Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/438

420 REPRODUCTION [ANIMAL. Vertcbrata. The segmentation is total and equal in Amphioxua, total and not markedly unequal in Mammalia, total and unequal in Amphibia, Acipenser, Petromyzon, partial in Birds and Reptiles, Teleosteaus and Elasmobranchs, but the limits between these types are not sharply defined, being mainly determined by the store of food-yolk. In Amphioxus there is no interference with the forma- tion of the typical gastrula, and this form is also readily recogniz- able in the Amphibia, Acipenser, and Petromyzon, where, however, the process of formation is less simple. At a point corresponding to the future hiud end of the embryo a true invagination takes place, forming, however, only the dorsal wall of the archenteron, at the same time the ectoderm cells have been growing round the yolk, and the floor of the enteric cavity is formed from yolk-cells. The blastopore is eventually closed, but a communication between the archeuteron and the neural canal persists for some time as the neurenteric canal. In the Elasmobranchs and Teleosteans, though there is no true invagination nor recognizable gastrula form, there are not wanting hints of its ancestral occurrence and of homologies with the typical form. The comparison of the different Ichthyop- sidau gastruke with the typical form and with that of the Saurop- sida, and such problems as the occurrence of an epibolic gastrula in Mammalia are still under discussion. The most important stages in the further Vertebrate development, the formation of the medullary groove and canal, the appearance of the notochord as an axial differentiation of hypoblast, the origin of the body- cavity from paired outgrowths from the archenteron, the occur- rence of gill-pouches opening externally from the throat, the ex- pansion of the medullary cord anteriorly to form the vesicles of the brain, the outpushing of a cerebral eye, and so on, are described in all text-books. (11) 2. But it is not enough empirically to record the ob- served phenomena of reproduction in the various groups, primarily essential though such labours are ; it is further necessary to generalize them. We should ascertain, that is to say, from a comprehensive and a comparative sur- vey, what degree of unity of structure is discovered actually to prevail within and among the various groups. Were this knowledge once definitely reached and incor- porated with the results of comparative anatomy and palaeontology, that complete conspectus of the animal kingdom which is the goal to which all morphological inquiries converge would come clearly into view. Such an exposition of the contributions of the study of de- velopment to morphological knowledge may evidently pro- ceed in either of two ways, either by starting from the later and more superficial similarities which are expressed in the classification of the different groups, and working downwards towards earlier and deeper unities in the various organs and tissues ; or by starting from the most primitive homology, and working upwards, so demonstrat- ing the unities which are observed to obtain in the suc- cessive stages, from segmentation and layer differentiation onwards. For our present purpose, it is more convenient to follow the latter course, and to note the main concrete generalizations of development in ascending order from the deepest and most general to the more superficial and specific. 1. Ovum Theory. The most fundamental unity upon which all the others really depend is the familiar fact that all organisms, plant and animal alike, develop from a single germinal cell or ovum essentially comparable with any ordinary cell. The import- ance of these results may be best expressed by a quotation from Agassiz's once famous Essay on Classification: Von Baer "thus showed for the first time that there is no essential difference in the mode of reproduction of the so-called viviparous and oviparous animals, and that Man himself developed in the same manner as animals. