and 87. THE CCELOM THEORY arc, in any case, older than the chorda ; since they also develop in the same way as in the chordoma in a number of inverte- brates which have no chorda (for instance, sagitta, Figs. 7q Si). Moreover, in the amphioxus the first outline of the chorda appears later than that of the ccelom-sacs. Hence we must, according to the biogenetic law, postulate a special intermediate form between the gastrula and the chordula, which we will call cceiomula, an unarticulated, worm-like body with primitive gut, primitive mouth, and a douhle body-cavity, hut no chorda. This embryonic form, the bilateral cceiomula (Fig. 84), may in turn be regarded as the ontogenetic reproduction (maintained by heredity) of an ancient ancestral form of the civlomaria, the ccelomaea (cf. Chapter XX. ). In sagitta and other helmintha the two ccelom-pouches (presumably the gonades or sex-glands of the ceeloma?a) are separated by a complete median partition, the dorsal and ventral mesentery (Fig. Si dm and vm) ; but in the vertebrates only the upper part of this vertical partition is maintained, and forms the dorsal mesenterv. This mesenterv afterwards takes the form of a thin membrane, which fastens the visceral tube to the chorda (or the vertebral column). At the under side of the visceral tube the ccelom-sacs blend together, their inner or median walls breaking down and disappearing. The body-cavity then forms a single simple hollow, in which the gut is quite free, or only attached to the dorsal wall by means of the mesentery (cf. Plate IY., Fig. 5). The development of the body-cavity and the formation of the chordula in the higher vertebrates is, like that of the gastrula, chiefly modified by the pressure of the food-yelk on the embryonic structures, which forces its hinder part into a discoid expansion. These cenogenetic modifications seem to be so great that until twenty years ago these important processes were totally misunderstood. It was generally believed that the body-cavity in man and the higher vertebrates was due to the division of a simple middle layer, and that the latter arose by cleavage from one or both of the primary germinal layers. The truth was brought to THE CCELOM THEORY light at last by the comparative embrvological research of the Hertwigs. They showed in their Coelom Theory (1881) that all vertebrates are true enteroccela, and that in every case a pair of ccelom-pouches are developed from the primitive gut by folding. The cenogenetic chordula-forms of the craniotes must therefore be derived in the same way from the palingenetic embryology of the amphioxus, as I had previously proved for their gastrula-forms. The chief difference between the ccelomation of the acrania (amphioxus J and the other vertebrates (craniotes) is that the two ccelom-folds of the primitive gut in the former are from the first hollow vesicles, filled with fluid, but in the latter are empty pouches, 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 embrvological literature ordinary logic does not count for very much. In many of the manuals and large treatises on this science it is proved that vesicles, pouches, or sacs deserve that name only when they are inflated and filled with a clear fluid. When they are not so filled (for instance, when the primitive gut of the gastrula is filled with yelk, or when the walls of the empty coelom-pouches are pressed together), these vesicles must not be cavities any longer, but " solid structures." The evolution of the large food-yelk in the ventral wall of the primitive gut (Figs. 88, 89) is the simple cenogenetic cause that converts the sac-shaped ccelom-pouches of the acrania into the leaf-shaped ccelom-streaks of the craniotes. To convince ourselves of this we need only compare, with Hertwig, the palingenetic ccelomula of the amphioxus (Figs. 83, 84) with the corresponding cenogenetic form of the amphibia (Figs. 92-94), and construct the simple diagram that connects the two (Figs. 90, 91). If we imagine the ventral half of the primitive gut-wall in the amphioxus embryo (Figs. 82-87) distended with food-yelk, the vesicular ccelom-pouches (lh) must be pressed together by this, and forced to extend in the shape of a thin double plate between THE CCELOM THEORY the gut-wall and body-wall (Figs. 89, cp^TaasUx^hsTorf follows a downward and forward direction. They arc not directly connected with these two 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. qo 11). At this important spot we have the source of embryonic development (blastocrene), or "zone of growth," from which the coelomation (and also the gastrulation) originally proceeds. Hertwig even succeeded in showing, in the ceelomula- embrvo of the water salamander ( ' Iriton J, between the first Fig. 00. 91. Figs. 9oand 91. Diagrammatic vertical section of ecelomula-embryos Of vertebrates. (From Hertwig.} Fig. 90, vertical section through the primitive mouth. Fig. 91, vertical section before the primitive mouth. 11 primitive mouth, net primitive gut, d yelk, ilk yelk-nuclei, dh gut-cavity, /// body-cavity, w/> medullary plate, ch chorda plate, uk and ik outer and inner germinal layers, pb parietal ami -.■!> visceral mesoblast. structures of the two middle layers, the relic of the body- cavity, which is represented in the diagrammatic transitional form (Figs. 90, 91). In sections both through the primitive mouth itself (Fig. 92) and in front of it (Fig. 93) 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. <>;, u) we can penetrate into them from without. It is only here at the border oi the primitive mouth that we can show the direct transition of the two middle layers into the two limiting lasers or primary germinal layers. THE CCELOM THEORY The structure of the chorda also shows the same features in these ccelomula-embryos of the amphibia (Fig. 94) as in the amphioxus (Figs. 82-85). It arises from the entodermic cell-streak, which forms the middle dorsal line of the primitive gut, and occupies the space between the flat ccelom-pouches (Fig. 94 A). While the nervous centre is formed here in the median line of the back and separated from the ectoderm as " medullary tube," there takes place at the same time, directly underneath, the severance of the chorda from the entoderm (Fig. 94, A, B, C). Under the chorda is formed (out of the ventral entodermic half of the Fig. 92. Fig. 93. Figs. 92 and 93.— Transverse section of ecelomula embryos of triton. (From Hertwig. ) Fig-. 92 .section through the primitive mouth, Fig. 93 section in front of the primitive mouth. 11 primitive mouth, dh gut-cavity, dz yelk-cells, d /"yelk-stopper, ak outer and ik inner germinal layer,/* parietal and vb visceral middle layer, m medullary plate, rh chorda. gastrula) the permanent gut or visceral cavity (enteron) (Fig. 94, B, d/i). This is done by the coalescence, under the chorda in the median line, of the two dorsal side-borders of the gut-gland-layer ( ik), which were previously separated by the chorda-plate (Fig. 94, A, c/t); these now alone form the clothing of the visceral cavity (dh ) (enteroderm, Fig. 94, C). 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) THE CCE/.O.U THEORY *3S remains for a long time the source of development (blasto- crenej, or the zone of fresh construction, in the further building-up of the organism. One has only to compare carefully the illustrations given (Figs. 88 <»4) to see that, as a fact, the cenogenetic coelomation of the amphibia can be deduced directly from the palingenetic .!./..<. Vertical section of the dorsal part of three triton- embryos. (From Hertvig.) In Fig. .1 tlx- medullary swellings (the parallel borders of the medullary plate) begin to rise ; in Fig. 11 they grow towards each other; in Fig. C they join and form the medullary tube. >»/> medullary plate, m/medullary folds, n nerve-tube, ch chorda, iM body-cavity, mk, and mk., parietal and visceral mesoblasts, uv primitive-segmeni cavities, ai ectoderm, it entoderm, A' /). and underneath the middle of the chorda ( c// ) and at each side of it a couple of broad mesodermic layers (sp). These enclose a narrow space or cleft (uwh), which is nothing else than the structure of the body-cavity. The two layers that enclose it — the upper parietal layer ( ' Iipl ' ) and the lower visceral layer (df) — are pressed together from without, hut clearly distinguishable. This is even clearer a little later, when the medullar)- furrow is closed into the nerve-tube (Fig. 96 mr). Here the mesoderm has divided into two sections by a longitudinal fold, an inner (median) primitive-segment plate ( 'una ) and an outer (Lateral) plate; the narrow- ccelom-eleft may be seen both in the former (uwh) and the latter (nip). It afterwards enlarges into - "V .J'P? el. Or* ao *'p . Gastrodermis Epithelia of the gut and j U. Nutrition. (nutritive the viscera] 1 layer 1. '. glands. J NINTH TABLE SYNOPSIS OF THE SIX FUNDAMENTAL ORGAN'S (A) AND THE THREE BODY-CAVITIES (B) OF THE CHORDULA, AND THEIR ORIGIN FROM THE GERMINAL LAYERS. A. The Fundamental Organs of the Chordula. The Two Separation of Six Products of the Primary , the Four Seeon- Primitive Germinal Germinal dary Germinal Embryonic Plates in Layers. Layers. Plates. Man. I. Primitive Covering. Epithelium of the outer or upper layer : Ectoderm or ectoblast (animal layer). Epiblast. . Outer skin of the chordula ( = ectoderm of the chordsea). , Dorsal median part of the 1 1. Cera b 1 a s t | horn-plate Ji. Outer skin, (covering-ecto- | hair, nails. I blast). I ' 2. Neuroblast C medullary plate 2. Brain, spinal (nerve-plate). - marrow, sense- Nerve -e c t o- cells. [ blast. l_ f 3. Parietal meso- C ■j and 4. The two blast (outer 1 3. Muscle-system, layers of the layer of the J skeletal system, ccelom-pouches ccelom-pouches)! corium. (outer and inner muscle-plate. plates 1. The 1 II. Primitive lateral parts of 4. Viscera] meso- I Gut. the dorsal wall blast (inner 4. Sex-glands, Epithelium of the of the primitive layer of the i vascular system, inner or lower gut. ccelom-pouches) heart, blood. germ-layer : k muscle-plate. 1 Entoderm or endoblast S. Median part of /"5. Chorda b ast 1 (chorda ■ 'ate) 15. Rudiment of the chorda in (vegetal layer) the dorsal wall hypoblast. of the primitive | (axial en do- | the vertebral gut. [ blast). ^ column. 6. Ventral wall of ,-6. Enteroblast ,6. Epithelium oi the primitive | (gland-endo- | alimentary gut. "j blast) (gut-K canal, lungs, | epithelium). A liver, etc. B. Primary Cavities in the Body of the Chordula. I. Animal Cavity. II. Vegetal Cavity. Wall formed of ectoderm- epithelia. Walls formed of entoderm- epilhelia. 1. Single tube. and 2b. Pair of ' ccelom-pouches. 3. Single tube. 1. Cavity oi' the nerve-tube. Medullary Canal. 2a and 2b. Right and left body- cavity. Cceloma. 3. Cavity of the permanent gut, Gastroeoel. TENTH TABLE SYNOPSIS OF THE FOUR CHIEF GROUPS OF THE METAZOA THAT MAY BE DISTINGUISHED ACCORDING TO THE NUMBER OF GERMINAL LAYERS Germ-group. Germ-layers. Germ-form. Animal-classes. I. One-1 a y e re d animate. Monoblastiea (without primitiv gut HI Blastoderma Blastula. .... , Blastsads \ esicular larva , . , . . (volvocina, (with embryonic , " catallacta, cavity or blasto- . magosphaera) coel). Gastrula. Gastraeads II. Two-layered i. Eetoderma Cup-larva (with (cyemaria, animals. (epiblast). primitive gut- olynthus, Diploblastica -\ Entoderma cavity and primi- tive mouth : hydra. ( with primitive gut I. (hypoblast). progaster and ccelenteria). 1 prostoma). III. Three-layered t. Eetoderma animals. Triploblastica (with gut-cavity— gastro-canal mv- tem — always with- out ami-., without bodj -cavity). skin-layer. Mesoderma Mesomula. Large larva or (in tin- shape embryo with mas- ofmesenchym) sive mesenchym middle layer. between the two 3. Entoderma primary layers. gut-layer. Most of the ccelenteria (sponges, speda, corals, ctenophora, platodes). Lowest ccelomaria. I\'. Four-layered animals. Tetrablastica (with gut - cavity ami body-cavity ; generally with anus and blood-vessels). 1. Neural layer skin-sense-laj er neuroblast .'. Parietal layer skin-fibre-layer myoblast .-,. Visceral layer gut-fibre-layer gonoblast. 4. Enteral layer gut-gland-layer enteroblast Coelomula. Pouch-larva or embryo with gut- cavity and body- cavity. Gut-wall formed ol the two inner layers (visceral layers). Body-wall of the two on 1 1 layers. Most of the ccelomaria : vermalia (great majority), mollusca, echinoderma, articulata (annelida, ciustacca, trach tunicata, \ ertebi (acrania, craniota). -'45 CHAPTER XI. THE VERTEBRATE CHARACTER OF MAN The association of comparative anatomy and ontogeny. Place of man in zoological classification. The types or steins of the animal kingdom. The phvlogenetic relations of the twelve animal stems. Protozoa and metazoa. Coelenteria and ccelomaria. Unity of the vertebrate stem, including' man. Essential features of the vertebrates. Amphioxus and the hypothetical primitive vertebrate (prospondylus). Division of the simple bilateral body into head and trunk. Axial rod or chorda. The antimera or symmetrical halves of the body. Medullary tube or nerve tube (brain and spinal marrow). Three pairs of sense-organs (nose, eyes, ears |. Chorda- sheath (perichorda). Muscles. Corium. Epidermis. Body-cavity. Alimen- tary canal. Gill-gut in the head-half of the body ; liver-gut in the trunk- half. Gills and lungs. Stomach and small intestine. Liver. Blood-vessels and heart. Pro-kidneys (pronephridia). Segmental sex-organs (gonades). Metamerism or articulation of the vertebrates. Monophyletic origin of the vertebrates and of the mammals. The milk apparatus in mammals. Redundant milk glands and nipples. Hypermastism and hyperthelism. Gynecomastism (large milk-forming breast-glands in the male sex). Apparent hermaphrodism. We have now secured a number of firm standing-places in the labyrinthine course of our individual development by our study of the important embryonic forms which we have called the cytula, morula, blastula, gastrula, ccelomula, and chordula. 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 body 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 go to the formation of the complex body of man and the higher animals. It is so difficult to understand this construction that we will first seek a companion who may help us out of many difficulties. This helpful associate is the science of comparative 246 THE VERTEBRATE CHARACTER OF MAN anatomy. Its task is, by comparing the fully-developed bodily tonus in the various groups of animals, to learn the general laws of organisation, according to which the body is constructed ; at the same time, it has to determine the affinities of the various groups by critical appreciation of the degrees oi difference between them. Formerly, this work was conceived in a teleological sense, and it was sought to find traces o\ the pre-formed plan of the Creator in the actual purposive organisation of animals. But comparative anatomy has gone much deeper since the establishment o( the theory of descent ; its philosophic aim now is to explain the variety of organic forms by adaptation, and their similarity by heredity. At the same time, it has to recognise in the shades o( difference in form the degree of blood-relationship, and make an effort to construct the ancestral tree of the animal world. In this way, comparative anatomy enters into the closest relations with comparative ontogeny on the one hand, and with the science of classification on the other. Now, when we ask wliat position man occupies among the other organisms according to the latest teaching of comparative anatomy and classification, and how man's place in the zoological system is determined by comparison of the developed bodily forms, we get a very definite and significant reply; and this reply gives us extremely important conclusions that enable us to understand the embryonic development and its phvlogenetic purport. Since Cuvier and Baer, since the immense progress that was effected in the early decades o\~ the nineteenth' century by these two great zoologists, the opinion has generally prevailed that the whole animal kingdom may be distributed in a small number of great divisions or types. They are called types because a certain typical or characteristic structure is constantly pre- served within each of these large sections. Since we applied the theory ot descent to this doctrine of types, we have learned that this common type is an outcome o( heredity ; a the animals of one type are blood-relatives, or members of one stem, and can be traced to a common ancestral form. Cuvier and Baer set up four of these types : the vertebrates, 248 THE VERTEBRATE CHARACTER OF MAN articulates, molluscs, and radiates. The former three of these are still retained, and may be conceived as natural phylogenetic unities, as stems or phyla in the sense of the theory of descent.1 It is quite otherwise with the fourth type — the radiata. These animals, little known as yet at the beginning- of the nineteenth century, were made to form a sort of lumber-room, into which were cast all the lower animals that did not belong to the other three types. As we obtained a closer acquaintance with them in the course of the last sixty years, it was found that we must distinguish among them from four to eight different types. In this way the total number of animal stems or phyla has been raised to eight or twelve (cf. Chapter XX.). These twelve stems of the animal kingdom are, however, by no means co-ordinate and independent types, but have definite relations, partly of subordination, to each other, and a very different phylogenetic meaning. Hence they must not be arranged simply in a row one after the other, as was generally done until thirty years ago, and is still done in some manuals. We must distribute them in three subor- dinate principal groups of very different value, and arrange the various stems phylogenetically on the principles which I laid down in my Monograph on the Sponges, and developed in the Study of the Gastrcea Theory. We have first to dis- tinguish the unicellular animals (protozoa) from the multi- cellular tissue-forming (metazoa J. Only the latter exhibit the important processes of segmentation and gastrulation ; and they alone have a primitive gut, and form germinal layers and tissues. The metazoa, the tissue-animals or gut-animals, then sub- divide into two main sections, according as a body-cavity is or is not developed between the primary germinal layers. We may call these the ccetenteria and coelomaria ; the former 1 According to the early theory of types, those of the animal kingdom are parallel and completely independent ; but according- to my gastrfea theory they are divergent stems, connected at their root. This view of the affinity of the lower and higher animal-stems, which I first advanced in 1872 (in the Mono- graph on the Sponges ), is further developed in my Systematic Phytogeny (1896), and compendiously stated in the tenth edition of the History of Creation (1902). THE VERTEBRATE CHARACTER OF MAN arc often also called zoophytes or coelenterata, and the latter bilaterals. This division is the more important as the ccelen- teria (without COelom) have no blood and blood-vessels, or an anus. The coelomaria (with body-cavity) have generally an anus, and blood and blood-vessels. There are tour stems belonging to the ccelentejia : the gastrseads ("primitive-gut animals"), sponges, cnidaria, and platodes. Of the coelomaria we can distinguish six stems: the vermalia at the bottom represent the common stem-group (derived from the platodes) of these, the other five typical stems oi the coelomaria — the molluscs, echinoderms, articulates, tunicates, and verte- brates— being evolved from them. Man is, in his whole structure, a true vertebrate, and developes from an impregnated ovum in just the same characteristic way as the other vertebrates. There can no longer be the slightest doubt about this fundamental fact, nor ot the fact that all the vertebrates form a natural phylo- genetic unity, a single stem. The whole of the members of this stem, from the amphioxus and the cyclostoma to the apes and man, have the same characteristic disposition, connection, and development of the central organs, and arise in the same way from the common embryonic form of the chordula. Without going into the difficult question of the origin of this stem, we must emphasise the fact that the vertebrate stem has no direct affinity whatever to five of the other ten stems ; these five isolated phyla are the sponges, cnidaria, molluscs, articulates, and echinoderms. On the other hand, there are important and, to an extent, close phylogenetic relations to the other five stems — the protozoa (through the amoeba?), the gastrseads (through the blastula and gastrula), the platodes and vermalia (through the ccelomula), and the tunicates (through the chordula). How we are to explain these phylogenetic relations in the present state o( our knowledge, and what place is assigned to the vertebrates in the animal ancestral tree, will be con- sidered later (Chapter XX.). For the present our task is to make plainer the vertebrate character of man, and especial ly to point out the chief peculiarities of organisation by which 25o THE VERTEBRATE CHARACTER OF MAX the vertebrate stem is profoundly separated from the other eleven stems of the animal kingdom. Only after these com- parative anatomical considerations shall we be in a position to attack the difficult question of our embryology. The development of even the simplest and lowest vertebrate from the simple chordula (Figs. 86-89) is so complicated and difficult to follow that it is necessary to understand the organic features of the fully-formed vertebrate in order to grasp the course of its embryonic evolution. But it is equally necessary to confine our attention, in this general anatomic characterisation of the vertebrate-body, to the essential facts, and pass by all the unessential. Hence, in giving you now an ideal anatomic description of the chief features of the vertebrate and its internal organisation, I omit all the subordinate points and restrict myself to the most important characteristics. Much, of course, will seem to the reader to be essential that is only of subordinate and secondary interest, or even not essential at all, in the light of comparative anatomy and embryology. For instance, the skull and vertebral column and the extremities are non-essential in this sense. It is true that these parts are very important physiologically ; but for the morphological conception of the vertebrate they are not essential, because they are only found in the higher, not the lower, vertebrates. The lowest vertebrates have neither skull nor vertebra?, and no extremities or limbs. Even the human embryo passes through a stage in which it has no skull or vertebra;; the trunk is quite simple, and there is yet no trace of arms and legs. At this stage of development man, like every other higher vertebrate, is essentially similar to the simplest vertebrate form, which we now find in only one living specimen. This one lowest vertebrate that merits the closest study — undoubtedly the most interesting of all the vertebrates after man — is the famous lancelet or amphioxus, to which we have already often referred (Plates XVIII. and XIX.). As we are going to study it more closely later on (Chapters XVI. and XVII.), I will only make one or two passing observations on it here. THE VERTEBRATE CHARACTER OF MAN The amphioxus lives buried in the sand of the sea, is from g 7 centimetres long, and has, when fully developed, the shape o( a very simple, longish, lancet-like leaf; hence its name of the laneelet. The narrow body is compressed on both sides, almost equally pointed at the fore and hind ends, without any traee of external appendages or articulation of 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 investigations have shown that it is of the greatest importance in connection with the comparative anatomy and ontogeny of the verte- brates, and therefore witli 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, wc get anatomic pictures of the utmost instructiveness (cf. Figs. 101-105 and Plates XVIII. and XIX.). In the main they correspond to the ideal which we form with the aid of comparative anatomy and ontogeny ot the primitive type or build of the vertebrate — the long extinct form to which the whole stem owes its origin. As we take the phylogenetic unity of the vertebrate stem to be beyond dispute, and assume a common origin from a primitive stem-form for all the vertebrates, from amphioxus to man, wc are justified in forming a definite morphological idea of this primitive vertebrate ( prospondylus or vcrlebnva ). Wc need only imagine a few slight and unessential changes in the real sections of the amphioxus in order to have this ideal anatomic figure or diagram of the primitive vertebrate form, as we see in bigs. 101-105. The amphioxus departs so little from this primitive form that we may, in a certain sense, describe il THE VERTEBRATE CHARACTER OF MAX as a modified "primitive vertebrate"1 (cf. Plates XVIII. and XIX. with Figs. 101-105). The outer form of our hypothetical primitive vertebrate was at all events very simple, and probably more or less similar to that of the lancelet. The bilateral or bilateral- symmetrical body is stretched out lengthways and compressed at the sides (Figs. 101-103), oval in section (Figs. 104, 105). There are no external articulation and no external appen- dages, in the shape of limbs, legs, or fins. On the other hand, the division of the body into two sections, head and trunk, was probably clearer in prospondylus than it is in its little- changed ancestor, the amphioxus. In both animals the fore or head-half of the body contains different organs from the trunk, and different on the dorsal from on the ventral side. As this important division is found even in the ascidia, the remarkable invertebrate stem-relatives of the vertebrates, we may assume that it was also found in the prochordonia, the common ancestors of both stems. It is also very pronounced in the young larva? of the cyclostoma (Plate XIX., Fig. 16); this fact is particularly interesting, as this palingenetic larva- form is in other respects also an important connecting-link between the higher vertebrates and the acrania. The head of the acrania, or the anterior half of the body (both of the real amphioxus and the ideal prospondylus), contains the 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, contains the liver-gut and sexual- glands in the ventral part, and the spinal marrow and most of the muscles in the dorsal part. In the longitudinal section of the ideal vertebrate (Fig. 101) we have in the middle of the body a thin and 1 The ideal figure of the vertebrate as given in Figs. 101-105 's a hypo- thetical scheme or diagram, that has been chiefly constructed on the lines of the amphioxus, but with a certain attention to the comparative anatomy and ontogeny of the ascidia and appendicularia on the one hand, and of the cyclostoma and selachii on the other. This diagram has no pretension what- ever to be an "exact picture," but merely an attempt to reconstruct hypo- thetieally the unknown and long extinct vertebrate stem-form, an ideal " architypus. " THE VERTEBRATE CHARACTER OF MAX nil «' MB If gh -v (( r ms ka kg is h Fig. ioi. I dvs «f f it / gh g an e nay _— - — ^T\ Figs. 101-105.— The ideal primi- tive vertebrate (prospondylus). Diagram. Fig. 101 side-view (from the left). Fig. 102 back-view. Fig. 103 front view. Fig. 104 transverse section Fig. 105. through tin- head (to the left through the gill-pouches, to the right through the gill-clefts). Fig. 105 transverse section of the trunk (to the right a pro-renal canal is affected), a aorta, o/*anu9,au eye, b side-furrow (primitive renal process), e cceloma (body-cavity), 1/ small intestine, e parietal eye (epiphysis), f fin border of the -.kin, g auditory vesicle, gh brain, /; heart, i muscular cavity (dorsal coelom-pouch), £ gill-gut, ka \ gill-folds, /liver, inn stomach, md mouth, ms muscles, na nose (smell pit), n renal canals, 11 apertures of same, o outer skin, /> gullet, r spinal narrow, s sexual glands (gonades), / corium, u kidney-openings (pores ^t the lateral furrow), v visceral vein (chief vein), x chorda, y hypophysis (urinary appendage), - gulli or gill-groove (hypobranchial groove). 254 THE VERTEBRATE CHARACTER OF MAN flexible, but stiff, 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 dorsal is, also called chorda vertcbralis, vertebral cord, axial cord, spinal cord, notochorda, 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 long-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. 101). When we make a vertical section through the whole length of this long-axis, the body divides into two equal and symmetrical halves, right 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 the two halves the sagittal ("arrow") axis, or "dorsoventral axis," goes from the back to the belly, corresponding to the sagittal seam of the skull. But when we make an 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 transverse, frontal, or lateral axis (cf. Plates VI. and VII.). THE VERTEBRATE CHARACTER OF MAN 255 The two halves ol~ the vertebrate body that are separated by this horizontal transverse axis and by the chorda are of 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 movement and sensation. The ventral half is essentially the vegetative half o\ the body, and contains the greater part of the vertebrate's vegetal organs, the visceral and vascular systems, sL-\ual system, etc. — the instruments of nutrition and reproduction. 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 that is engaged. Each o( the two halves developes in the shape of a tube, and encloses a cavity in which another tube is found. The dorsal hall 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 organ 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 (Plate VII., Figs, n-16 n). Even in the primitive vertebrate this composition is plainly indicated. The fore half of the body, which corresponds to the head, encloses a knob-shaped vesicle, the brain (g/i); this is pro- longed backwards into the thin cylindrical tube of the spinal marrow ( r ). Hence we find here this very important psychic organ, which accomplishes sensation, 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 filled with fluid (Figs. 101-105 r). We still find the medullary tube in this verv simple form for a time in the THE VERTEBRATE CHARACTER OF MAX embryo of all the vertebrates (cf. Plate VII., Figs. 11-13), and it retains this form in the amphioxus throughout life ; only in the latter case the cylindrical medullary tube barely indicates the separation of brain and spinal cord. The lancelet's medullary tube runs nearly the whole length of the body, above the chorda, in the shape of a long thin tube of almost equal diameter throughout (Plate XIX., Fig. 15), and there is only 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 larva? (Plate XVIII., Fig. 5) on the one hand and the young cyclostoma (Plate XIX. , Fig. 16) on the other clearly show a division of the vesicular brain, or 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, has probably had three pairs of sense- organs, though of a simple character, a pair of, or a single olfactory depression, right in front (Figs. 101, 102, 11a), 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 halves or antimera of the body. With this partition is connected the mass of connec- tive 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 ccelom-pouches which we shall call the skeleton plate or " sclerotom " in the craniote embryo. In the latter the chief part of the skeleton — the vertebral column and skull — developes from this chord-sheath ; in the THE VERTEBRATE CHARACTER OF MAX -'57 acrania it retains its simple form as a soft connective matter, from which are formed the membranous partitions between the various muscular plates or myotomes (Figs. 101, 102, ms). To the right and left of the cord-sheath, at each side of the medullary tube and the underlying axial rod, we find in all the vertebrates the large masses of muscle that constitute the musculature of the trunk and effect its movements. Although these are very elaborately differentiated and connected in the developed vertebrate (corresponding to the many differentiated parts of the bony skeleton), in our ideal primitive vertebrate we can distinguish only two pairs of these principal muscles, which run the whole length of the body parallel to the chorda. These are the upper (dorsal) and lower (ventral) lateral muscles of the trunk. The upper (dorsal) muscles, or the original dorsal muscles (Fig. 105 ms), form the thick mass of flesh on the back. The lower (ventral) muscles, or the original muscles of the belly, form the fleshy wall of the abdomen. Both sets are articulated, and consist of a double row of muscular plates (Figs. 101, 102 //is); the number of these myotomes determines the number of joints in the trunk, or metamera. The myotomes are also developed from the thick wall of the coelom-pouches (Fig. 105 /). Outside this muscular tube we have the external envelope of the vertebrate body, which is known as the corium or cutis ( Plate VI. /). This strong and thick envelope consists, in its deeper strata, chiefly of fat and loose connective tissue, and in its upper layers of cutaneous muscles and firmer connective tissue. It covers the whole surface of the fleshy body, and is of considerable thickness in all the craniota. But in the acrania the corium is merely a thin plate of connective tissue, an insignificant " corium-plate " (lamella con'/, Figs. 101-105 I )• Immediately above the corium is the outer skin (epidermis, a), the general covering of the whole outer surface. In the higher vertebrates the hairs, nails, feathers, claws, scales, etc., grow out of this epidermis. It consists, with all its appendages and products, of simple cells, and has no blood- vessels. Its cells are connected with the terminations of the THE VERTEBRATE CHARACTER OF MAN sensory nerves. Originally, the outer skin is a perfectly simple covering of the outer surface of the body, composed only of homogeneous cells — a permanent horn-plate. In this simplest form, as one-layered epithelium, we find it, at first, in all the vertebrates, and throughout life in the acrania. It afterwards grows thicker in the higher vertebrates, and divides into two strata — an outer, firmer horn-layer and an inner, softer mucus-layer ; also a number of external and internal appendages grow out of it : outwardly, the hairs, nails, claws, etc., and internally, the sweat-glands, fat- glands, etc. It is probable that in our primitive vertebrate the skin was raised in the middle line of the body in the shape of a vertical fin border (fj. A similar border, going round the greater part of the body, is found to-day in the amphioxus and the cyclostoma ; we also find one in the tail of fish-larva? and tadpoles. Now that we have considered the external parts of the vertebrate and the animal organs, which mainly lie in the dorsal half, above the chorda, we turn to the vegetal organs, which lie for the most part in the ventral half, below the axial rod. Mere we find a large body-cavity or visceral cavity in all the craniota. The spacious cavity that encloses the greater part of the viscera corresponds to only a part of the original cceloma, which we considered in the tenth Chapter ; hence it may be called the metacosloma. As a rule, it is still briefly called the cceloma ; formerly it was known in anatomy as the pleuroperitoneal cavity. In man and the other mammals (but only in these) this cceloma divides, when fully developed, into two different cavities, which are separated by a transverse partition — the muscular diaphragm. The fore or pectoral cavity (pleura cavity) contains the oesophagus, heart, and lungs ; the hind or peritoneal or abdominal cavity contains the stomach, small and large intestines, liver, pancreas, kidneys, etc. But in the verte- brate embryo, before the diaphragm is developed, the two cavities form a single continuous body-cavity, and we find it thus in all the lower vertebrates throughout life. This THE VERTEBRATE CHARACTER OF MAN body-cavity is clothed with a delicate layer of cells, the ccelom- epithelium. In the acrania the ccelom is articulated both dorsal ly and ventrally, as their muscular pouches and primitive genital organs plainly show (Fig. 105). The chief o( the viscera in the body-cavity is the alimen- tary canal, the organ that represents the whole body in the gastrula. In all the vertebrates it is a long tube, enclosed in the body-cavity and more or less differentiated in length, and has two apertures — a mouth for taking in food (Figs. 101, to,'', /in/) and an anus for the ejection of unusable matter or excrements (a/). With the alimentary canal (Plates IV., Y. ) lies below the gut, in the middle line of its ventral side, and is therefore also called the vena subintestinalis ; it conducts carbonised or venous blood back from the body to the gills. At the branchial section of the gut in front the two canals are connected by a number of branches, which rise in arches between the gill-clefts. These " branchial vascular arches " (kg) run along the gill-arches, and have a direct share in the work of respiration. The anterior continuation of the principal vein which runs on the ventral wall of the gill-gut, and gives off these vascular arches upwards, is the branchial artery (ka). At the border of the two sections of the ventral vessel it enlarges into a contractile spindle-shaped tube (Figs. 101, 103 h). This is the first outline of the heart, which afterwards becomes a four- chambered pump in the higher vertebrates and man. There THE VERTEBRATE CHARACTER OF MAN 263 is no heart in the amphioxus, probably owing to degenera- tion. In prospondylus the ventral gill-heart probably had the simple form in which we still find it in the ascidia and the embryos of the craniota (Figs. 101, 10,} h). The kidneys, which act as organs of excretion or urinary organs in all vertebrates, have a very different and elaborate construction in the sections of this stem; we will consider them further in the twenty-ninth Chapter. Here I need only mention that in our hypothetical primitive vertebrate they probably had the same form as in the actual amphioxus — the fore-kidneys (protonephra). These are originally made up of a double row ot little canals, which directly convey the used-up juices or the urine out of the body-cavity (Fig. 105 //). The inner aperture of these pronephridial canals opens with a vibratory funnel into the body-cavity ; the external aperture opens in lateral grooves of the epidermis, a couple of longitudinal grooves in the lateral surface of the outer skin (Fig. 105 b). The pronephridial duct is formed by the closing of this groove to the right and left at the sides. In all the craniota it developes at an early stage in the horn- plate (Plate VI., Figs. 4 a, 5 u) ; in the amphioxus it seems to be converted into a wide cavity, the atrium, or pcribranchial space (Plate XV I II., Fig. 13 c). Xext to the kidneys we have the sexual organs of the vertebrate. In most of the members of this stem the two are joined together in a unified urogenital system ; it is only in a few groups that the urinary and sexual organs are separated (in the amphioxus, the cyclostoma, and some sections of the fish-class). In man and all the higher vertebrates the sexual apparatus is made up of various parts, which we will consider in the twenty-ninth Chapter. But in the two lowest classes of our stem, the acrania and cyclostoma, they consist merely of simple sexual glands or gonades, the ovaries of the female sex and the testicles (spermatid) of the male ; the former provide the ova, the latter the sperm. In the craniota we alwavs find only one pair of gonades; in the amphioxus several pairs, metamerically arranged. They must have had the same form in our hypothetical prospondylus (Figs. 101, 10,-5 •»")• 264 THE VERTEBRATE CHARACTER OF MAN These segmental pairs of gonades are the original ventral halves of the ccelom-pouches. The organs which we have now enumerated in this general survey, and of which we have noted the characteristic disposition, are those parts of the organism that are found in all vertebrates without exception in the same relation to each other, however much they may be modified. We have chiefly had in view the transverse section of the body (Figs. 104, 105), because in this we see most clearly the distinctive arrangement of them. But to complete our picture we must also consider the articulation or metamera- formation of them, which has yet been hardly noticed, and which is seen best in the longitudinal section. In man and all the more advanced vertebrates the body is made up of a series or chain of similar members, which succeed each other in the long axis of the body — the segments or metamera of the organism. In man these homogeneous parts number thirtv-three in the trunk, but they run to several hundred in many of the vertebrates (such as serpents or eels). As this internal articulation or metamerism is mainly found in the vertebral column and the surrounding muscles, the sections or metamera were formerly called pro-vertebrae. As a fact, the articulation is by no means chiefly determined and caused by the skeleton, but by the muscular system and the segmental arrangement of the kidneys and gonades. How- ever, the composition from these pro-vertebrae or internal metamera is usually, and rightly, put forward as a prominent character of the vertebrate, and the manifold division or differentiation of them is of great importance in the various groups of the vertebrates. But as far as our present task — the derivation of the simple body of the primitive vertebrate from the ehordula — is concerned, the articulate parts or metamera are of secondary interest, and we need not go into them just now. The characteristic composition of the vertebrate body developes from the embryonic structure in the same way in man as in all the other vertebrates. As all competent experts now admit the monophyletic origin of the vertebrates THE I 'ER TEBR. I IF t II. 1 R. I ( ' TER OF MAX 265 on the strength o( 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 all questions." We may, moreover, point out that this answer is just as certain and precise in the case of the origin of man from the mammals. This advanced vertebrate class is also monophyletic, or has evolved from a common stem-group of lower vertebrates (reptiles, and, earlier still, amphibia). This follows from the fact that the mammals are clearly distinguished from the other classes of the stem, not merely in one striking par- ticular, hut in a whole group of distinctive characters. It is only in the mammals that we find the skin covered with hair, the breast-cavitv separated from the abdominal cavity by a complete diaphragm, and the larynx provided with an epiglottis. The mammals alone have three small auscultory bones in the tympanic cavity — a feature that is connected with the characteristic modification of their maxillary joint. Their red blood-cells have no nucleus, whereas this is retained in all other vertebrates. Finally, it is only in the mammals that we find the remarkable function of the breast-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 are 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 number of young at a time, have several mammary glands at the breast. Hedge-hogs and sows have five pairs, mice four to the pairs, dogs and squirrels four pairs, cats and bears three pairs, most of the ruminants and many of the rodents two pairs, each provided with a teat or nipple ( maslos ). In the various genera of the half-apes (lemures) the number varies a good deal. On the other hand, the bats and apes, which only beget one young at a time as a rule, have only one pair of mammary glands, and these are found at the hrca^t as in man. These variations in the number or structure of the 266 THE VERTEBRATE CHARACTER OF MAX mammary apparatus ( mammarium ) have become doubly interesting 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 (hypermastism) and Fig. 106 A, B, C, D.— Instances of redundant mammary glands and nipples^ hypermastism ). As. pair of small redundant breasts (with two nipples on the left) above the large normal ones; from a 45-year-old Berlin woman, who had had children 17 times (twins twice). (From Hansemann.) 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 Warschau. (From Netigebaur.) C three pairs of nipples : two pairs on the normal glands and one pair above; from a 19-year-old Japanese maiden. D four pairs of nipples: one pair above the normal and two pairs of small accessory nipples underneath; from a 22-year-old Baden soldier. (From Wicderslnim.) corresponding teats (hyperllielism) in both sexes. Fig. 106 shows four cases of this kind — A, B, and Cof three women, and D of a man. They prove that all the above-mentioned numbers may be found occasionally in man. Fig. 106 A THE VERTEBRATE CHARACTER OF MAN 267 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 abo\e the breast is not infrequently represented in ancient statues of Venus. In Fig. 106 C we have the same phenomenon in a Japanese girl o\~ nineteen, who has two nipples on each breast besides (three pairs altogether). Fig. 106 D 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 oi five pairs (as in the pig and hedge-hog) was found in a Polish servant of twenty-two who had had several children ; milk was given by each nipple ; there were three pairs of redundant nipples above and one pair under- neath the normal and very large breasts (Fig. 106 B). A number of recent investigations (especially among recruits) have shown that these things arc not uncommon in the male as well as the female sex. They can only be explained by phylogeny, which attributes them to atavism and latent heredity. The earlier ancestors of all the primates (including man) were lower placentals, which had, like the hedge-hog (one of the oldest forms of the living placentals), several mammary glands (five 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 develop- ment of the atrophied structures. Special notice should be taken of the arrangement of these accessory mammae ; they form, as is clearly seen in Fig. 106 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 polymastic lower placentals are arranged in similar lines. The phylogenetie explanation of polymastism, 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 15 mm. long) the microscopic traces Fig. 107.— A Greek gyneeomast. 77//; VERTEBRATE CHARACTER OF MAN 269 of five pairs of 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 polvthelic lower primates (lemures). But the milk-gland of the mammal has a great morpho- logical 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 sexes. However, it is usually active only in the female sex, and yields the valuable " mother's milk"; in the male sex it is small and inactive, a real rudi- mentary organ of no physiological interest. Nevertheless, in certain cases we find the breast as fully developed in man as in woman, and it may give milk for feeding the young. We have a striking instance of this gynecomastism (large milk-giving breasts in a male) in Fig. 107. I owe the photograph (taken from life) to the kindness of Dr. Ornstein, of Athens, a German physician, who has rendered service by a number of anthropological observations (for instance, in several cases of tailed men). The gynecomast in question is a Greek recruit in his twentieth vear, who has both normally developed male organs and a very pronounced female breast. It is noteworthy that the other features of the structure are 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 sculptors often produced. Hut the man would only be a real hermaphrodite if he had ovaries internally besides the (externally visible) testicles. I observed a very similar case during my stay in Ceylon (at Belligemnia) in 188 1. A young Cinghalese in his twenty- fifth year was brought to me as a curious hermaphrodite, half- man and half-woman. His large breasts gave plenty o( 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 in Fig. 107. As the Cinghalese are small o( stature THE VERTEBRATE CHARACTER OF MAN 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 part) and the dressing of the hair (with a comb), I first took the beardless youth to be a woman. The illusion was greater, as in this remarkable case gynecomastism was associated with cryptorchism — 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 (as in Fig. 107). It was clear that this apparent hermaphrodite also was a real male. Another case of 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-born child on his own breast in despair ; and the continuous stimulus of the child's sucking movements had revived the activity of the mammary glands. It is possible that nervous suggestion had some share in it. Similar cases have been often observed in recent years, even among other male mammals (such as sheep and goats). The great scientific interest of these facts is in their bearing on the question of heredity. The stem-history of the mammarium rests partly on its embryology (Chapter XXIV.) and partly on the facts of comparative anatomy and physiology. As in the lower and higher mammals (the monotremes, and most of the marsupials) the whole lactiferous apparatus is only found in the female ; and as there are traces of it in the male only in a few younger marsupials, there can be no doubt that these important organs were originally found only in the female mammal, and that ihey were acquired by these through a special adaptation to habits of life. Later, these female organs were communicated to both sexes by heredity ; and they have been maintained in all THE VERTEBRATE CHARACTER OF MAN persons o( either sex, although they are not physiologically active in the males. This normal permanence of the female lactiferous organs in both sexes of the higher mammals and man is independent of any selection, and is a fine instance of the much-disputed " inheritance of acquired characters." ELEVENTH TABLE SYNOPSIS OF THE CHIEF ORGANS OF THE PROVERTEBRATES (THE HYPOTHETICAL PRIMITIVE VERTEBRATES) AND THEIR DEVELOPMENT (PROSPONDYLUS) Four Secondary Germinal Layers. Synonyms of the Layers. Fundamental Organs of the Primitive Vertebrates. I. Sensory layer (skin-sense-layer) neuroblast. Lamina neuralis outer limiting' layer. (Sensation. J Skin-layer (Baer). Primary animal layer. Outer skin (epidermis) (simple cell-layer on the outer surface of the bodv). Nervous system (sensorium). 2. A. Medullary tube (nervous centre). 2. B. Peripheral nervous system. Sense-organs (sensillaj. 3. A. Nose (olfactory pits). 3. B. Eyes. 3. C. Auscultory vesicles (stato- cysts). II. (skii Muscular layer i-fibrous-layer) mvoblast. Lamina parietalis outer middle layer. (Movement.) Fleshv-layer (Baer). (Mainly used for construction of the episomites and somato- pleura. ) 4. Corium (cutis-plate). 5. Muscular wall of the trunk (motorium) (metamerous lateral muscles). 6. Chord-sheath (perichorda) (skeletal base). III. Sexual layer (gut-fibrous-layer) gonoblast. Lamina viseeralis inner middle layer. (Reproduction . ) Vascular layer (Baer). (Mainly used for construction of the hypo- somites and the splanchno- pleura. ) 7. Fore kidneys (pronephridia) (metamerous ccelom-canals). 8. Sexual glands (gonades) (metamerous ventral coelom- pouehes). 9. Vascular system (vasorium). 9. A. Ventral principal vein. Heart. 9. B. Dorsal aorta (principal artery). 10. Ventral muscular wall and mesentery (fibrous wall of the gut). 10. A. Skeleton and muscles of the gill-arches (visceral skeleton ). 10. B. Muscular wall of the hepatic IV. Glandular layer (gut-gland-la ver) enteroblast. Lamina enteralis inner limiting' layer. (Nutrition. ) M ucous laver (Baer). Primary vegetal layer. 1 1 . Chorda dorsalis (notochorda) (axial rod), unarticulated. 12. Gut-epithelium (gastro- dermis). 12. A. Epithelium of the head or gill-gut. 12. B. Epithelium of the trunk or liver-gut. CHAPTER XII. EMBRYONIC SHIELD AND GERMINATIVE AREA Cenogenetic characteristics of amniote embryology. The classic hen's egg as .1 source of error. False antithesis of germ and yelk. The yolk belongs to the vegetal half. Yelk-germ and yelk-glajids of the amphibia. Flal germinal disk o',' the birds and reptiles. Severance of it from the yelk-sac. Primary, secondary, and tertiary embryonic si tges of the vertebrate. The so-called blastula of the mammal (germinal gut-vesicle or blastocyst). Its origin by modification of the feeding of the young. Descent of the viviparous mammals from oviparous. Envelopes of their epigastrula (covering layer). Conversion of the two-layered into the four-layered germinal disk. Dark and tight germinative area. Embryonic shield ( embryaspis ) or dorsal shield ( notaspis), embryonic formation. Relation of the germinative area to the permanent gut (menosoma). The continued inheritance and subsequent loss of the food-yelk in the vertebrates. Influence of these cenogenetic processes on the modification of the gastrula. THE three higher classes of vertebrates which we call the amniotes — the mammals, birds, and reptiles — are notably distinguished by a number of peculiarities of their develop- ment from the five lower classes of the stem — the animals without an amnion (anamnia or ichthyopoda). All the amniotes have a distinctive embryonic membrane known as the amnion (or " water-membrane "), and a special embryonic appendage — the allantois. They have, further, a consider- able yelk-sac, which is filled with food-yelk in the reptiles and birds, and with a clear corresponding fluid in the mammals. In consequence of these cenogenetic structures, the original features of the development of the amniotes are so much altered that it is very difficult to reduce them to the pa j in- genetic embryonic processes of the lower amnion-less verte- brates. The gastraea theory shows us how to do this, by representing the embryology of the lowest vertebrate, the skull-less amphioxus, as the original form, and deducing from it, through a series oi gradual modifications, the gastru- lation and ceelomation of the craniota. It was somewhat fatal to the true conception of the chief -7.1 T EMBRYOXIC SHIELD AND GERMIXATIVE AREA embryonic processes of the vertebrate that all the older embryologists, from Malpighi (1687) and Wolff (1750) to Baer (182S) and Remak (1S50), 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 embryological research is, as we have seen, a source of dangerous errors. The large globular food-yelk of the bird's egg causes, in the first place, a flat 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 antithesis of germ and yelk. The latter was regarded as a foreign body, extrinsic to the real germ, whereas it is really a part of it, an embryonic organ of nutrition. Many authors said there was no trace of the embryo until a later stage, and outside the yelk ; sometimes the two-layered embryonic disk itself, at other times only the central axial portion of it (as distinguished from the germinative area which we will describe presently) was taken to be the first outline of the embryo. 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 therefrom, belong to the embryo. As the large original yelk-mass in the undivided egg of the bird only represents an inclosure in the greatly enlarged ovum, so the later content of its embryonic yelk-sac (whether yet segmented or not) is only a part of the entoderm which forms the primitive gut. This is clearly shown by the amphiblastic ova of the amphibia and cyclos- toma, which explain the transition from the archiblastic yelk-less ova of the amphioxus to the large yelk-filled ova of the reptiles and birds. It is precisely in the study of these difficult features that we see the incalculable value of phylogenetic considerations in explaining complex ontogenetic facts, and the need of EMBRYONIC SHIELD .IX/> GERMINATIVE AREA 275 separating cenogenetic phenomena from palingenetic. This is particularly clear as regards the comparative ontogeny of the vertebrates, because here the phylogenetic unity of the stem has been already established by the well-known facts of paleontology 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 constantly recurring. A wrong idea of the formation of the yelk not only led astray the most and best of the older embryologists, but the same thing not infrequently happens in our time. We have a recent instance in the excellent work, On the Embryology and Anatomy of the Ceylon Ichthyophis Glutinosus. Those admirable observers, the brothers Paul and Fritz Sarasin, formulated the thesis, in the third part of this work (18S9), that "the two germinal layers of the gastrula do not corre- spond to the entoderm and ectoderm, but to the blastoderm and yelk of the vertebrate," and thought they had thus " provided the foundation for a comparative embryology of the animal kingdom." On their view, " the gastrula consists of two layers, of which the inner is the lecithoblast and the outer the blastoderm." The misinterpretation oi facts and confusion of ideas which lie at the bottom of these opinions are due to the supposition that in every case the yelk is a part of the vegetal half of the embryo. As the undivided food-yelk is only a portion of the contents of the vegetal hemisphere of the ovum in the unicellular germ (the stem-cell), so we must always regard the divided food-yelk as a part of the ventral wall of the primitive gut in the multicellular embryo. The yelk embryo, or lecithoblast, of Sarasin is only a limited portion of the entoderm — that portion which developes in the ventral wall of the primitive gut from its central part; as " yelk-gland " ( lec it hade nia ) it is just as much a subordinate glandular part of the whole gut-tube as the visceral glands (liver, lungs, etc.) that afterwards grow out of it. On the other hand, the dorsal part of the embryo, which Sarasin opposes as " blastoderm " to the ventral lecithoblast, is by no means the original embryonic membrane (embracing all the 276 EMBRYONIC SHIELD AND GERM/NATIVE AREA embryonic cells), the real blastoderm, but the relic of the entoderm and the whole of the ectoderm. In many other cases also the cenogenetic relation of the embryo to the food-yelk has until now given rise to a quite wrong idea of the first and most important embryonic processes in the higher vertebrates, and has occasioned a number of false theories in the ontogeny of them. Until thirty years ago the embryology of the higher vertebrates always started from the position that the first structure of the embryo is a flat, leaf- shaped disk ; it was for this reason that the cell-layers that compose this germinal disk (also called germinative area) are called " germinal layers." This flat germinal disk (blasto- discusj, which is round at first and then oval, and which is often described as the scar or cicatricula in the laid hen's egg, is found at a certain part of the surface of the large globular food-yelk. I am convinced that it is nothing else than the discoid, flattened gastrula of the birds (dtscogastrula). At the beginning of germination the flat embryonic disk curves outwards, and separates on the inner side from the underlying large yelk-ball. In this way the flat layers are converted into tubes, their edges folding and joining together (Fig. 10S). As the embryo grows at the expense of the food-yelk, the latter becomes smaller and smaller ; it is 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 (saccus vite/linus or vestcula umbilicalis, Fig. 108 nb). This is enclosed by the visceral layer, is connected by a thin stalk, the yelk-duct (ductus vitellinus), with the central part of the gut-tube, and is 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 atrophies, 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 vertebrates was mainly based on the chick, and regarded the antithesis of EMBRYONIC SHIELD AND GERMINATIVE AREA embryo (or formative-yelk) and food-yelk (or yelk-sac) as original, it had also to look- upon the flat leaf-shaped structure oi the germinal disk as the primitive embryonic form, and emphasise the fact that hollow grooves were formed of these flat layers by folding, and closed tubes by the joining together oi their edges. Fig. ioS. -Severance of the discoid mammal embryo from the yelk-sac, in transverse section (diagrammatic). .1 The germinal disk (li, hf) lies flat on one side of the gill-gut vesicle (kb). />' In the middle of the germinal disk we find the medullary groove (tar), and underneath it the chorda ( ch ). C The gut-fibre-layer fdf) has been enclosed by the gut-gland- layer (ad). l> The skin-fibre-layer (hf) and gut-fibre-layer ft//') divide at the periphery ; the gu( I 1/ ) begins to separate from the yelk-sac or umbilical vesicle /■'. The medullary tube ( mr j is closed; the body-cavity (c) begins to form. /•' The provertebrae ( w) begin to grow round the medullary tube (mr) ami the chorda ( rh ) : the gut (a) is cul off from the umbilical vesiclefni/ // The vertebrae I w) 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 : /; horn-plate, mr medullary tube, hf skin-fibre-layer, tc provertebrae, ch chorda, c body-cavity or cceloma, fibre-layer, dd gut-gland-layer, d gut-cavity, nb umbilical vesicle. This idea, which dominated the whole treatment o( the embryology oi the higher vertebrates until thirty years ago, was totally false. The gastraea theory, which has its chief application here, teaches us that it is the very reverse of the truth. The cup-shaped gastrula, in the body-wall oi which the two primary germinal lasers appear from the first as closed tubes, is the original embryonic form of all the EMBRY0X1C SHIELD AXD GERMIXATIVE AREA vertebrates, and all the invertebrate metazoa ; and the flat germinal disk with its superficially expanded germinal layers is a later, secondary form, due to the cenogenetic formation of the large food-yelk and the gradual spread of the germ- layers over its surface. Hence the actual folding of the germinal layers and their conversion into tubes is not an original and primary, but a much later and tertiary, evolu- tionary process. In the phylogeny of the vertebrate embryonic process we may distinguish the following three stages : — A. First Stage : B. Second Stage : C. Third Stage : Primary Secondary Tertiary (palingenetic) (cenogenetic) (cenogenetic) embryonic process. embryonic process. embrj'onic process. The germinal layers form from the first closed tubes, the one-layered blastula being converted into the two - layered gastrula b}' invagination. No food-yelk. (A mph ioxus. ) The germinal layers spread out leaf-wise, food-yelk gathering in the ventral entoderm, and a large yelk-sac being formed from the middle of the gut-tube, (Amphibia.) The germinal layers form a flat germinal disk, the borders of which join together and form closed tubes, separating from the central yelk-sac. ( Amniotes. J As this theory, a logical conclusion from the gastraea theory, has been fully substantiated by the comparative study of gastrulation in the last few decades, we must exactly reverse the hitherto prevalent 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 permanent or after-gut (metagaster)^ or the permanent alimentary canal, and the yelk-sac ( lecithoma ) or umbilical vesicle. This is very clearly shown by the com- parative ontogeny of 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 of EMBRYONIC SHIELD AND GERMINATIVE AREA the primitive gut. Bui it afterwards becomes so large that a part of the yelk does not divide, and is used up in the yelk- sac that is cut off outside. When we make a comparative study of the embryology of the amphioxus, the frog, the chick, and the hare (Plates II., III.), there cannot, in my opinion, be any further doubt as to the truth of this position, which I have held for thirty years. Hence in the light of the gastraea theory we must regard the features of the amphioxus as the only and real primitive structure, departing very little from the palingenetic embrvonic form, among all the vertebrates. In the cyclostoma and the frog these features are, on the whole, not much altered cenogeneticallv, but very much so in the chick, and most of all in the hare. In the bell-gastrula of the amphioxus and in the crested gastrula of the petromyzoa and the frog the germinal layers are found to be closed tubes or vesicles from the first (Plate II., Figs. 6, 11). On the other hand, the chick-embrvo (in the new laid, but not yet hatched, egg) is a flat circular disk, and it was not easy to recognise this as a real gastrula. Rauber and Goette have, however, achieved this. As the discoid gastrula grows round the large globular yelk, and the after-gut or permanent gut then separates from the outlying velk-sac, we find all the processes which we have shown (diagrammatically) in Fig. 108 — processes that were hitherto regarded as principal acts, whereas they are merely secondary. The oldest, oviparous mammals, the discoblastic mono- tremes, behave in the same way as the sauropsida (reptiles and birds). But the corresponding embryonic processes in the viviparous mammals, the marsupials and placentals, are very elaborate and distinctive. They were formerly quite misinterpreted ; it was not until the publication of the studies of Edward van Beneden (1875) and the later 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 embrvonic forms of the lower vertebrates. Although there is no independent food-yelk, apart from the formative yelk, 2So EMBRYOXIC SHIELD AXD GERMIXAT1VE AREA in the mammal ovum, and although their segmentation is total on that account, nevertheless a large yelk-sac ( lecithoma ) 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 foqd-yelk and partial segmen- tation. In the mammals, as well as in the latter, the flat, leaf-shaped germinal disk (blastodiscus ) separates from the yelk-sac, and its edges join together and form tubes. How, then, can we explain this curious anomaly? Only as a result of very characteristic and peculiar cenogenetic modifications of the 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 (1884) that the monotremes, the lowest and oldest of the mammals, still lav eggs, and that these develop like the discoblastic ova of the reptiles and birds. Their nearest descendants, the marsupials, formed the habit of retaining the eggs, and developing them in the oviduct ; the latter was thus converted into a womb (uterus). A nutritive fluid that was secreted from its wall, and transuded 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 meroblastic monotremes was gradually atrophied, and at last disappeared so completely that the partial ovum- segmentation of their descendants, the rest of the mammals, once more became total. From the discogastrula of the former was evolved the distinctive epigastrula of the latter. It is only by this phylogenetic explanation that we can understand the formation and development of the peculiar, and hitherto totally misunderstood, blastula of the mammal. This vesicular condition of the mammal embryo was dis- covered 200 years ago (1677) by Regner de Graaf. He found in the uterus of a hare four days after impregnation small, round, loose, transparent vesicles, with a double EMBRYONIC SHIELD AND GERM/NATIVE AREA envelope. However, Graaf's discovery passed without recog- nition. It was not until 1827 that these vesicles were re-discovered by Baer, and then more closely studied in 1S4J by Bischoff in the hare (Figs. 109, no). They are found in the womb of the hare, the dog, and other small mammals, a lew days after copulation. The mature ova o\ the mammal, when they have left the ovary, are fertilised either here or in the oviduct immediately afterwards by the invading sperm-cells.1 (As to the womb and oviduct see Chapter XXIX.) The cleavage and formation of the gastrula take place in the oviduct. Either here in the oviduct or after the mammal gastrula has passed into the Fig, 109. Fig. i 10. Fig. 109. The visceral embryonic vesicle (blastocysts or gastrocystis) of a hare 11 Ik- " blastula" or vesicula blastodermica of other writers). " outer envelope (ovolemma), 6 skin-layer or ectoderm, forming tin' entire wall ol the yelk-vesicle, c groups of dark cells, representing the visceral layer or entoderm. Fig. mo. The same in sections, Letters as above. ) lias withdrawn ;i little from the smooth ovolemma In tlio middle ot~ the ovolemma we see the round germinal disk (blast odiscus, c), at the edge of which (at . The yelk-sac and vessels of the germinative area, which soon spread over its whole surface, are, therefore, real embryonal organs, or temporary parts of the embryo, and have a transitory importance in connection with the nutrition of the growing later body ; the latter may be called the " permanent body " (menosoma) in contrast to them. The relation of these cenogenetic features of the amniotes to the palinijenetic structures of the older non-amniotic vertebrates may be expressed in the following theses: The original gastrula, which completely passes into the embryonic bodv in the acrania, cyclostoma, and amphibia, is early divided into two parts in the amniotes — the embryonic shield (embtyaspis), which represents the dorsal outline of the permanent body (menosoma); and the temporary 29o EMBRYOXIC SHIELD AND GERMIXATIVE AREA embryonic organs of the germinative area and its blood- vessels, which soon grow over the whole of the yelk-sac. The differences which we find in the various classes of the vertebrate stem in 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 modifications 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 polyphyletic increase and decrease of the nutritive yelk causes in the form of the gastrula, and especially in the situation and shape of the primitive mouth. The primitive mouth or prostoma is originally a simple round aperture at the lower (aboral) pole of the long axis ; its dorsal lip is above and ventral lip below. In the holo- blastic amphioxus this primitive mouth is a little eccentric, or shifted to the dorsal side (Fig. 41). The aperture increases with the growth of the food-yelk in the cyclostoma 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. 122 b). In the wide-mouthed, circular discoid gastrula of the selachii or primitive fishes, which spreads quite flat on the large food-yelk, the anterior semi-circle of the border of the disk is the ventral, and the posterior semi- circle the dorsal lip (Fig. 122 A). The amphiblastic amphibia are directly connected with their earlier fish-ancestors, the dipneusts and ganoids, and further the oldest selachii (cestracion) ; they have retained their total unequal segmenta- tion, and their small primitive mouth (Fig. 122, C, ab) is blocked up by the yelk-stopper, lies at the limit of the dorsal and ventral surface of the embryo (at the aboral pole of its equatorial axis), and there again has an upper dorsal and a lower ventral lip (a, b). The formation of a large food-yelk followed again in the stem-forms of the amniotes, the protaminotes or proreptilia, descended from the amphibia (Fig. 122 D). But here the accumulation of the food-yelk took place only in the ventral wall of the primitive-gut, so that EMI! R YONIC SHIELD . I X/> GBRMIN. I II I rE . I RE. 1 291 the narrow primitive mouth Lying behind was forced upwards, and came to lie on the back of the discoid " epigastrula " in the shape of the "primitive groove"; thus (in contrast to the case of the selachii, Fig. 122 J) the dorsal lip (b) had to be in front, and the ventral \\p(i/J behind (Fig. 122 D). This feature was transmitted to all the amniotes, whether they retained the large food-yelk (reptiles, birds, and monotremes), or lost it by atrophy (the viviparous mammals). - Fig. 122. -Median longitudinal section of the gastrula of four vertebrates. (From RabT.) .1 discogastrula of a shark (pristiurus). 11 amphigastrula of a sturgeon ( accipenser ). C amphigastrula of an ampbibium (tritniij. D epigastrula of an amniote (diagram), a ventral, b dorsal lip o\ the primitive mouth. This phvlogenetic explanation oi gastrulation and ccelo- mation and the comparative Study oi them in the various vertebrates throw a clear and full light on many ontogenetic phenomena, as to which the most obscure and confused opinions were prevalent thirty years ago. In this we see especially the high scientific value o ventral nonv narrow hinder end ; it is in the median line o\ this that the primitive streak appears (Fig. 124 ps). The narrow longi- tudinal groove or meridian furrow in it — the so-called •• primitive groove" — is, as we have seen, t he primitive mouth Fig. 1 23. -Embryonic vesicle of a seven-days' old hare with oval embryonic shield (ag)- -< seen from above, B from the side. (From Kolliker.) ay dorsal shield (notaspis)or embryonic spot ( un-u embryonalis). In li the upper half of the vesicle is made up of the two primary germinal layers, the lower (up to e») only from the outer layer. Fig. 124. Oval embryonic shield of the hare (Fig. 124 •' i>|' six days eighteen hour-, il of eight days). (From KSUiker.) ps primitive streak, />/• primitive groove, arg area germi- nalis, sw sickle-shaped terminal growth. o( the gastrula. In the gastrula-embryos of the mammals, which are much modified cenogenetically, this cleft-shaped prostoma is lengthened so much that it soon traverses the whole o\ the hinder hall" of the dorsal shield ; as we find in a DORSAL BODY AND VENTRAL BODY hare-embryo of six to eight days (Fig. 125 pr). The two swollen parallel borders that limit this median furrow are the lateral lips of the primitive mouth, right and left. In this way the bilateral, dipleurous, or bilateral-symmetrical 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-half of the dorsal shield a median furrow quickly makes its appearance (Fig. 125 rf). This is the broader dorsal furrow or medul- lary groove, the first structure of the central nervous system. The two parallel dorsal or medullary swel- lings that enclose it grow together over it afterwards, and form the me- dullarv tube. As is seen in trans- verse sections, it is formed only of the outer germinal layer (Figs. 139, 140). The 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 coelom pouches grow between the primary germinal layers. Thus the median primitive furrow (pr) in the hind-half 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 observer to be merely the forward continuation of the former. Hence they were formerly Fig. 125.— Dorsal shield (ag) and germinative area of a hare-embryo of eight days. (From Kblliker.) pr primitive groove, //"dorsal furrow. />OA'S.l/. BODY AND VENTRAL BODY always contused, and in the oldest and much-copied illustra- tion of the dorsal shield of the hare which Bischoff gave in [842 (Fig. 120) one simple longitudinal furrow goes the whole length of the middle line. This error was the more pardonable as immediately afterwards the two grooves do actually connect in a very remarkable way. The two parallel dorsal swellings, which pass into each other arch-wise in front, diverge in the rear and embrace the anterior end of the primitive groove (Fig. 125). They then grow together over it in such a way that the primitive groove (or the hindermost cavity of the primitive gut) passes directly into the closing medul- lary tube. The point of transi- tion is the remarkable neurenteric Fig. 126. Fk- I27- Fig. .-•<•. Embryonic shield of a hare of eight days. (From Van Beneden.) ^-primitive groove, en canalis neurentericus, nk nodus neurentertcus (or • Hensen's knot "), */ head-process (chorda). Fig 1 -t. Longitudinal section of the ccelomula of amphioxus (from the left), lentoderm, d primitive gut, en medullary duct, n nerve-tube, m mesoderm, s Brst primitive segment, c ccelom-pouches. (From Hatschek. ) canal (Fig. 127 en). The thickened mass at the border of the primitive mouth, which surrounds it, is the neuren- teric knot (or "Hensen's knot." Fig. 126 nk). The direct connection which is thus established between the two cavities of the primitive gut and the medullary tube does not last long; the two are soon definitely separated by a partition. The enigmatic canalis neurentericus is a very old embryonic organ, and of great phylogenetic interest, because it arises in the same way in all the chordoma (both tunieates and vertebrates). In every case it touches or embraces like DORSAL BODY AXD VEXTRAL BODY an arch the posterior end of the chorda, which has been developed here in front out of the middle line of the primitive gut (between the two coelom-folds of the sickle-groove) ("head-process," Fig. 126 kf). These very ancient and strictly hereditary structures, which have no physiological significance to-day, deserve (as "rudimentary organs ") our closest attention. The tenacity with which the useless neurenteric canal has been transmitted down to man through the whole series of vertebrates is of equal interest for the theory of descent in general, and the phylogeny of the chordonia in particular. Fig. 12S. Fig. 129. r • Fig. 128.— Longitudinal section of the ehordula of a frog. (From Balfour. ) nc nerve-tube, x canalis neurentericus, al alimentary canal, _vk yelk- cells, m mesoderm. Fig. 129.— Longitudinal section of a frog-embryo. (From Goette.) m mouth, / liver, an anus, ne canalis neurentericus, mc medullary tube, pn pineal gland (epiphysis), eh chorda. The connection which the canalis neurentericus (Fig. 127 en) establishes between the dorsal nerve-tube (11) and the ventral gut-tube ( d ) is seen very plainly in the amphioxus in a longitudinal section of the ccelomula, as soon as the primitive mouth is completely closed at its hinder end. The medullary tube has still at this stage an opening at the forward end, the neuroporus (Fig. 86 np). This opening also is afterwards closed. There are then two completely closed canals over each other — the medullary tube above and the gastric tube below, the two being separated by the chorda. The same features as in the acrania are exhibited by the related tunicates, the ascidia (Plate XVIII., Figs. 5, 6). poks.i/. hoiiv axd ventral nunv Again, we find the neurenteric canal in just the same form and situation in the amphibia. A longitudinal section of a young tadpole (Fig. i 2S) shows how we may penetrate from the still open primitive mouth (x) 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. i2(), ne) represents the arched connec- tion between the dorsal medullary canal ( ' mc ) and the ventral gastric canal. In the amniotes this original curved form of the neurenteric canal cannot be found at first, because here the J>V Fig. 130. Figs. 130 and 131.- Dorsal shield Of the Chick. (From Bal/bur.) The medullary furrow (me), which is not \vt visible in Ki_<. 130, encloses with it> hinder end the fore >-nil of the primitive groove (pr) in Fig. 131. primitive mouth travels completely over to the dorsal surface of the gastrula, and is converted into the longitudinal furrow we call the primitive groove. Hence the primitive groove (I'ig. 131 pr), examined from above, appears to be the straight continuation of the fore-lying and younger medullary furrow (me). The divergent hind legs of the latter embrace the anterior end of the former. Afterwards we have the complete closing of the primitive mouth, the dorsal swellings joining to form the medullary tube and growing over the prostoma. The canalis neurentericus then DORSAL BODY AND VENTRAL BODY leads directly, in the shape of a narrow arch-shaped tube (Fig. 132 ne), from the medullary tube (sp) to the gastric tube (pag ). Directly in front of it is the latter end of the chorda fc/ij. Fig. 132.— Longitudinal section of the hinder end of a chick. (From Balfour.) sp medullar}- tube, connected with the terminal gut (pag) by the neurenteric canal ( ne ), ch chorda, pr neurenteric (or Hensen's) knot, al allan- tois, ep ectoderm, hy entoderm, so parietal laj-er, sp visceral layer, «« amis- pit, am amnion. While these important processes are taking place in the axial part of the dorsal shield, its external form also is changing. The oval form (Fig. 120) becomes like the sole of a shoe or sandal, lyre- shaped or finger biscuit- shaped (Fig. 133). The middle third does not grow in width as quickly as the posterior, and still less than the anterior third ; thus the shape of the per- manent body becomes somewhat narrow at the waist. At the same time the oval form of the ger- minative area returns to a circular shape, and the inner pellucid area separates more clearly from the opaque outer area (Fig 134 a). The completion of the circle in the area marks the limit of the formation of blood-vessels in the mesoderm. Fig. 133.— Germinal area or ger- minal disk of the hare with sole- shaped embryonic shield, magnified about ten times. The clear circular field (d) is the opaque area. The pellucid area (c) is lyre-shaped, like the em- bryonic shield itself ( b). In its axis is seen the dorsal furrow or medullary furrow (a). ( From Bisrhoff. ) /XWS.I/. /:<)/>)■ AND VENTRAL HODY fir- A- Fig. 134. .Medullary ~A groove Neurenteric ™ Fig. 1 j& Fig. 134. —Embryo of the opossum, sixty hours old, lour mm. in diameter. I From Selenia. ) ^' the globular embryonic vesicle, u the round ger- minative area, h limit of the ventral plates, r dorsal shield, o its fore part, u the first primitive seg- ment, c/; chorda, chr its fore-end, />>- primitive groove (or mouth). Fig. 135. Sandal- shaped embryonic shield of a hare of eight days, with the fore part of the germina- tive area (<"' opaque, ap pellucid area). (From Kiilliker. ) rf dorsal fur- row, in the middle of the medullary plate, /;, />r primitive groove I mouth 1, Sta dorsal (Mem) zone, pa ventral (parietal I /one. In the narrow middle part the first three primi- tive segments may be Fig. 136. Human embryo at the sandal-stage, two mm. long, d of the second week, magnified twenty-five times. (From Count Spec in. long, from ih» 1 DORSAL BODY AXD VEXTRAL BODY The characteristic sandal-shape of the dorsal shield, which is determined by the narrowness of the middle part, and which is compared to a violin, lyre, or shoe sole, persists for a long time in all the amniotes. All mammals, birds, and reptiles have substantially the same construction at this stage, and even for a longer or shorter period after the division of the primitive seg- ments into the ccelom-folds has begun (Fig. 1 35). The human embryonic shield assumes the sandal-form in the second week of develop- ment ; towards the end of the week our sole-embryo has a length of about one line or two millimetres (Fig. 136). (Cf. Plates IV. and V.) The complete bilateral svmmetrv of the vertebrate bodv is very early indicated in the oval form of the em- bryonic shield (Fig. 120) by the median primitive streak ; in the sandal-form it is even Fig. 137. — Sandal-shaped em- bryonie shield of a hare of nine more pronounced (Figs. 134- days. (From Kiillikcr.) (Back view „, T1 - . r from above. I sts stem-zone or dorsal I3b>- -1 lle axial Organs Ot shield (with eight pairs of primitive seg- the mjdclle plane (the primi- merits), ps parietal or ventral zone, tip l \ r pellucid area, af amnion-fold, h heart, tive streak behind, the me- p/i pericardial cavity, t« omphalo- . mesenteric vein, n/, eye-vesicles, ;•/; tore dtlllary tube in front, and the brain, ink middle brain, //// hind brain, , , j„ „„,.u\ ^<-.'ll uw primitive segments (or vertebrae). chorda underneath) are Still more clearly differentiated in the sole-shaped embryonic shield, and so are the lateral organs that develop symmetrically to the right and left of them. In these lateral organs of the embryonic shield a darker central and a lighter peripheral zone become more obvious ; the former is called the stem-zone (Fig. 137 stz), SANDAL-EMBRYOS OF SAUROPSIDA The Evolution of Man. 1'. Ed. PL IV SANDAL-EMBRYOS OF MAMMALS The Evolution of Man. V.Ed. PL V. U Man (homo) DORSAL BODY AND VENTRAL BODY and the latter the parietal zonefps); from the first we gel the dorsal and from the second the ventral half of the body-wall. The stem-zone o( the amniote embryo would be called more appropriately the dorsal zone or dorsal shield ; from it developes the whole of the dorsal half o( the later body (or permanent body) — that is to say, the dorsal body (episoma). Again, it would be better to call the "parietal zone" the ventral zone or ventral shield ; from it develop the ventral "lateral plates," which afterwards separate from the embryonic vesicle and form the ventral body (hypotonia ) — 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 construction which Fig. [37 illustrates in the hare and Fig. i,yS in the opossum, so like each other that we can either not distinguish them at all or only by means of quite subordinate peculiarities in the size of the various parts. Moreover, the human sandal-shaped embryo cannot at this stage be distinguished from those of other mammals, and it particularly resembles that of the hare. I have given on Plates IV. and V. the sandal-shaped embryos of six different amniotes for the purpose of comparison, and have reduced them to the same size ; all of them are highly magnified. Plate IV. shows the sandal-shaped embryonic shield (at three stages of development) of three of the sauropsids : E lizard (lacerta), C tortoise (chelonia), II hen (gallus). Plate V. gives the embryos of three mammals: 5" pig (sits). A' hare (lepus), M man (homo). On the other hand, the outer form of these Hat sandal-shaped embryos is very different from the corresponding form oi the holoblastic lower animals, especially the acrania (amphioxus). Nevertheless, the body is just the same in the essential features of its structure as that we find in the chordula ol' the latter (Figs. NO -89), and in the segmented embryonic forms which immediately develop from it. The striking external difference is here again due to the fact that in the palingenetic embryos o( the amphioxus (Figs! 80, 87) and the amphibia DORSAL BODY AXD VEXTRAL BODY (Figs. 88. 89) the gut-wall and body-wall form closed tubes from the first, whereas in the cenogenetic embryos of the amniotes they are forced to expand leaf-wise on the surface owing: to the great extension of the food-velk. Fig. 138.— Sandal-shaped embryonic shield of an opossum (dideU phys), throe days old. (From Selenia.) (Back view from above.) s/c stem- zone or dorsal shield (with eight pairs of primitive segments), ps parietal or ventral zone, ap pellucid area, no opaque area, /;/; halves of the heart, v lore- end, // hind-end. In the median line we see the chorda (ch) through the transparent medullary tube (m). 11 primitive segment, pr primitive streak (or primitive mouth). It is all the more notable that the early separation of dorsal and ventral halves takes place in the same rigidly hereditary fashion in all the vertebrates. In both the acrania and the craniota the dorsal body is about this period separated nOA'S.l/. /,'(>/> V .l.\/> VENTRAL BODY from the ventral body. In the middle part o( the body this division has already taken place by the construction of the axial chorda between the dorsal nerve-tube and the ventral canal. But in the outer or lateral part of the body it is only brought about by the division o( the coelom-pouches into two sections by a frontal constriction — a dorsal episomite (dorsal segment or provertebra) and a ventral hyposomite (or ventral segment). In the amphioxus each of the former makes a muscular pouch, and each of the latter a sex-pouch or gonad. (Cf. the transverse section of the vertebrate. Figs. 104, 105, and Figs. 3-7 on Plate VI.) These important processes of differentiation in the mesoderm, which we will consider more closely in the next Chapter, proceed step by step with interesting changes in the ectoderm, while the entoderm changes little at first. We can study these processes best in transverse sections, made vertically 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 food-yelk (Fig. 139 tltl). 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 ( inch ) ; this separates the two plates of the ccelom-pouches, the lower (visceral) and upper (parietal). The broad dorsal furrow (Rf) formed by the medullary plate (in) is still wide open, but is divided from the lateral horn-plate ( It ) by the parallel medul la r v swell i n gs. As the medullary swellings rise and bend towards each other (Fig. 140 in), one of these parallel longitudinal furrows, the lateral furrow (sulcus lateralis), is formed in the mesoderm on each side. In this lateral furrow we find at first the pro- renal duct (Fig. 141 ung). As the lateral furrow cuts com- pleted through the middle layer, this falls into two sections : the inner or middle part ( 11 ) is the primitive segment piece, which forms the greater part of the Stem-zone, and afterwards DORSAL BODY AND VENTRAL BODY divides by articulation into the chain of somites (in Figs. 137 and 1 38 with eight pairs of somites already). The outer or lateral section is the lateral plate (Fig. 140 sp) ; when we look at it from above it appears as the parietal zone, and afterwardsjdivides into the two fibrous layers. In the fore half of the embryonic shield, which corresponds to the later Fig. 139.— Transverse section of the embryonic shield of a chiek, at the end of the first (.lay of incubation). (From Kolliier.) h horn-plate, m medullary plate, forming the dorsal furrow (Rf), ch chorda, uwh ccelom-cleft, mvp dorsal part of the mesoderm, sp ventral part (lateral plates), dd gut-gland layer. head, there is no separation between the inner provertebral mass and the outer lateral plates. The median innermost part of the lateral plates, which touches the primitive segment piece or provertebral plate, is called the middle plate (Fig. 141, nip). Underneath it we find the first two blood-vessels, the primitive aortas ( ' ao ). During these processes important changes are taking place in the outer germinal layer (the "skin-sense layer"). The continued rise and growth of the dorsal swellings causes Fig. 140.— Transverse section of the embryonic disk of a ehiek at tlie end of the first day of incubation, a little more advanced than Fig. 139, magnified about twenty times. The edges of the medullary plate (m). the medullary swellings (w), which separate the medullary from the horn-plate ( h ). are bending towards each other. At each side of the chorda (ch) the primitive segment plates (u) have separated from the lateral plates (sp). A gut-gland layer. (From Remak.) their higher parts to bend together at their free borders, approach nearer and nearer (Fig. 140 w), and finally unite. Thus in the end we get from the open dorsal furrow, the upper cleft of which becomes narrower and narrower, a closed cylindrical tube (Fig. 141 nir). This tube is of the utmost importance ; it is the first rudiment of the central nervous DOh'SM. /;<)/>)■ AND VENTRAL BODY system, the brain and spinal marrow, the medullary tube (tubus mednllaris ). This ontogenetic fact was formerly looked upon as very mysterious. We shall sec presently thai in the light o( the theory o\ descent it is a thoroughly natural process. The phylogenetic explanation oi~ 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-perception, are accomplished ; hence it was bound to develop originally from the outer and upper surface of the body, or from the epidermis. The medullary tube afterwards separates completely from the outer germinal layer, and is surrounded by the middle parts of the provertebrae and forced inwards (Fig. 151). The remaining portion of the skin-sense layer (Fig. 141 //) is now called the horn-plate or /.,/ eA u*r ao *'p J J iff Fig. 141. -Transverse section of the embryonic shield (of a chick, on the second day o( incubation), magnified about one hundred times. (From KoUiier.) h horn-plate, mr medullary tube, ung prerenal duct, «;.• primitive segments, ///>/ skin-fibre layer, «//> middle plate, df gut-fibre layer, sp coelom- folds, no primitive aorta, dd gut-gland layer. horn-layer, because from it is developed the whole of the outer skin or epidermis, with all its horny appendages (nails, hair, etc.). (Cf. Plates VI. and VII. and the explanation.) A totally different organ, the prorenal (primitive kidney) duct ( ung J, is found to be 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 of the provertebrae (on the outer side, Fig. 141, ung). It originates, it seems, out o( the horn-plate at the side oi 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 o\ the medullary lube from the horn-plate. Other observers think that the first trace oi it does not come from the skin-sense layer, but the skin-fibre layer. DORSAL BODY AA'D VENTRAL BODY The inner germinal layer, or the gut-fibre layer (Fig. 141 dd), remains unchanged during these processes. A little later, however, it shows a quite flat, groove-like depression in the middle line 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, the alimentary canal, in the same way as the medullary groove grows into the medullary tube. The gut- fibre layer (Fig. 142 /), which lies on the gut-gland layer (d), naturally follows it in its folding. Moreover, the incipient gut-wall consists from the first of two layers, internally the gut-gland layer and externally the gut-fibre layer. The formation of the alimentary canal resembles that of the medullary tube to this extent — in both cases a straight groove or furrow arises first of all in the middle line of a flat layer. The edges of this furrow then bend towards each other, and join to form a tube (Fig. 142). But the two processes are really very different. The medullary tube closes in its whole length, and forms a cylindrical tube, whereas the alimentary canal remains open in the middle, and its cavity continues 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, the construction of the navel. The closing of the medullary tube is effected from both sides, the edges of the groove joining together from right and left. But the closing of the alimentary canal is not only effected from right and left, but also from front and rear, the edges of the ventral groove growing together from every side towards the navel. Throughout the three higher classes of vertebrates the whole of this process of the secondary construction of the gut is closely connected with the formation of the navel, or with the separation of the embryo from the yelk-sac or umbilical vesicle. (Cf. Fig. 108, and Plate VII., Figs. 14, 15.) In order to get a clear idea of this, we must understand carefully the relation of the embryonic shield to the germinative DORSAL BODY AND VENTRAL HOI'Y area and the embryonic vesicle. This is done best by a comparison of the five stages which are shown in longitudinal section in Pigs. 143-147. The embryonic shield fc), which at first projects very slightly over the surface o( the germina- tive area, soon begins to rise higher above it, and to separate from the embryonic vesicle. At this point the embryonic shield, looked at from the dorsal surface, shows still the original simple sandal-shape (Figs. 135-138). We do not yet see any trace oi articulation into head, neck, trunk, etc., or limbs. But the embryonic shield has increased greatly in Fig. 14-— Three diagrammatic transverse sections of the em- bryonic disk of the higher vertebrate, to show the origin of the tubular organs from the bending germinal layers. In Fig. A the medullary tube (n J and the alimentary canal ( a J 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 closed ; the prerenal duets ( it ) mc cut oil' from the horn-plate ( h ) and internally connected with segmental prerenal canals. Iii Fig. C both the medullary tube and the dorsal wall above and the alimentary canal and ventral wall below are elosed. All the open grooves have become closed tubes ; the primitive kidneys are directed inwards. The figures have the same meaning in all three figures: /; skin-sense layer, n medullary tube, n prerenal duets, .r axial rod, i primitive-vertebra, r dorsal wall, b ventral wall, r body- cavity or coeloma, /' gut-fibre layer, / primitive artery (aortal. -.< primitive vein (subin testinal vein), d gut-fibre layer, a alimentary canal. (Cf. Plates VI. and VII.) thickness, especially in the anterior part. It now has the appearance of a thick, oval swelling, strongly curved over the surface of the germinative area. It begins to sever com- pletely from the embryonic vesicle, with which it is connected at the ventral surface. As this severance proceeds, the back bends more and more; in proportion as the embryo grows the embryonic vesicle decreases, and at last it merely hangs as a small vesicle from the belly of the embryo (Fig. 147 ]■ .l.\/> VENTRAL HODY severance, a groove-shaped depression is formed at the surface of the vesicle, the limiting furrow, which surrounds the vesicle in the shape of a pit, and a circular mound or dam (Fig. 144 ks) is formed at the outside of this pit by the eleva- tion of the contiguous parts of the germinal vesicle. In order to understand clearly this important process, we may compare the embryo to a fortress with its surrounding rampart and trench. The ditch consists of the outer part of the germinative 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 proceeds peripherally beyond the borders of the embryo over the whole germinative area. At first this middle layer reaches as far as the germinative area ; the whole of the rest of the embryonic vesicle consists in the beginning only of the two original limiting layers, the outer and inner germinal layers. Hence, as far as the germinative area extends the germinal layer splits into the two 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. 145 am). The inner plate, the gut-fibre layer, remains on the inner layer oi~ the embryonic vesicle (on the gut-gland layer). The outer plate, the skin-fibre layer, lies close on the outer layer o( the germinative area, or the skin-sense layer, 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. 144- 147 am). To return to our illustration o( the fortress, we yrelk-sac ( ds ) smaller. In Fig. 1 47 1 In- embryo shows the gill-clefts and the outline of tlu' two legs; the chorion has formed branching villi Hurts). In all lour figures *' embryo, " outer germinal layer, m middle germinal layer, 1 inner germinal layer, am amnion lis head-sheath, ss tail-sheath), ah amniotic cavity, as amniotic sheath of the umbilical cord, kh embryonic vesicle, ds yolk- sac (umbilical vesicle), o^p vitelline duct, skin-fibre layer, m = mu — mp muscle-plate, itiv provertebral plate [wh cutaneous rudiment of the body of the vertebra, ivb of the arch of the vertebra, ivq the rib or transverse continuation), uwh provertebral cavity, ch axial rod or chord, sh chorda-sheath, bh ventral wall, g hind and v fore root of the spinal nerves, a=af=am amniotic fold, p body-cavity or cceloma, c(/"gut-fibre layer, ao primitive aortas, sa secon- dary aorta, vr cardinal veins, d=dd gut-gland layer, dr gastric groove. In Fig. 14S the larger part of the right half, in Fig. 149 the larger part of the left half, of the section is emitted. Of the velk-sac or remainder of the embrvonic vesicle only a small piece of the wall is indicated below. (Cf. the sections in Plate VI., Figs. 3-S.) nOKSA/. /{()/))■ .l.V/> VENTRAL />■()/>)' Fig. rS the plates meet from both sides above and below the chorda, they completely enclose it, and so form the tubular, outer chord-sheath, the skeleton-forming sheath from which the 316 DORSAL BODY AXD VEXTRAL BODY vertebral column is formed {perichorda, Fig. 142 C, s ; Figs. 150 wh, 151). (Cf. Figs. 3-8 on Plate VI. and the following Chapters.) We find below in the construction of the ventral wall precisely the same processes as in the formation of the dorsal wall (Fig. 142 b, Fig. 149 hp, Fig. 151 bh). It is formed on the flat embryonic shield of the amniotes from the upper plates of the parietal zone, or the parietal lamella of the lateral plates, which is covered with the horn-plate. 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 ventral wall or the lower wall of the body, the two lateral plates bending considerably on the inner side of the amniotic fold, and growing towards each other from right and left. While the alimentary canal is closing, the body-wall also closes on all sides. Hence the ventral wall, which embraces the whole ventral cavity below, consists of 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 navels, an inner and an outer one. The internal or intestinal navel is the definitive point of the closing of the alimentary wall, wrhich puts an end to the open communication between the ventral cavity and the cavity of the yelk-sac (Fig. 108). The external or cutaneous navel is the definitive point of the closing of the ventral wall ; this is visible in the developed body as a small depression. In each case two secondary germinal layers take part in the coalescence — in the gut-wall the gut-gland layer and gut-fibre layer ; in the ventral wall the skin-fibre layer and skin-sense layer. With the formation of the internal navel and the closing of the alimentary canal is connected the formation of two cavities which we call the capital and the pelvic sections of the visceral cavity. As the embryonic shield lies flat on the wall of the embryonic vesicle at first, and only gradually separates from it, its fore and hind ends are independent in the beginning ; on the other hand, the middle part of the DDRSA/. BODY AND VI-.XTRAI. BODY ventral surface is connected with the yolk-sac by means o( tin.- vitelline or umbilical duet (Fig. 152 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 belly. We see this very clearly in the excellent old diagram- matic illustration given by Baer (Fig. [52), a median longi- tudinal section of the embryo of the chick in which the dorsal body or episoma is deeply shaded. The embryo seems to be trying to roll up, like a hedgehog protecting itself from its pursuers. This pronounced curve of the back Fig. 152. -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 (episoma) dark, with convex outline, middle plate, ug prorenal duct, lh body-cavity, <• ectoderm, Ith ventral skin, /;_/" skin- fibre layer, df gut-fibre layer. ( From Kallmann. ) Fig. i.,().— Transverse section of a shark-embryo (or young selachius). mr medullary tube, ch chorda, u aorta, d gut, vp principal (or subintestinal) vein, ml myotome, mm muscular mass o( the provertebra, m/> middle plate, ug prorenal duet, lh body-cavity, e ectoderm of the rudimentary extremities, ma mesenchymic cells, a point where the myotome and nephrotome separate. I From II. E. Ziegler. ) other in consequence of special growth movements (Figs. 14S 150 ung). The direction they take in this corresponds to the limit between the dorsal body and the ventral body (cf. Figs. 155 and 156). While they advance between the stem-zone and parietal zone of the embryonic shield of the amniote, they depart more and more from their point of origin, and approach the gut-gland layer. In the end they lie deep in the interior, on either side of the mesentery, DORSAL BODY AND VENTRAL BODY underneath the chorda (Fig. 150 ung). At the same time the two primitive aortas change their position (cf. Figs. 141- 150 ao); they travel inwards underneath the chorda, and there coalesce at last to form a single secondary aorta, which is found under the rudimentary vertebral column (Fig. 150 ao). Fig. 157.— Transverse section of a duek-embryo with twenty-four primitive segments. (From Balfour.) From a dorsal lateral joint of the medullary tube (spc) the spinal knots (spg) grow out between it and the horn- plate, ch chorda, ao double aorta, liy gut-gland layer, sp gut-fibre layer, with blood-vessels in section, ms muscle plate, in the dorsal wall of the myoccel (episomite). Below the cardinal vein (cav) is the prerenal duct (tvd) and a segmental prerenal canal fs/J. The skin-fibre layer of the body-wall (so) is continued in the amniotic fold (am). Between the four secondary germinal layers and the structures formed from them there is formed embryonic connective matter with stellate cells and vascular structures. [Her twig's " mesenchym.") The cardinal veins, the first venous blood-vessels, also back towards each other, and eventually unite immediately above the rudimentary kidneys (Figs. 150 vc, 157 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 /HMW.l/. /;<)/>)■ .l\/> VENTRAL BODY ovarj in the female and the testicle in the male. Both develop from a small part of the ceelous epithelium, the cell- covering o( 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 assume most important relations to it, is only secondary. (Cf. Chapter XXIX. and Plate VI., Figs. 4-8.) THIRTEENTH TABLE SYNOPSIS OF THE COMPOSITION OF THE VERTEBRATE-BODY FROM DORSAL AND VEN- TRAL BODY, HEAD-HALF AND TRUNK-HALF Dorsal and Ventral Body. Episoma and hyposoma. Head and Trunk. Caput and truncus. Skull-less Animals. Skulled Animals. Cra n iota. I. Dorsal body. Episoma' ( = dorsal shield or notaspis in the amniote embrj'o). " Stem-zone " ( = prevertebral plates). (Animal hemi- sphere of the amphigastrula, Figs. 43-50.) Neural region. I. A. Head-hall' of the dorsal body. (Episoma capitale.) a. Si raple pro- cerebral ves- icles. b. Three pairs of simple organs of sense. c. No rudimen- tary brain. a. Brain (with five cerebral ves- icles). b. Three pairs of complex organs of sense. c. Cartilaginous rudiment a ry brain. /'a. Spinal marrow. fa. Spinal marrow. I. B. b. Simple unarti- ' b. Segmental ver- Trunk-half of the culated peri- 1 tebral column. dorsal body. chorda. 1 c. Dorsa 1 a n d (Episoma c. Dorsal trunk- ventral trunk- truneale.) muscles with muscles with- v myoccel. V out myoccel. Horizontal Frontal Septum between Episoma and Hyposoma; Axial, the Endoblastie Chorda— Lateral, the Eetoblastic Prorenal Duets. 11. Ventral body. Hyposoma ( = lateral plates and yelk-sac, besides the allantois in the amniote embryo). " Parietalzone " ( = lateral plates). (Vegetal hemi- sphere 'of the amphigastrula, Figs. 43-S°- ) Gastric region. II. A. Head-half of the ventral body. (Hyposoma capitale.) a. Head-wall per- manent, with numerous gill- clefts. b. S e g m e n t a 1 pronephridia. c. Mouth. Branchial gut and hypobran- chial groove. No floating bladder or lungs. One - chambered II. B. Trunk-half of the ventral body, (Hyposoma' truneale.) a. Ve n t ral wall (belly- plates). (Parietal layer of the hypso- mites). b. Several se g- mental prone- phridia. c. Several s e g- mental gonades. d. Stomach. Simple hepatic tube. Smallintestine. Anus. a. Head-wall em- bryonal with from five to seven pairs of gill-clefts. b. Head-kidneys (pronephros). c. Mouth. Gullet (j a w - c avit y) a n d thyreoidea. Floating blad- der or lungs. Many-chambered "heart. a. Ventral wall (belly-plates). ( Parietal layer of the lateral plates. ) b. A pair of com- pact kidneys. c. One pair of gonades. d. Stomach. Compact liver. Pancreas. Small intestine. Large intestine. Anus. I: Embryonic glands (sex-glands). is Gill-clefts (gullet-clefts). / Corium. lb Liver (kepar). lr Wind-pipe (trachea). hi Lung (fin/mo ). mil Mammary gland (mamma). mg Stomach (stomachus). mil Mouth-cavity. mp Muscular plate (muscularis). n Neural or medullary tube. », Fore-brain (cerebrum). ;;.. Intermediate brain (sphere of vision). // Middle brain. «4 Cerebellum. it- Hind-brain. ALPHABETICAL TABLE IN EXPLANATION OF THE LETTERS ON PLATES VI. AND VII. N.B. The ectoderm (skin-sense layer) is coloured orange, the dorsal mesoderm (in the episoma) blue, the ventral mesoderm (in the hyposoma) red, and the entoderm (gut-gland layer) green. a Anus. ii/i Amniotic cavity. /;/ Allantois (urinary sac). am Amnion (water-vesicle). ao Aorta. on Primitive mouth (prostoma). b Ventral muscles. bb Breast-bone (sternum ). c Body-cavity (eeeloma). c. Chest or pleural cavity (cavitas file uric ). <., Peritoneal cavity (cavitas peri- tonei). Gonocoel (ventral eeeloma I. cli Axial rod ( chorda). cm Myoccel (dorsal eeeloma). en Neurenteric canal. it Ccelom-pouches. cp Ccelom polar cells (cells of the primitive mesoderm). ex Seroccel (extra-foetal ccelom). cl Alimentary canal ( trttchus ). dc Large intestine (colon), dil Small intestine (ileum). df Cut-fibre layer. its V elk-sac (umbilical vesicle). du Primitive gut. <• Ectoderm. em Embryo. f Womb (uterus). Sexual glands (gonades). Sexual plates (embryonic epithe- lium). h Horn-plate ( eerablnstiis ). lib Bladder (vesica urinaria ). hf Skin-fibre layer. Ilk Heart-ventricle (ventri cuius). h! Left I arterial I heart. hr Right (venous) heart. hv Heart-auricle (atrium ). h: Heart (cur). i Entoderm. VO Call-bladder ( vesica fellea ). S Spinal marrow (medulla spinalis). Mouth ( osculum ). Pancreas. Organs of sense. Dorsal muscles. Ribs ( custic ). Skull (cranium ). Pubic bone ( os pubis ). Gullet (pharynx). Skeleton-plate. Oesophagus. Mesentery. Prerenal duct ( ncpliroiluctus). Prerenal tubes ( prnniphridiei ). Prerenal groove ( nephrusuli us J. Primitive segments (prevertebral somites). Rudimentary vein. Cardinal veins. Vagina. Vertebra. Vertebral arch. Body of vertebra. Legs (limbs). Diaphragm. EXPLANATION OF PLATES VI. AND VII. The Plates VI. and VII. are intended to give a partly ontogenetic and partly phylogenetic explanation of the construction of the human body from the germinal layers. Plate VI. contains only diagrammatic transverse sections (through the saggital and the transverse axis); Plate VII. contains only diagrammatic longitudinal sections (through the sagittal and the long axis), seen from the left. The primary layers and their products are marked by the same colours throughout, the skin-sense layer orange and the gut-gland layer green. The mesoderm and its products are blue in the episoma, or dorsal body ; and red in the hyposoma, or ventral body. The letters have the same meaning throughout. In all the figures the dorsal surface of the body is upward, and the ventral surface downward. Plate VI. DIAGRAMMATIC TRANSVERSE SECTIONS OF VERTEBRATES. Fig. i. Transverse section of the gastrula of a primitive verte- brate (amphioxus, of. Fig. 10, Plate VII., longitudinal section, and Figs. 40 and 41 1. The whole body is an alimentary canal (d) ; the wall of it consists only of the two primary layers. Fig. 2. Transverse section of the eoelomula of a primitive verte- brate (amphioxus) at the commencement of ccelomation. The dorsal wall of the primitive gut (du) divides into the rudiments of the median chorda (ch) and the two ccelom-pouches(V/J. The neural tube ( 11) begins to separate from the corneous plate (c). (Cf. Figs. S2-S4. ) Fig. ,3. Transverse section of the chordula (Figs. S6-89). The axial chorda (ch) lies between the dorsal nerve-tube (n) and the ventral gut- tube (d). The ccelom-pouch still simple in the left (younger) half ( ct ) ; in the right (older) half it is divided by the lateral furrow into a dorsal muscular pouch (myoccel, cm) and a ventral sexual pouch (gonoccel, eg), nip muscle- plate, gp sexual-plate, / corium-plate, /; horn-plate (outer skin). Fig. 4. Transverse section of an ideal primitive vertebrate (pro- spondylus or vertebrcea, p. 251). The ccelom-pouch is still simple in the left (younger) half, and opens outwardly by a prorenal canal fas J into the lateral prorenal groove fur J; in the right (older) half the dorsal part, or muscular pouch frw J, is divided from the ventral part, or sexual pouch (eg) ; the latter opens by a prorenal canal (us) into the prorenal duct ( u ), which has sepa- rated from the the horn-plate (h). The right and left body-cavities arc -.till separate. In the gut-fibre wall we see the first blood-vessels, the arteries above (aorta, an) and veins below (principal or subintestinal vein, hi). ell chorda, 11 medullary tube, d alimentary tube, gp sexual plate, nip muscular plate, / corium-plate, /; horn-plate. Fig. 5. Transverse section of a primitive fish embryo (selachii). The features of construction are almost the same as in the preceding ; only the right and left ccelom-pouches have united. This has given rise to the simple body-cavity (metaccel or pleuro-peritoneal cavity). The skeletal plate 326 VM TRANSVERSE SECTION PI. 17. ■ L qlation of Nan V.Ed LONGITUDINAL SECTION I' I VII e.n n'P t EXPLAN. I T/OJV OF /'/.. I /'/■: 1 7. 327 also (formed from the middle part of the dorsal ccelom-pouch) is more advanced, and forms independent "prevertebral halves" (lak). As in Fig. 4, it is assumed as a matter of hypothesis that the cceloma originally opens outwards (to the left ?) by segmental canals (pronephridia), but afterwards (to the right ') the dorsal and ventral ccelom-pouches are quite separate. (Cf. the section in Fig. i.s(>. 1 Fig. t>. Transverse section of the germinal disk of an amniote (or higher vertebrate), with rudiments of the first organs. (Cf. the section of the chick on the second day of incubation, Fig. 141. 1 Tin- medullary tube (») and tin- prerenal ducts (u)art separated from the horn-plate (h). At each siilo of tin' chorda (eh) the provertebrae ( n-r ) and the lateral plates are differentiated. Between the skin-fibre layer (hf) and the gut-fibre layer (df) we see the first formation of t le body-cavity or cceloma (eg); underneath it aro the two primary aortas (no). Fig. 7. Transverse section of the germinal disk of the same amniote, a little further advanced than Fig. .;. (Cf. the section of the chick- embryo on the third day of incubation, Fig. 148. 1 Medullary tube ( 11 ) and chorda ( 'eh ) already begin to be enclosed by the provertebrae (ma). The prerenal ducts (u) are already completely separated from the horn-plate (h ) bv the corium-plate ( I )■ c body-cavity, PnJ neuro- porus, dv fore pouch of the gut, ch chorda, mf mesodermic fold, pin polar cells of the mesoderm ( >>is ), e ectoderm. somites give rise, from their uppermost section, to the prone- phridia or prerenal canals, and from the lower to the segmental rudiments of the sexual glands or gonades. The partitions of the muscular dorsal pieces (myotomes ) remain, and determine the permanent articulation of the vertebrate organism. But the partitions of the extensive ventral pieces ( gonotomes ) become thinner, and afterwards disappear in THE ARTICULATION OF THE BODY 339 part, so that their cavities run together to form the metacoel, or the simple permanent body-cavity. The articulation proceeds in substantially the same way in the other vertebrates, the craniota, starting from the ecelom- pouches. But whereas in the former case there is first a transverse division oi the coelom-sacs (by vertical folds) and then the dOrso-ventral division, the procedure is reversed in the craniota; in their case each of the long ccelom-pouches first divides into a dorsal (primitive segment plates) and a ventral (lateral plates) section by a lateral longitudinal fold. Only the former are then broken up into primitive segments by the sub- sequent vertical folds; while the latter (segmented for a time in the amphioxus) remain undivided, and, by the divergence of their parietal and visceral plates, form a body- cavity that is unified from the first. In this case, again, it is clear that fig. 167. —Transverse „,. _„_♦ „^T., .. 1 .1,, 1%.,,,,., ,r .1, , section of the middle of an we must regard the features of the amphioxus -embryo with younger craniota as cenogeneticallv eleven primitive seg- ° ° ■ ments. ( 1- rom Hatschck. | To modified processes that can be traced the left the segment is -.tin , . ,, , , 1 • simple, to the ritrht alreadv palmgenetically to the older acrania. divided by the lateral fold We have an interesting inter- (^ into dorsal and ventral B halves. ak, ink, ik outer, mediate stage between the acrania middle, and inner germinal . layers, « neural tube, ch and the fishes in these and many chorda, dh alimentary canal, ^t . • .« * sd dorsal somite, sv ventral other respects in the cyclostoma somite> c coeloma; (myxinoides and petromyzontes, cf. Chapter XXL). In particular, the development of their muscular segments (from the dorsal somites) is nearer to that oi the amphioxus than of the other vertebrates (the gnathostoma). This is connected with the fact that the cyclostoma, like the acrania, have no vertebral column, and that the articulation of the body is very simple and primitive in both groups ; the formation of the head, especially, remains at a very low stage, and there arc- no pairs of limbs. These embryonic processes are much more complex in the fishes, with which begins the long THE ARTICULATIOX OF THE BODY series of gnathostome ("jaw-mouthed") vertebrates with two pairs of extremities. Among the fishes the selachii, or primitive fishes, yield the most important information on these and many other phylogenetic questions (Figs. 168, 169). The careful studies of Riickert, Van Wijhe, H. E. Ziegler, and others, have given us most valuable results. The products of the middle Fig. 16S. Figs. 168 and 169.— Transverse section of shark-embryos (through the region of the kidneys). (From Wijhe and Hertwig.) In Fig. 16c, the dorsal segment-cavities (h) are already separated from the body-cavity ( II: j. but they are connected a little earlier (Fig. 16S). nr neural tube, ch chorda, sch subchordal string, no aorta, sk skeletal plate, nip muscle-plate, cp cutis-plate w connection of latter (growth-zone), vn primitive kidneys, Kg-prorenal duct, uk prerenal canals, lis point where they are cut off', tr prerenal funnel, ink middle germ-layer {mk2 parietal, ink., visceral), it inner germ-layer (gut-gland layer). germinal layer are partly clear in these cases at the period when the dorsal primitive segment cavities (or tnyocoels, //) are still connected with the ventral body-cavity (l/i : Fig. 168). In Fig. 169, a somewhat older embryo, these cavities are separated. The outer or lateral wall of the dorsal segment yields the cutis-plate (cp), 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 ARTICULATION OF THE BODY the skeletal plate, the formative matter of the vertebral column ( sk ). In the amphibia, also, especially the water-salamander ( triton ), we can observe very clearly the articulation of the coelom-pouches and the rise of the primitive segments from their dorsal half (ci. Fig. 04, A, B, C). The cavity o( the originally simple coelom-sacs (Fig. 94 A and right half of H ) remains visible both in the dorsal and ventral segments, even after the two have been separated by the lateral fold (Fig. 04 C and left half oi li ). A hori- zontal longitudinal or frontal section of this salamander-embryo (Fig. 170) shows very clearly the scries of paii> of these vesicular dorsal segments, which have been cut off on each side from the ventral side-plates, and lie to the right and left of the chorda. The metamerism of the amniotes agrees in all essential points with that of the three lowerclasses of vertebrates we have Fig. 170.— Frontal (or horizontal - longitudinal) section of atriton-embryo with three pairs ol' primitive segments. eh chorda, us primitive segments, ush their cavity, ak horn plate. Fig. 171.— Transverse section of a chick-embryo of the second day of incubation. (From KSUiker.) mr medullary tube, ch chorda, uw protovertebra, ung prorenal ducts, u" primitive aorta, //:.'// prevertebral cavity, mi primitive kidneys, h horn-plate, o/amniotic told, lip skin-fibre layer, <^ gut- fibre layer, /> COilom, till yelk-gland layer. considered ; but it varies considerably in detail, in conse- quence of cenogenetic disturbances that are due in the first place (like the degeneration o( 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 ARTICULATION OF THE BODY the solid structure of 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 first of solid laminae of cells (Figs. 97-100). And when the articulation of the somites begins in the sole- shaped embryonic shield, and a couple of protovertebrae are Fig. 172.— Transverse section of the embryo of a chick of the fourth day, magnified about one hundred times. The protovertebrse have split into the outer muscle-plate ( mpj and the inner skeletal plate. The latter begins to enclose the chorda ( ch ) below as the body of the vertebra; f'v/ij, and the medullar}- tube ( m ) above as the arch of the vertebrae (ivb), the cavity of the medullary tube ( m/i J being- now very narrow. At «-y the muscular plate advances into the ventral wall ( lip ), hpr corium-plate or dorsal wall, h horny plate, a amnion, ung prorenal duct, un prorenal canals, ao primitive artery (aorta), vc cardinal vein, df gut-fibre layer, dd gut-gland layer, dr alimentary groove. developed in succession, constantly increasing in number towards the rear, these cube-shaped somites (formerly called protovertebrae, or primitive vertebrae) have the appearance of solid dice, made up of mesodermic cells (Fig. 141). Never- theless, there is for a time a ventral cavity, or prevertebral cavity, even in these solid "protovertebrse" (Fig. 157 uwh). This vesicular condition of the provertebra is of the greatest THE ARTICULATION OF THE liOltY phylogenetic interesl ; we must, according to the coelom theory, regard it as an hereditary reproduction of the vesicular dorsal somites of the amphioxus (Figs. 161-167) and the lower vertebrates (Figs, 168-170). This rudimentary " prevertebral cavity " has no physiological significance whatever in the amniote-embryo ; it soon disappears, being tilled up with cells of the muscular plate. Another variation in the formation of the segments in the amniotes is that the development of the muscular plates from the inner (median) wall of their somites spreads to the outer (lateral) wall ; hence here the cell-stratum of the " skin-fibre layer," which lies directly below the cutis-plate (the later corium-plate, Fig. 169 cp), also takes a lively part in the further growth of the muscular plate. It grows out on all sides from this point, especially downwards into the lateral plates of the ventral wall (the ventral plates). The innermost median part of the primitive segment plates, which lies immediately on the chorda (Fig. 172 c/i) and the medullary tube f»ij, forms the vertebral column in all the higher vertebrates (it is wanting in the lowest) ; hence it may be called the skeleton plate. In each of the pro- vertebra; it is called the "sclerotome" (in opposition to the outlying muscular plate, the " myotome"). From the phylo- genetic 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) divides into two lamina;, which grow- round the chorda, and thus form the foundation of the body of the vertebra (wh). The upper lamina presses between the chorda and the medullary tube, the lower between the chorda and the alimentary canal (Fig. 142 C). As the laminae of two opp site prevertebral pieces unite from the right and left, a circular sheath is formed round this part of the chorda. From this developes the body oi' a vertebra — that is to say, the massive lower or ventral half of the bony ring, which is called the "vertebra" proper and surrounds the medullary tube (Figs. 173 173). The upper or dorsal half of this bony ring, the vertebral arch (Fig. 172 wb) arises in just the same THE ARTICULATIOX OF THE BODY way from the upper part of the skeletal plate, and therefore from the inner and upper edge of the cube-shaped primitive vertebra. As the median 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 of two pro- vertebral pieces 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, second, cartilaginous stage, and finally into a third, permanent, bony stage. These three stages can generally be distinguished in the greater part of the skeleton of the higher vertebrates ; at first most parts of V - h'lG. '73- Fig. l73- Fig. 174- Fig. ■75- Fig. 174. Fig, -The third cervical vertebra (human). -The sixth dorsal vertebra (human). -The second lumbar vertebra (human). the skeleton are softer, tender, and membranous ; they then become cartilaginous in the course of their development, and finally ossify. At the head part of the embryo in the amniotes there is not generally a cleavage of the middle germinal layer into provertebral and lateral plates, but the dorsal and ventral somites are blended from the first, and form what are called "the head-plates" (Fig. 153 k). From these are formed the skull, the bony case of the brain, and the muscles and corium of the body. The skull developes in the manner of the membranous vertebral column. The right and left halves of the head curve 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 cartilaginous primitive skull, THE ARTICULATION OF THE BODY such as we find permanently in many o\ the fishes. 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 differentiated and modified than that of the lower vertebrates, the amphibia and fishes. But as the one has arisen phylogenetically from the other, we must assume that in the former no less than the latter the skull was originally formed from the sclerotomes of a number oi (at least nine) head-somites. While the typical articulation of the vertebrate body is always obvious in the episoma or dorsal body, and is clearly expressed in the metamerism of the muscular plates and vertebrae (myotomes and sclerotomes), it is more latent in the hyposoma or ventral body. Nevertheless, these ventral hypo- somites of the vegetal half of the body are not less important than the episomites of the animal half. The segmentation in the ventral cavity affects the following principal systems of organs : i. The gonades or sex-glands (gonotomes) ; 2. The nephridia or kidneys (nephrotomes) ; and 3. The head-gut with its metamerous gill-clefts (branchiotomes). (Plate VII., Fig. 12.) The metamerism of the hyposoma is less conspicuous because in all the craniotes the gonocoels — the cavities of the ventral segments, in the walls of which the sexual products are developed — have long since coalesced, and formed a single large body-cavity, owing to the disappear- ance of the partition. This cenogenetic process is so old that the metaccel in the lateral plates of the craniota has everywhere the appearance from the first of a simple unseg- mented cavity, and that the rudiment of the gonades also is almost always unsegmented. It is the more interesting to learn that, according to the important discovery of Riickert, this sexual structure is at first segmental even in the actual selachii, and the several gonotomes only blend into a simple sexual gland on either side secondarily. Amphioxus, the sole surviving representative of the acrania, once more yields us most interesting information; in this case the sexual glands remain segmented throughout 346 THE ARTICULATION OF THE BODY life, and so do the ventral body-cavities. 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 gonades* are originally nothing else than the real gonotomes, separate body-cavities, formed from the hyposomites of the trunk. The reason why they have hitherto generally been misunder- stood, and the amphioxus has wrongly been credited with a simple body-cavity, is that the latter has been confused with the large peribranchial space. The gonades are the most important segmental organs of the hyposoma, in the sense that they are phylogenetically the oldest. We find sexual glands (as pouch-like appendages of the gastro-canal system) in most of the coelenteria, even in the cnidaria (medusa;), which have no nephridia. The latter appear first (as a pair of prorenal canals or excretory tubes) in the platodes (turbellaria), and have probably been inherited from these by the articulates (annelids) on the one hand and the unarticulated prochordonia on the other, and from these passed to the articulated vertebrates. The oldest form of the renal system in this stem are the segmental pro- nephridia or the metamerous prorenal canals, in the same arrangement as Boveri found them in the amphioxus. They are small canals that lie in the frontal plane, on each side of the chorda, between the episoma and hyposoma (Fig. 176 n); their internal funnel-shaped opening leads into the various body-cavities, their outer opening is the lateral furrow of the epidermis. Originally they must have had a double function, the carrying away of the urine from the myoccel of the episomites and the release of the sexual cells from the gonoccel of the hyposomites. The recent investigations of Riickert and Van Wijhe on the mesodermic segments of the trunk and the excretory system of the selachii show that these " primitive fishes " are closely related to the amphioxus in this further respect. The transverse section of the shark-embryo in Fig. 168 shows the dorsal and ventral halves of the ccelom-pouches still openly connected. In the middle of the section, in the frontal axis, THE ARTICULATION OF THE JiODY the narrow myoccel (or cleft-like " muscular cavity " of the dorsal segment) passes by a narrow connecting channel (vb ) directly into the wide gonoccel (Hi) or the body-cavity of the ventral segment, from the epithelium o( which sexual cells develop. The narrow connecting channel (vb) becomes the pronephridium, or prerenal canal, which carries away the excretory products of both body-cavities (the urine of the dorsal muscular cavity and the sexual cells of the ventral sexual cavity). Afterwards (Fig. 169) the two cavities are divided by a partition. Then the inner opening of the renal canal only leads into the lower ventral cavity. The outer opening was in the surface of the outer skin, probably in the lateral furrow of the epidermis, from which the prerenal duct developes in the craniotes by constric- tion (Fig. 171 ting). In the amphioxus, as Boveri dis- covered, they still open in the corresponding part of the secondary "mantle- cavity." In other higher verte- brates, also, the kidneys de- velop (though verydifferently formed later on) from similar structures, which have been secondarily derived from the segmental pronephridia of the acrania. The parts of the mesoderm at which the first traces of them are found are usually called the middle or mesenteric plates, and their segmental parts mesornera. As the first traces of the gonades make their appearance in the ccelous epithelium o( 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. Fig. 176. Transverse section of the trunk of a primitive vertebrate (prospondylusj. a aorta, 6 lateral furrow (prorenal duct), rf small intestine,/" float- ing border of the skin, i muscular cavity (dorsal ccelom-pouch), ms muscles, n renal canals, 11 outer skin, r spinal marrow, s sexual glands (gonades), / corium, -' principal vein, v chorda. 348 THE ARTICULATION OF THE BODY The chief and oldest organ of the vertebrate hyposoma, the alimentary canal, is generally described as an unsegmented organ. But we could just as well say that it is the oldest of all the metamerous organs of the vertebrate ; the double row of the ccelom-pouches grows out of the dorsal wall of the gut, on either side of the chorda. In the brief period during which these segmental ccelom-pouches are still openly connected with the gut, they look just like a double chain of metamerous visceral 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 gut, or " branchiomerism." V Hid p Fig. 177.— Optical longitudinal section of the primitive vertebrate ( prospondyljts ). a aorta, of anus, au eye, d small intestine, e parietal eye (epiphysis),/' floating border of skin, g auditor}' vesicle, gh brain, h heart, k gill- gut, ka branchial (gill) artery, kg branchial vascular arches, ks gill-clefts, / liver, ma stomach, nid mouth, ms muscles, na nose (olfactory pit), o outer skin, p gullet, r spinal cord, s sexual glands (gonades), t corium, v principal vein, x chorda, y hypophysis (urinary appendage). The gill-clefts, which originally in the older acrania pierced the wall of the fore-gut and the gill-arches that separated them, were presumably also segmental, and distributed among the various metamera of the chain, like the gonades in the after-gut and the nephridia (Fig. 177 ksj. In the amphioxus, too, they are still segmentally formed. Probably there was a division of labour of the hyposomites in the older (and long extinct) acrania, in such wise that those of the fore-gut took the function of breathing and those of the after-gut repro- duction. The former developed into gill-pouches, the latter into sex-pouches. There may have been pronephridia in both. Branchiomerism is so much changed in the living vertebrates, and so reduced in the amniotes, that it has been THE ARTICULATION OF THE BODY denied altogether by some scientists. Moreover, in the amniotes their respiratory function has disappeared. Never- theless, certain parts of them have been generally maintained in the embryo by a tenacious heredity. At a very early stage we notice in 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 -ill-clefts (Plates VIII. -XIII., Figs. 178-1S1/O. Theybelong to the characteristic and inalienable organs of the amniote- embrvo, and are found always in the same spot and with the same arrangement and structure. There are formed to the right and left in the lateral wall of the fore-gut cavity, in its fore- most part, first a pair and then several pairs of sac-shaped inlets, that pierce the whole thickness of the lateral wall of the head. They are thus converted into clefts, through which one can penetrate freely from without into the gullet. The wall thickens between these branchial folds, and changes into an arch-like or sickle-shaped piece — the gill, or gullet-arch. In this the muscles and skeletal parts of the branchial gut separate ; a blood-vessel arch arises afterwards on their inner side (Fig. 177 kaj. The number of the branchial arches and the clefts that alternate with them is four or five on each side in the higher vertebrates (Fig. 181 d, /,/',/ )• In some of the fishes (selachii) and in the cyclostoma we find six or seven of them permanently. These remarkable structures had originally 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 at the sides of the gullet. In the higher vertebrates they afterwards disappear. The branchial arches are converted partly into the jaws, partly into the bones of the tongue and the car. From the first gill-cleft is formed the tympanic cavity of the ear. (Cf. Plates I., VIII. XIII., first and second row.) The primary articulation of the vertebrate body, which proceeds from the primitive segments of the mesoderm, affects most of its chief systems of organs : in the episoma especially the muscles and skeleton, in the hyposoma the kidneys and THE ARTICVLATIOX OF THE BODY gonades and the branchial gut. Then there is a secondary articulation of other systems of organs, which is dependent Fig. 178.— Head of a shark embryo ffiristiurusj, eight mm. long, magnified Lwcnty tinier. (From Parker.) Seen From the ventral side. Fig. 179. Figs. 179 and 1S0.— Head of a chick embryo, of the third day. Fig, 179 from the front, Fig. 1S0 from the right, n rudi- mentary nose (olfactory pit), / rudimentary pIG ,g,_ eye (optic pit, lens-cavity), g rudimentary ear (auditory pit), v fore-brain, gl eye-cleft Of the three pairs of gill-arehes the first has passed into a process of the upper jaw (o) and of the lower jaw (u). (From Kolliker.) FlG. 181.— Head Of a dog embryo, seen from the front. it the two lateral halves of the foremost cerebral vesicle, b rudimentary eye, c middle cerebral vesicle, de first pair of gill-arches (e upper-jaw process, d lower-jaw process), /■/'■/'' second, third, and fourth pairs of gill-arches, g // //-heart [gright, h left auricle ; i left, k right ventricle I, / origin of the aorta with three pairs of arches, which go to the gill-arches. (From Bischoff. ) THE ARTICULATION OF THE BODY on and determined by the preceding one. Thus we have in the later stages the development of a segmental arrangement o\ the peripheral nerves and blood-vessels ; the one starts from the episoma, the other from the hvposoma. Especially important is the fact that in man and all other vertebrates the psychic organ is subject to this "secondary metamerism." It is readily recognisable in the human embryo in the fourth week, the eetodermic nerve-roots connecting with the corres- ponding mesodermic muscle-plates of the provertebrae (Fig. iSj). There are few parts of the vertebrate organism that are not subject to metamerism, like the outer covering- or integument oi the body. The outer skin (epidermis ) is unsegmented from the first, and proceeds from the uniformly disposed horny plate. Moreover, the underlying cutis is also not metamerous, although it developes from the segmental structure of the cutis-plates (or lateral lamina? of the episo- mites, Figs. 1 68, 169 cp). The vertebrates are strikingly and profoundly different from the articulates in these respects also. Further, most of the vertebrates still have a number of unarticulated or monomeric organs, which have arisen loeallv, 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 hvposoma. The heart is originally only a local spindle-shaped enlarge- ment of the large ventral blood-vessel or principal vein, at the point where the subintestinal passes into the branchial artery, at the limit oi the head and trunk (figs. iSt, [82). The three higher sense-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. 180 //)• The organ oi sight, the eye, is found at the side of the head, also in the shape of a depression (Figs. 180 /, 1S1 b), to which corresponds a considerable vesicular THE ARTICULATION OF THE BODY hollowing of the foremost cerebral vesicle on each side. Farther behind, at each side of the head, there is a third depression, the first trace of the organ of hearing (Fig. iSog-). Rudiment of ear (labyrinthic vesicles) Pneumogastric nerve X. Vagus Terminal nerve XI. Accessorius Twentieth spinal nerve Fig. 182.— Human embryo of the fourth'week (twenty-six days old), six mm. long-, magnified twenty times. (From Moll.) The rudiments' of the cerebral nerves and the roots of the spinal nerves are especially marked. Underneath the four gill-arches (left side) is the heart (with auricle, I", and ventricle, A"), under this again the liver ( L). As yet we can see nothing of the later elaborate structure of these organs, nor of the characteristic build of the face (cf. Plate I., Figs. 1-5). When the human embryo has reached this stage of THE ARTICULATION OF THE /,'()/) Y development, it can still .scarcely be distinguished from that o( any other higher vertebrate (c\. Plate 1. and p. 356). All the chief parts o( the body are now laid down : the head with the primitive skull, the rudiments of the three higher Sense-organs and the live cerebral vesicles, and the ^ill- arches and clefts ; the trunk with the spinal cord, the rudiment of the vertebral column, the chain of metamera, 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 embryo of the mammals, the birds, the reptiles- etc. (cf. p. 356, Plates VJU.-XIIL, top row). This is an ontogenetic fact of the utmost significance. From it we can gather the most important phylogenetic conclu- sions. There is still no trace of the limbs. Although head and trunk are separated and all the principal internal organs are laid down, there is no indication whatever of the "extremities" at this stage; they are formed later on. Here again we have a fact of the utmost interest. It proves that the older vertebrates had no feet, as we find to-day in the lowest living vertebrates (amphioxus and the cvclostoma). The descendants of these ancient footless vertebrates only acquired extremities — two fore-legs and two hind-legs — at a much later 2A i"ic;. 183. Transverse section of the shoulder and fore-limb (wing) of a chick- embryo of the fourth day, magnified about twenty times. Beside the medullary tube we ran seeon each side three clear streaks in the dark dorsal wall, which advance into the rudimentary fore-limb or wing 1 1 1. The upper- most of them is the muscular plate; tin- middle is the hind and the lowest On- lor,' rool of a spinal nerve. Under the chorda in the middle is 1 ho single aorta, and at each side o( ii a cardinal vein, and below these the primitive kidneys. The gut is almost closed, ventral wall advances into the amnion, encloses the embryo. (From Remai.) The hich THE ARTICULATION OF THE BODY stage of development. These were at first all alike, though they afterwards vary considerably in structure — becoming fins (of breast and belly) in the fishes, wings and legs in the birds, fore and hind legs in the creeping 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. 183, 184). They have always the appearance of two pairs of small birds, which re- present at first simple roundish knobs or plates. Gradually each of these plates be- comes a large projection, in which we can dis- tinguish a small inner part and a broader outer part. The latter is the rudiment of the foot or hand, the former that of the leg or arm. The simi- larity of the original rudiment of the limbs in different groups of vertebrates will be seen on Plates VIII. -XIII. How the five fingers or toes with their blood-vessels gradually differentiate within the simple fin-like structure of the limbs can be seen in the instance of the lizard in Fig. 185. They are formed in just the same way in man ; in the human embryo of five weeks the five fingers can clearly be distin- guished within the fin-plate (Fig. 186). The careful study and comparison of human embryos Fig. 184.— Transverse section of the pelvic region and hind legs of a chick-embryo of the fourth day, magnified about forty times, h horn-plate, •hj medullary tube, n Canal of the tube, u primitive kidneys, x chorda, e hind legs, b allantois canal in the ventral wall, / aorta, v cardinal veins, a gut, d gut- gland layer, /'gut fibre layer, ^embryonic epithelium, r dorsal muscles, c body-cavity or cceloma. (From Waldeyer. ) THE ARTICULATION OF THE BODY with those of other vertebrates at this stage o( development is very instructive, and reveals more mysteries to the impartial student than all the religions in the world put together. For instance, let us compare attentively the three successive Stages of development that are represented, in twenty Fig. 185. Development of the lizard's legs (lacerta agUis), with special relation 10 their blood-vessels. r, j, 5, 7, 9, // right fore-leg; /,;. /■; left fore-leg ; -'. -/. '', 8, m, u right hind-leg ; 14, 16 left hind-leg ; SS I ' lateral veins of the trunk, VU umbilical vein. (From /•'. ffochstetter.) different amniotes, in the six following Plates (VIII. -XI II.). When we see that as a fact twenty different amniotes of such divergent characters develop from the same embryonic form, we can easily understand that they may all descend from a common ancestor. 356 THE ARTICULATION OF THE BODY In the first stage of development (the first row, I.), in which the head with the five cerebral vesicles is already clearly indicated, but there are no limbs, the embryos of all the vertebrates, from the fish to man, are only incidentally or not at all different from each other. In the second stage (the Fig. i86. — Human embryo, five weeks old, eleven mm. long-, seen from the right, magnified ten times. ( From Russel Bardeen and Harmon Lewis. ) In the undissected head we see the eye, mouth, and ear. In the trunk the skin and part of the muscles have been removed, so that the cartilaginous vertebral column is free ; the dorsal root of a spinal nerve goes out from eaeli vertebra (towards the skin of the back). In the middle of the lower half of the figure part of the ribs and intercostal muscles are visible. The skin and muscles have also been icmoved from the right limbs; the internal rudiments of the ft\o 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 the extremities. The tail projects under the foot, and to the right of it is the first part of the umbilical cord. THE ARTICULATION OF THE BODY middle row, II.). which shows the limbs, we begin to see differences between the embryos of the lower and higher vertebrates ; but the human embryo is still hardly distin- guishable from that of the higher mammals. In the third Stage (lowest row, III.), in which the gill-arches have nk 1 Fig. 187. I'u.-. 187 <). Embryos of the bat (,ves- pertilio murinus) at three different stages. (From Oscar Schultze.) Fit;. 1S7. Rudimentary limbs if fore-leg, // hind-leg). I lenticular depression, r olfactory pit, nk upper jaw, nk lower jaw, k... £„, k- , first, second, and third gill-arches, » amnion, >i umbilical vessel, 5 A-C). In the opinion o( many travellers and anthropologists, the atavistic tail-formation is hereditary in many isolated tribes .1 c Kii.. 195. Tail of a six months' old boy. -I stretched out, # contracted, Cdrawjiout. it rom Granville Harrison.) (especially in south-eastern Asia and the archipelago), so that we might speak of a special race or "species" of tailed men (homo Cauda tus). Battels has " no doubt that these tailed men will be discovered in the advance of our geo- graphical and ethnographical knowledge o( the lands in question " {A rchvof&r A nthropologiey Band XV., p. 129). When we open a human embryo of one month (Fig. 196), we find the alimentary canal formed in the body-cavity, and FCETAL MEMBRAXES AXD CIRCULATIOX for the most part cut off from the embryonic 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 heart shows all its four sections ; it is very large, and almost fills the whole of the pectoral cavity (Fig. 196 ov). Behind it are the very small rudimentary Fig. 196.— Human em- bryo, 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 g-ut. /; eye, 3 nose, 4 upper jaw, •7 lower jaw, 6 second, 6" third gill-arch, ot' heart (0 right, o left auricle ; v rig-ht, v' left ventricle), b origin of the aorta, /' liver in umbilical vein), e gut (with vitelline artery, cut off at a'), y vitelline vein, /// primitive kidneys, t rudimentary sexual glands, r terminal gut (cut off at the mesentery e), 11 um- bilical artery, it umbilical \ ein, Q fore-leg, 1/ hind-leg. (From Coste. ) Fig. 197— Human em- bryo five weeks old, opened from the ventral side(as inFig. 196). Breast and belly-wall and liver are removed. 3 outer nasal process, 4 upper jaw, 5 lower jaw, e tongue, •:• right, v' left ventricle of heart, »' left auricle, b origin of aorta, 6', b" , b'" ;'u" '96- rK;- '97- first, second, and third aorta-arches, c, <"', c" vena cava, ae lungs (y pulmonary artery ), e stomach, m primitive kidneys (./left vitelline vein, s cystic vein, a right vitelline artery, >i umbilical artery, u umbilical vein), X vitelline duct, i rectum, S tail, □ fore-leg, 9' hind-leg. The liver is removed. (From Cosh:) lungs. The primitive kidneys (m ) are very large ; they fill the greater part of the abdominal cavity, and extend from the liver ( f) to the pelvic gut. Thus at the end of the first month all the chief organs are already outlined. But there are at this stage no features bv which the human embryo materially /■>/•• /'. I I. MEMBR. I NES A \D C '/HI Y 7..-I TION 37 1 differs from that oi the dog, the hare, the ox, or the horse — in a word, oi any other higher mammal. All these embryos have the same, or at least a very similar, form ; they can at the most be 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 is larger in pro- portion to the trunk than in the ox. The tail is rather longer in the dog than in man. These are all negligible differences. On the other hand, the whole internal organisation and the form and arrangement of the various organs are essentially the same in the human embryo of four weeks as in the embryos of the other mammals at corresponding stages. It is otherwise in the second month of human develop- ment. Fig. 1 l> 1 represents a human embryo of six weeks (VI.), one of seven weeks (VII.), and one of eight weeks (VI 1 1.) at natural size. The differences which mark oi'\ the human embryo from that of the dog and the lower mammals now begin to be more pronounced. We can see important difference;, at the sixth, and still more at the eighth, week, especially in the formation of the head (Plate XIII., Fig. Mill, etc.). The size of the various sections of the brain is greater in man, and the tail is shorter. Other differences between man and the lower mammals are found in the relative si/e of the internal organs. But even at this stage the human embryo differs very little from that of the nearest related mammals, the apes, especially the anthropomorphic apes. The features by means of which we distinguish between them are not clear until later on. Even at a much more advanced stage of development, when we can distinguish the human foetus from that o( the ungulates at a glance, it still closely resembles that of the higher apes. At last we get the distinctive features, and we can distinguish the human embryo confidently at the first glance from that of all other mammals during the last four months of foetal life — from the sixtli to the ninth month of pregnancy. Then we begin to find also the differences between the various races of men, especially in regard to the formation of the skull and the face. (Cf. Chapter XXIII.) FCETAL MEMBRANES AXD CIRCULATION The striking resemblance that persists so long between the embryo of man and of the higher apes disappears much earlier in the lower apes. It naturally remains longest in the large anthropomorphic apes (gorilla, chimpanzee, orang, and gibbon). The physiognomic similarity of these animals, which we find so great in their earlier years, lessens with the increase of age. On the other hand, it remains throughout life in the remarkable long-nosed ape of Borneo (nasalis lai~vatus, Plate XXV.). Its finely-shaped nose would be regarded with envy by many a man who has too little of that organ. ■ If we compare the face of the long-nosed ape with that of abnormally ape-like human beings (such as the famous Miss Julia Pastrana, Fig. 198), it will be admitted to represent a higher stage of development. There are still people among us who look especially to the face for the "image of God in man." The long-nosed ape would have more claim to this than some of the stumpy- nosed human individuals one meets. This progressive divergence of the Fig. 198.— Theheadof , . - . c , . , . Miss Julia Pastrana, human from the animal form, which is JKMte) phot°Krsiph by based on the law of the ontogenetic connection between related forms, is found in the structure of the internal organs as well as in external form. It is also expressed in the construction of the envelopes and appendages that we find externally to the foetus, and that we will now consider more closely. Two of these appendages — the amnion and the allantois — are only found in the three higher classes of vertebrates, while the third, the yelk-sac, is found in most of the vertebrates. 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, we find it just the same in man as in the higher mammals. The ovum is, you will remember, first surrounded bv the transparent structureless ovolemma or zona FCET. I / MEMBR. I .VAN . I ND ( '/A7 7 7. I TION 373 pellucida (Figs, i, 14). But very soon, even in the first week of development, it is replaced by the permanent chorion. This arises from the external layer o( the amnion, the sero/emma, or " serous membrane," the formation ot which we shall consider presently; it surrounds the foetus and its appendages as a broad, completely-closed sac ; the space between the two, tilled with clear watery fluid, is the scro- ccelom, or interamniotic cavity ("extra-embryonic body- cavity"). But the smooth surface of the sac is quickly covered with numbers o\ tiny tufts, which are reallv hollow out-growths like the fingers of a glove (Figs. 199, 204, 217 (//:). They ramify and push into the corresponding- depressions that are formed by the tubular glands of the mucous membrane o( the maternal womb. Thus, the ovum secures its permanent seat (Figs. 199-207). In human ova of eight to twelve days this external mem- brane, the chorion, is already covered with small tufts or villi, and forms a ball or spheroid of six to eight millimetres in diameter (Figs. 199-201). As a large quantity of fluid gathers inside it, the chorion expands more and more, so that the embryo only occupies a small part of the space within the vesicle. The villi of the chorion grow larger and more numerous. They branch out more and more. At first the villi cover the whole surface, but they afterwards dis- appear from the greater part of it ; they then develop with proportionately greater vigour at a spot where the placenta is formed from the allantois. When we open the chorion of a human embrvo of three weeks, we find on the ventral side of the foetus a large round sac, filled with fluid. This is the yelk-sac, or "umbilical vesicle," the origin of which we have con- sidered previously. The larger the embrvo becomes the smaller we find the yelk-sac. Afterwards we find the remainder of it in the shape of a small pear-shaped vesicle, fastened to a long thin stalk (or pedicle), and hanging from the open belly of the foetus (Fig. 207). This pedicle is the vitelline duct, and is separated from the body at the closing of the navel. The wall of the umbilical vesicle consists, you FCETAL MEMBRANES AND CIRCULATION will remember, of an inner plate, the gut-gland layer and an outer plate, the gut-fibre layer. It is therefore made up of the same constituents as the gut-wall itself, and really forms a direct continuation of it. In birdsand reptiles, in which the yelk-sac is much larger, it contains a considerable quantity of nutritive material, albuminous and fatty substances. Fig. Fig. 203. Fig. 199.— Human OVUm of twelve to thirteen days (?). (From Allen Thomson.) 1. Not opened, natural size. 2. Opened and magnified. Within the outer chorion the tiny curved foetus lies on the large embryonic vesicle, to the left above. Fig. 200.— Human ovum of ten days. (From Allen Thomson.) Natural size, opened ; the small foetus in the right half, above. Fig. 201. — Human foetUS of ten days, taken from the preceding ovum, magnified ten times, a yelk-sac, b neck (the medullary groove already closed), c head (with open medullary groove), d hind part (with open medullary groove), e a shred of the amnion. Fig. 202.— Human OVUm of twenty to twenty-two days. (From Allen Thomson.) Natural size, opened. The chorion forms a spacious vesicle, to the inner wall of which the small fetus (to the right above) is attached by a short umbilical cord. Fig. 203.— Human foetUS of twenty to twenty-two 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 of same, e second gill-arch (two smaller ones behind). Three gill-clefts are clearly seen, /'rudimentary fore-leg, P" auditory vesicle, /; eye, i heart. FCETAL MEMBRANES AND CIRCULATION These pass by the vitelline duct into the visceral cavity, and serve as food, as in the oviparous monotremes. In the other (viviparous) mammals the yelk-sac is much less important for the nutrition of the embryo, and it atrophies at an early stage. Fig. 204.— Human embryo of sixteen to eighteen days. (From Coste. ) Magnified. The embryo is surrounded by the amnion ( » ), lies Free with this in tin- opened embryonic vesicle. The belly is drawn up by the large yelk-sac ( (I ), and fastened to the inner wall of the embryonic membrane by the short and thick pedicle (b). Hence the normal convex curve of the back (Fig. 20.^) is here changed into an abnormal concave surface. // heart, hi parietal mesoderm. The spots on the outer «.ill ol the serolemma are the roots of the branching chorion-villi, which are free at the border. Behind the yelk-sac a second appendage, of much greater importance, is formed at an early stage at the belly oi the mammal embryo. This is the allantois or" primitive u ni nary sac," an important embryonic organ, only found in the three 376 FCETAL MEMBRANES AXD CIRCCLATIOX higher classes of vertebrates. In all the amniotes the allantois quickly appears at the hinder end of the alimentary canal, growing out of the cavity of the pelvic gut (Fig. 208, ;-, //, Fig. 209 ALC '). Umbilical vesicle (yelk-sac I Umbilical cord (pedicle) Fig. 205. -Human embryo of the fourth week, seven and a-half mm. long-, lying in the dissected chorion. The allantois originated as a prolongation of the urinary bladder of the amphibia ; in their descendants, the protam- niotes (the ancestors of the amniotes), it has grown out of the ccelom of the embryo, and has henceforth to take a part in its nutri- tion. The first trace of it is a small vesicle at the edge of the cavity of the pelvic gut ; it represents a fold of the gut, and has (like the yelk-sac) a two-layered ' .. * wall. The cavity o\ Fig. ,06. -Human embryo of the fourth the vesicle is clothed week, with its membranes, like Fig-. 205, but with the gilt - gland a little older. The yelk-sac is rather smaller, the amnion and chorion larger. laver, and the Outer FOETAL MEMBRANES .l\/> CIRCULATION Fig. 207. -Human embryo with its membranes, six weeks old. Tin- outer- envelope of the whole ovum is ilu- chorion, thickly covered with its branching villi, a product of the serous membrane. The embryo is enclosed in the delicate amnion-sac. The yelk-sac is reduced to a small pear-shaped umbilical vesicle; its thin pedicle, the long vitelline duet, is enclosed in the umbilical cord. In the latter, behind the vitelline duet, is the much shorter pedicle of the allantois, the inner lamina of which (the gut-eland layer) forms a large vesicle in most of the mammals, while the outer lamina is attached to the inner wall of the outer embryonic coat, and forms the placenta there. (Half diaerrammatii Fig. jos.— Median longitudinal section of the embryo ofa chick (fifth day of incubation}, seen from the right (head to the right, tail to the left). Dorsal body (episoma) dark, with convex surface, d gut, 0 mouth, a anus, A liver, g mesentery, / lungs, ;• aurielo of heart, i- ventricle, b arterial arches, / aorta, c yelk-sac, m vitelline duct, u allantois, r pedicle of the allantois, " amnion, w amniotic cavity, s serous membrane, (From Burr.) .178 FCETAL MEMBRANES AND CIRCULATION lamina of the wall is formed of the thickened gut-fibre layer. The little vesicle gets bigger and bigger, and grows into a large sac, filled with fluid, in the wall of which large blood- vessels are formed. It soon reaches the inner wall of the foetal cavity, and spreads along the inner surface of the chorion (Fig. 209 ALC). In many mammals the allantois is so large that at last it surrounds the whole embryo and the other appendages as a wide membrane, and spreads over the whole of the inner surface of the prochorion. When we open Fig. 209.— Diagram of the embryonic organs of the mammal (fcetal membranes and appendages). (From Turner.) E, J/, H, outer, middle, and inner germ layer of the embryonic shield, which is figured in median longitudinal section, seen from the left, am amnion, AC amniotic cavity, UV yelk-sac or umbilical vesicle, ALC allantois, al periccelom or serocoelom (inter- amniotic cavity), ss serolemma (or serous membrane), pc prochorion (with villi). an ovum of this character, we encounter first a large cavity filled with fluid ; this is the amniotic cavity. Only when this membrane is removed do we reach the amniotic vesicle which encloses the embryo proper. The further development of the allantois varies con- siderably in the three sub-classes of the mammals. The two lower sub-classes, monotremes and marsupials, retain the simpler structure of their ancestors, the reptiles. The wall of the allantois and the enveloping serolemma remains smooth I- (KIM. MEM UK. WIS AND CIRCULATION 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 o( the allantois, richly supplied with branches of the umbilical vessel, presses into these serous villi of the primary chorion, and forms the "secondary chorion." lis embryonic blood-vessels are closely correlated to the contiguous maternal blood-vessels of the environing uterus, Fig. 210.— Embryo of a dog, from the right. ,; first, /> second, <• third, d fourth cerebral vesicle, e eye,fi auditory vesicle, ,j,r// first gill-arch (g lower jaw, // upper jaw), i second gill-arch, klm heart { I- right auricle, / right and m left ventricle), >i origin of aorta, « heart-pouch, /> liver, allantois, q fore-leg, r hind-leg. The curved embryo lias been straightened out. (From Bisrhoff.) importance and useful in classification. There is only one of these that need be specially mentioned — the important fact established by Selenka in 1890 that the distinctive human placentation is confined to the anthropoids. In this most advanced group of the mammals the allantois 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 FXETAL MEMBRANES AND CIRCULATION )8i pedicle." Until very recently this was regarded as a structure peculiar to man. We now know from Selenka that the much- discussed ventral pedicle is merely the pedicle of the allantois, combined with the pedicle o\ the amnion and the rudimentary pedicle oi the yelk-sac. It has just the same structure in the orang and gibbon (Figs. 213-216), and very probable in the chimpanzee and gorilla, as in man ; it is, therefore, not a disproof, but a striking fresh proof, o( the blood-relationship of man and the anthropoid apes. Hence the allantois is in- terestingin three ways in connec- tion with man's geneal ogi cal tree : firstly, be- cause this ap- pendage is want- ing in the lower classes of verte- brates, and is developed only in the three higher classes of the stem, the reptiles, birds, and mammals ; secondly, be- cause the placenta developes from the allantois only in the placentals, or the higher mammals and man, and not in the lower mammals (marsupials and monotremes) ; thirdly, because the remarkable peculiarities of human placentation are only found outside man in the anthropoid apes, not in the other placentals. 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 charac- terises man (Fig. 217). In this case there is at an early Fie.. 212. Diagrammatic frontal section of the pregnant human womb. ( From Longet. 1 The embryo hang? by the umbilical cord, which encloses the pedicle of the allantois ( al ' ). nb umbilical vessel, am amnion, ch chorion, ds decidua serotina, villi of the placenta, c cervix uteri, u uterus. 382 FCETAL MEMBRANES AXD CIRCULATION Fig. 215. Figs. 213-215.— Embryos of the kalawet-gibbon 01 Borneo (hyiobates concolorj. Fig". 213 embryo of seventeen mm. from head to buttocks, magnified four times ; seen from the left. Fig-. 214 the same, seen from the front. Fig-. 215 embryo of one hundred mm. from head to buttocks, three-fourths natural size, in the same position as found in uterus, with which it is still connected by the umbilical cord. Only the dorsal half of the dissected uterus is shown, and the placenta is attached to the central part of this. FCETAL MEMBRANES AND CIRCULATION 383 stage an intimate blending of the chorion of the embryo and the part o( the mucous lining o( the womb to which it attaches. The villi o( the chorion with the blood-vessels they contain grows so completely into the tissue of the uterus, which is rich in blood, that it becomes impossible to separate them, and they form together a sort of cake. This comes away as the "after-birth " at parturition ; at the same time the part o( the mucous lining of the uterus that has united inseparably with the chorion is torn away ; hence it is Fig. ji6.— Male embryo of the Siamang-gibbon (hyhbates siamanga) of Sumatra, two-thirds natural size: to the left 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. Both this embryo and the preceding (Fig. 215) take the head-position in the womb, and ibis is normal in man also. called the decidua (" falling-away membrane"), and also the " sieve-membrane," because it is perforated like a sieve. We find a decidua 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 decidua. The external or true decidua (Fig. 212 tin. Fig. 218^) is the part o\ the mucous lining of the womb that clothes the inner surface o\ the uterine cavity wherever it is not connected with the placenta. The placental or spongy decidua 3«4 FCETAl. MEM BR AXES A. YD CIRCULATION ( placcntalis or serotina, Fig. 212 ds, Fig. 218 d ) is really the placenta itself, or the maternal part of it (placenta uterina ) — 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 decidua (interna or reflexa, Fig. 212 dr, Fig. 218 f) is that part of the mucous lining of the womb which encloses the remaining: surface of the ovum, Foetal placenta Amniotic cavity Chorion (lreve) Uterine cavity End of the decidua Fig. 217.— Frontal section of the pregnant human womb. (From Turner.) The embryo (a month old) hang's in the middle of the amniotic cavity by the ventral pedicle or umbilical cord, which connects it with the placenta (above). the smooth chorion (chorion Iceve), in the shape of a special thin membrane. The origin of these three different deciduous membranes, in regard to which quite erroneous views (still retained in their names) formerly prevailed, is now quite clear; the external decidua vera is the specially modified and subse- quently detachable superficial stratum of the original mucous lining of the womb. The placental decidua serotina is that /■ c yav v v.. / nox part of the preceding which is completely transformed by the ingrowth o( the chorion-villi, and is used for constructing the placenta. The inner decidua reflexa is formed by the rise of a circular fold of the mucous lining (at the border of the decidua vera and serotina), which grows over the foetus (like the amnion) to the end. Fig. 218.— Human foetus, twelve weeks old, with its membranes, natural size. The umbilical cord goes from its navel to tin- placenta, b amnion, ^chorion, d placenta] d' relics or \illi on smooth chorion] /' internal or reflex deciduat g external or true decidua. (From B. Schultze. ) The peculiar anatomic features that characterise the human foetal membranes are found in just the same way in the higher apes. The lower apes and the other disco- placentals show more or less considerable variations, and, in general, simpler features. This applies especially to the delicate structure of the placenta itself, the blending of the chorion-villi with the decidua serotina. The mature human placenta is a circular (less frequently oval) disk of a soft, spongy texture, six to eight inches in diameter, aboui cue FCETAL MEMBRANES AXD CIRCULATION inch thick, and one to one and a half pounds in weight. Its convex outer surface (uniting with the uterus) is very uneven and tufted. Its concave inner surface (facing the uterine cavity) is quite smooth, and covered by the amnion. As a rule, the umbilical cord (funiculus umbilicalis ) starts from about the middle of the placenta; we have considered the origin of this from the ventral pedicle. This also is covered or sheathed by the amnion, which passes directly into the abdominal skin at the navel end of the cord (Fig. 218). The mature umbilical cord is a cylindrical string, twisted spirally Fig. 219.— Mature human foetus (at the end ot pregnancy, in its natural position, taken out of the uterine cavity). On the inner surface of the latter (to the left) is the placenta, which is connected by the umbilical cord with the child's navel. (From Bernhard Schultze.) on its axis, generally about twenty inches long and half an inch thick. It consists of a gelatinous connective tissue (the " Whartonian jelly "), in which we find the remainder of the vitelline vessels and the large umbilical vessels — the two umbilical arteries which conduct the blood of the embryo to the placenta and the strong umbilical vein that conveys the blood from the latter to the heart. The countless fine branchlets of this foetal umbilical vessel enter the ramified chorion-villi of the foetal placenta, and finally join in a peculiar way with these to form the wide blood-filled cavities that expand in the uterine placenta and contain the maternal FOSTAL MEMBRANES AND CIRCULATION 387 blood. The very complicated and difficult anatomic relations that develop here between the foetal and maternal placenta are found in this form only in man and the anthropoid ape; they differ more or less considerably in all the other deciduates. The umbilical cord is also proportionately longer in man and the apes than in the other mammals. 1 Pancreas and / ""' " portal vein Inner mouth of the womb (greatly distended)... Bladder Fig. j.'o. Median section of the lower half of the trunk of a woman in advanced pregnancy. The head of the child is already in the pelvis in, ili.- normal head-position. The foetal vesicle (the size of an apple) is siill whole in (In- vagina : the foetal water lias not yel escaped. 1 From Braune.) Until recently it was thought that the human embryo was distinguished by its peculiar construction of a solid allantois and a special ventral pedicle, and that the umbilical cord developed from this in a different way from 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 FCETAL MEMBRAXES AND CIRCULATIOX all the other placentals. But the remarkable discoveries published by the distinguished zoologist Selenka in 1890 proved that man shares these peculiarities of placentation with the 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. The new facts that Selenka discovered during his investi- gation of this question in India are so important, and yield such far-reaching conclusions, that I will give the results in his own words : — Some embryonic organs are developed earlier and some later in the apes and man than in the other mammals. Among the anticipated structures are: ( 1) the innumerable chorion-villi, (2) the coelom-sacs, by the expansion of which the yelk-sac is early removed and the amnion closed, and (3) the pedicle of the allantois. On the other hand, we have the following- retarded structures: (1) the yelk-sac. It is true that it quickly separates from the wall of the embryonic vesicle, but its vascular network only developes later on. As it has completely lost its earlier function of respiratory and nutritive organ, it must be regarded as a rudimentary organ. It sends no vessels into the chorion, all the blood- vessels of which are exclusively allantoic. (2) The rise of the allantoic cavity also is delayed, and (3) the differentiation of the germinative area. As special structures we may designate: (1) the looser texture of the somatopleura, which lines the chorion ; (2) the persistence of the pedicle of the allantois ; (3) the expansion of the amnion and its blending with the chorion ; (4) the forma- tion of two placenta; side by side, one of which may remain rudimentary; (5) the degeneration of the yelk-sac into a rudimentary organ ; and (6) the attach- ment of the non-placental part of the foetal membrane — whether it be the chorion laeve or the decidua reflexa — to the surrounding wall of the uterus. A third embryonic appendage, which we have already mentioned — the amnion or "water-membrane" — is also, like the allantois, one of the characteristic features of the three higher classes of vertebrates. We have introduced the amnion when dealing with the severance of the embryo from the embryonic vesicle (p. 308). We found that its walls rise about the embryonic body in the form of a circular fold. In front this fold rises to some height in what is called the hood or sheath of the head (Fig. 222 ks); behind also it curves over considerably as the hood or sheath of the tail fss); to the right and left the fold is at first lower, and is known as the side-hood or sheath (Fig. 226). All these " hoods " or " sheaths " are merely portions of a continuous circular fold Fig. 225. Figs. 221-225. Five diagrammatic longitudinal sections of the maturing mammal embryo and its envelopes. In Kitr^- 221-224 ,lu' longitudinal section goes through the sagittal or middle plane of the body, which cuts it into right and left halves 1 in li.y. 225 the foetus is seen from the left. In Fig. 221 the prochorion I ' is larger, the yelk-sac (ds) smaller. In FCETAL MEMBRAXES AXD CIRCVLATIOX that runs round the embryo. It grows higher and higher, rises up like a rampart, and at last curves like a grotto over the body of the embryo. The edges of the circular fold touch and join (Fig. 227). Thus in the end the embryo is enclosed in a membranous sac, which is filled with the amniotic fluid (Figs. 224, 225 ah). When the sac is completely closed, the inner plate of the fold, which forms the real wall of the amniotic sac, separates altogether from the outer. The latter attaches itself internally to the prochorion, replaces it, and becomes itself the permanent outer envelope of the embryo, described by Baer as the " serous membrane." This serolemma consists, like the thin wall of the amnion-sac, of two layers — the neural and the parietal germ-layers. The latter is in this case very thin and delicate, but can easily be recognised as a direct continua- tion of the skin-fibre layer. Naturally, in harmony with the folding process, the parietal middle layer is turned inwards in the serolemma and outwards in the amnion. The space between it and the allantois is the periccelom or the inter- amniotic cavity (the extra-embryonic body-cavity, Fig. 209 al). The phylogenetic cause of this ontogenetic formation of the amnion is to be sought on mechanical lines in the fact that the body of the embryo has gradually sunk into the underlying yelk-sac, thus leaving a circular fold of membrane around it. The growth of the latter into a completely closed sac, filled with fluid, is explained on the theory of selection by the great service which so admirable a protective structure offers to the delicate embryo. Of the three vesicular appendages of the amniote embryo which we have now described the amnion has no blood- vessels at any moment of its existence. But the other two Fig-. 225 the embryo already shows the gill-clefts and the rudiments of the two pairs of legs ; the chorion has branched villi. In all five figures : e embryo, a outer germinal layer, m middle germinal layer, i inner germinal layer, am amnion (is head sheath, ss tail sheath), ah amniotic cavity, as amniotic sheath of the umbilical cord, kh embryonic vesicle, i/s yelk-sac (umbilical vesicle), dg- vitelline duct, df gut-fibre layer, dd gut-gland layer, al allantois, vl=hh place of heart, d ovolemma or prochorian, d' villi of same, sh serous membrane (serolemma), sz villi of same, ch chorion, chs villi of same, st terminal vein, / periccelom or seroccelom (the space between the amnion and chorion, filled with fluid). (From Kullikcr.) Cf. Plate VII., Figs. 14 and 15. FOETAL MEMBRANES AND CIRCULATION 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 observations on the ' "■ --'•• Transverse section of the embryo of a chick (a little behind the anterior opening' of the first Circulation in gut) at the end ofthe first day of incubation. Above rl-io wnhrvn -irul its ls ""' medullary groove, below the gut-groove, still tiic em Dry o ana its wkl>. open 0n each side we siv the oullim. ol tlu. central or "an the body-cavity between the skin-fibre layer and the gut- 6 ' _ fibre layer. To the right and left of it outwards the heart. The first lateral hoods ofthe amnion are beginning lo rise. ,, , . . ( From Remak.) blood-vessels, the heart, and the first blood itself, are formed from the 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 laser were taken up only with the for- mation of blood- \ essels. Neither of these supposi- tions is true. Blood-vessels may Fig. 227. Transverse section of the embryo be formed indepen- ofa chick in the region of the navel (of the fifth day , , _ ■ ,*!,_- of incubation). The amniotic folds (am) almost dentlj in othe. touch above over the back of the embryo. Thegut n^pts, especially in fdj, -.till open, passes below into the yelk-sac. r > 1 (//" sfut-iihn- layer, sA chorda, sa aorta, vc cardinal the various pro- veins, bh ventral wall, nol yet closed, v fore, g hind - , roots of spinal nerves, mii muscle-plate, lip cutis- ducts Ot the SKin- plate, A horny-plate. (From Remai.) fibre layer. The tissue that composes the blood-vessels belongs to those secondary products oi the mesoderm that do not divide as FCETAL MEMBRANES AND CIRCULATION epithelial plates, but may arise anywhere in holes between the epithelial products of the germ-layers, and were marked off by Hertwig under the title of intermediate layer or mesenchyma. However, according to some observers, the inner vascular epithelium originates from the entoderm. The heart and the blood-vessels and the vascular system generally are by no means among the oldest parts of the animal organism. Aristotle believed that the heart was one of the first organs to be formed in the chicken ; and many later writers adopted this opinion. But this is not at all the case. The chief parts of the body — the four secondary germ- layers, the medullary tube and chorda — are formed long before there is any trace of the vas- cular system. As we shall see later, this fact is in complete harmony with the phylogeny of the animal kingdom. The ccelenteria (gas- trasads, sponges, cni- daria, and platodes), to which a section of our earliest animal ancestors belonged, have neither blood nor heart. The vermalia were developed at a comparatively late date from these bloodless ccelenteria, and the higher vermalia in which a vascular system of the simplest form developes (frontonia ) later still from the non-vascular lower vermalia (rotatoria); from the higher vermalia are descended the much younger vertebrates. The first blood-vessels of the mammal embryo have been considered by us previously in the transverse sections on Figs. 148-151 (p. 314). They are, firstly, the two primitive Fig. 228.— Transverse section of the embryo of a chick in the region of the shoul- der (of the fifth day of incubation). The section passes through the rudimentary fore- leg (or wing, e). The amniotic folds are joined over the back of the embryo. (From Remak.) Cf. Figs. 225, 226, and 227; also Plate VII., Fig. 14. FOETAL MEMBRANES AND CIRCULATION arteries or aortas, which lie in the narrow longitudinal clefts between the provcrtebne, the lateral plates, and the gut-gland layer (Figs. 141 t/o, 14800) ; and, secondly, the two principal or cardinal veins, which appear a little later, farther out than the former, above the primitive renal ducts (Figs. 140 157 cur). The heart arises in just the same way and in connection with these first vessels, in the lower wall of the fore- gut, at the throat, where the heart remains throughout life in the fish. The heart of the vertebrate is originally only a local enlargement of the median visceral vessel, which runs on the lower wall of the gut, and which we have called the principal vein in our study of the primitive vertebrate (Figs. 101, 10^ 7'). The simple, spindle-shaped heart, that we assume to have been here at the limit of the head and trunk, is found at the same spot, immediately behind the gill-gut, in the embryos of the acrania and the cyclostoma (Plate XIX., Fig. 16 //) and the fishes. By Fig. 229. —Human ., , . . ., ' embryo of fourteen to the contraction 01 its muscular wall the eighteen days, opened on venous blood that is brought by the |!u> (VL'",r;;' side- u"der & ' the frontal process ot the subintestinal vein is driven forward into lu'atl (') tlu- lleart (<~) >* . seen in the cardiac Cavity the branchial artery (on the under side (/>), with the base o( the c ,1 1 11 »\ aorta ( b I. The yelk-sac ot the branchial gut). (n) lias bivn ronioVl.d for The rudimentary heart is single in tho m;,st part (at * the ° inosculation ot the fore- the amphibia also. In the amniotes, arm), g primitive aortas , ...... . (lying under the primitive however, it is double trom the hrst, vertebra), > rectum, « having two distinct halves (Fig. 1 -,7 /;). aUantois ('" its P%di^< '' a \ t> 0/ / amnion. [From Caste.) But the two halves soon degenerate and unite, in the ventral middle line of the wall of the fore-gut, to form a single simple tube. The double structure is a later cenogenetic phenomenon, mechanically deter- mined by the flat expansion of the embryonic shield on the large yelk-vesicle. FCETAL MEMBRAXES AXD CIRCULATIOX The simple, spindle-shaped structure of the heart, which separates from the ventral wall of the head-gut, consists of the two germinal layers of the gut-wall, a small fold of the gut-gland layer being taken into the tube. From this is formed the endocard, the epithelial inner cellular lining of the heart. Its thick muscular wall, the myocard, is formed by the cells of the gut-fibre layer or visceral middle layer. From this also come the red blood-cells, and the first traces of the vessels that are connected with the heart. These also are at first solid, round strings of cells. They are then hollowed out by the secretion of fluid at their axis. Some of the cells are detached and float in the fluid, and thus become blood-cells. This applies both to the arteries (which convey the blood from the heart) and the veins (which convey it to the heart). The white blood-cells (lymph-cells or leucocytes) are travel- ling cells, originating in the mesenchyma and passing subsequently into the blood-vessels. The heart of every vertebrate lies at first in the ventral wall of the fore-gut, or in the ventral (or cardiac) mesentery, by which it is connected for a time with the wall of the body. But the heart soon severs itself from the place of ts origin, and lies freely in a cavity — the cardiac cavity (Fig. 230 c). For a short time it is still connected with the former by the thin plate of the mesocardium (hg). Afterwards it lies quite free in the cardiac cavity, and is only directly connected with the gut-wall by the vessels which issue from it (Fig. 230). Fig. 230.— Diagrammatic transverse Section Of the head of a mammal em- bryo. /; horny plate, m medullary tube (cerebral vesicle), tnr wall of same, / cutis- plate, s rudimentary skull, c/i chorda, /• gill- arches, nip muscular plate, c cardiac cavity, foremost part of the body-cavity (cceloma), d alimentary canal, dd gut-gland layer, df visceral muscular plate, hg mesocardium, /iw wall of heart, hi ventricle of heart, ab aorta-arch, a section of aorta-stem. FCETAL MEMBRANES AND CIRCULATION 395 The fore-end of the spindle-shaped tube, winch soon bends into an S-shape (Fig. 232), divides into a right and left branch. These tubes are bent upwards arch-wise, and represent the first arches of the aorta. They rise in the wall of the fore-gut, which they enclose in a sense, and then unite above, in the upper wall of the fore gut-cavity, to form a large single artery, that runs backward immediately under the Fig. 231. Vitelline vessels in the germinative area of a chick- embryo, at the close of the third day of incubation. (From Balfour.') The 1 germinative area is seen from the ventral side : the arteries are dark, tin- wins light. // In-art, .l.l aorta-arches, Ao aorta, ROf.A right omphalo- mesenteric artery, & '/'. sinus terminalis, L.DfumX R.OfTighX and left omphalo- mesenteric veins, 5. V. sinus venosus, ZXC ductus Cuvieri, S.CaK and I'.Cn fore and hind cardinal veins. chorda, and is called the aorta (Fig. 231 Ao). The first pair of aorta-arches rise on the inner wall of the first pair oi gill- arches, and so lie between the first gill-arch ( k ) and the fore- gut (d), just as we find them throughout life in the fishes. The single aorta, which results from the upper conjunction of these two first vascular arches, divides again immediately 396 FCETAL MEMBRAXES AXD CIRCVLATIOX into two parallel branches, which run backwards on either side of the chorda. These are the primitive aortas which we have already mentioned ; they are also called the posterior vertebral arteries. These two arteries now give off at each side, behind, at right angles, four or five branches, and these pass from the embryonic body to the germinative area ; they are called omphalo-mesenteric or vitelline arteries. They represent the first rudi- ment of a foetal circula- tion. Thus, the first blood-vessels pass over the embryonic body and reach as far as the edge of the germinative area. At first they are confined to the dark or " vas- cular " area. But they afterwards extend over the whole surface of the embryonic vesicle. In the end, the whole of the yelk-sac is covered with a vascular net-work. These vessels have to gather food from thecon- Fig. 232.— Boat-shaped embryo of the dog, from the ventral side, magnified about ten times. In front under the forehead we can see the first pair of g*ill-arches ; under- litory 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 protovertebrse, the primitive aortas, from which five pairs of vitelline arteries are t;iven off. (From Bhchoff. ) neath is the S-shaped heart, at the sides of tents of the velk-sac and which are the auditorv vesicles. The heart _ ' convey it to the em- bryonic body. This is done by the veins, which pass first from the ger- minative area, and after- wards from the yelk-sac, to the farther end of the heart. They are called vitelline, or, frequently, omphalo- mesenteric, veins. Thus, the first embryonic circulation (Figs. 231-234) is arranged in the following simple way in the three higher classes of vertebrates. The simple tubular heart (Fig. 234 d) divides, both in front and behind, into two vessels. The hind ■wws*, /■(/■: TAL MEMBRANES AND CIRCULATION 397 vessels are tlie afferent vitelline veins. They absorb nutritive matter from the embryonic vesicle or the yelk-sac, and convey it to the embryonic body. The anterior vessels are the efferent branchial arteries, which pass round the fore part of the gut in the shape of the rising aortic-arches ; they unite to form the aorta. The two branches that are formed by the splitting of the main artery — the primitive aortas — give o\( vitelline arteries to right and left, and these pass from the Fig. j^;,. -Embryonic shield and germinative area of a hare, in which wo see the first outline of the blood-vessels, seen from the ventral side. magnified about ton times. The hind end of the simple heart ( n j divides into two strong vitelline veins, and those form a vascular network in the dark area (which looks light on the blaek ground). At the bead-end we can see the fore brain with the two optic vesleles ( b. h ). The darker middle ot the embryo is the wide-open visceral cavity. On each side of the chorda wo see ten proto- vertebrse. (From Bishoff.) body of the embryo to the germinative area. Here, and in the periphery of the umbilical vesicle, we distinguish two layers of vessels, the surface-layer of arteries and the lower layerof veins. The two are connected. At first this vascular system only extends over the periphery of the germinative area to its border. Here, at the edge of the dark vascular area, all the branches unite in a large terminal vein (Fig. 2,^4 a). This vein disappears later on, when the 398 FCETAL MEMBRANES AXD CIRCULATION formation of vessels proceeds further in the course of development, and then the vitelline vessels cover the whole of the yelk-sac. These vessels naturally atrophy with the degeneration of the umbilical vesicle ; their importance is restricted to the first period of the life of the embryo. This vitelline circulation is afterwards replaced by a second, that of the allantois. Large blood-vessels are Fig. ^34.— Embryonic shield and germinative area of a hare, in which the first vascular system is fully formed, seen from the ventral side, magnified about five times. The posterior end of the S-shaped heart (d) divides into two strong vitelline veins, each of which gives off a fore (b ) and hind (c) branch. The ends of these unite in the circular terminal vein (a). In the germinative area we see the coarser (deeper-lying) venous net and the finer (more super- ficial) arterial net. The vitelline arteries ( f ) open into the two primitive aortas ( e ). The dark area, which surrounds the head like an aureole, corresponds to the depression of the head-hood. (From Bischoff.) developed in the wall of the urinary sac or the allantois, as before, from the gut-fibre laver. 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 F(I-:t. I L MEMBR. I SES . I ND ( '//>'( 7 '/.. 1 TION 31 n 1 converted into the placenta, its blood-vessels alone effect the nourishment of the embryo. They are called umbilical \esseN, and are originally double — a pair of umbilical arteries and a pair oi umbilical veins. The two umbilical veins (Fig. iq6 //), 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 goes with 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 (Figs. 196 /i, 197 «), are merely the ultimate termi- nations of the primitive aortas, which are stongly developed afterwards. This umbilical circulation retains its importance until the nine months of embryonic life are over, and the human embryo enters into the world as an independent indi- vidual. The umbilical cord (Fig. 212 al), in which these large blood-vessels pass from the embryo to the placenta, comes away, together with the latter, in the after-birth, and with pulmonarv respiration begins an entirely new form of circulation, which is confined to the body of the infant. There is a great phylogenetic significance in the perfect agreement which we find between man and the anthropoid apes in these important features of embryonic circulation, and the special construction of the placenta and the umbilical cord. We must infer from it a close blood-relationship of man and the anthropomorphic apes, a common descent of them from one and the same extinct group of lower apes. Huxley's " pithecometra-principle " 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 apes 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 confirmed in an interesting and unexpected way from the side of the experimental physiology of the blood. The experi- ments of Hans Friedenthal at Berlin have shown that human blood, mixed with the blood of lower apes, has a poisonous FCETAL MEMBRANES AXD CIRCULATION effect on the latter ; the serum of the one destroys the blood- cells of the other. But this does not happen when human Fig. 235. — Lar OF White-handed gibbon (hylobates lar or albimanus) from the Indian main-land. (From Brehm.) blood is mixed with that of the anthropoid ape. As we know from many other experiments that the mixture of two different kinds of blood is only possible without injury in the case of /••2. Gut gland- plate. Lamella ente, 1. Outer skin. Epidermis. 2. Nervous system. Medullarytube, 3. Sense- organs. Sensilla. 4. Corium. 5. Lateral muscles of the trunk (myotomes). 6. Ctiorda- sheath and its processes (perichorda). 7. Prone- phridia. Prerenal ca- nals (later pri- mitive kidneys and kidneys). ■s. Gonades' (ovaries and spermaries). 9. Dorsal artery aorta ventral vein (heart). 10. Mesentery and muscular wall of the gut. [Epithelial Tissue of ' the outer skin, the I mouth, and the Ganglionic cells and nerve-fibres. Differentiated sense-epithelia. :md 1 Cutis, connective ' tissue, and smooth I muscles of the I mesenchyma. ( Animal muscular \ tissue (striated). /' Supporting tissue I of the skeleton, I cartilage and hones. I Urinary epithe- I hum of the prone- I phridia and the later renal canals. Gonidia (ova and sper- matozoa). Tissues of the vascular walls. Lymph-cells. Smooth muscles and mesonchvm ofthe gut. 1 . Chorda (axial rod). I Chorda-tissue. Chorda ,1,., -sails. \ 12a. Head-gut, Cephalogaster branchial scut. 12b. Trunk-gut, J Hepatogaster, Liver-eut. 1 2a. Respiratory epithelium of the gullet and ^ill- crate, the hypo- branchial groove, and the lungs. -12b. Digestive epithelium of stomach, liver, small and large intestines. The Evolution of Man. V.Ed. PLJV. The Evolution of Man /'/'. PLXV1. - ■ EXPLANATION OF PLATES XV. AND XVI. Hum. in embryos in the foetal membranes. The six figures of these Plates are copied from the fine steel engravings illustrating The Development of Man and the Chick hi the Egg, which Professor Erdl (Munich) published in 1845. All six figures represent human embryos in their natural size, enveloped in tlu'ir membranes. In the first four figures from the second to the sixth week ol development) the mallochorion is cut away, and we see the tiny embryo enclosed in the amnion. The small umbilical vesicle (or rudimentary yelk- sac) hangs by a thin stalk out ol' the belly of the embryo, and lies in the n or seroccelom (the extra-embryonic body-cavity). (Cf. Plate XIV". and p. 365. 1 Plate XV., Fig. 1. A human embryo with the total membranes of about the tenth clay, natural size [Erdl, Plate III.. Fig. 1 |. Plate XV., Fig. -•. A human embryo with the foetal membranes of about the fourteenth day, natural size (Erdl, Plate III., Fig. 2). Plate XV., Fig. 3. A human embryo with the foetal membranes of three weeks, natural size (Erdl, Plate III.. Fig. j). Plate XV., Fig. 4. A human embryo with the foetal membranes of Six weeks, natural size (Erdl, Plate III., Fig. 5). Plate xv.. Fig. 5. A human embryo of twelve weeks, within the foetal membranes, natural size (Erdl, Plate XI., Fig. z). The embryo is com- pletely enclosed in the amniotic sac, filled with water, as in a bath. The umbilical cord, which passes from the navel of the embryo to the chorion, is sheathed with a continuation of the amnion, which makes folds at its points oi juncture. Above, the thickly clustered and branched chorion-villi form the placenta. The lower part of the chorion (cut away ami lying in delicate folds) is smooth and tuftless. Underneath it the uterine decidua, also cut away and spread out, hangs in coarser folds. Head and limbs are already far advanced. Plate XVI. A human embryo of five months, natural size (Erdl, Plate XIV.). The embryo is enclosed in the delicate, transparent amnion. which is evil open in front, so that the face and limbs stand out. The back is curved, the limbs drawn up. so that the embryo takes up as little space as possible in the ovum. The eyes are closed. From the navel the thick umbilical cord passes, in serpentine folds, over the right shoulder to the back, and trout there to the spong) placenta (to the right below). The thin outer- most membrane, lying in many folds, is the external foetal membrane, the chorion. ••"'Sfe "*WrV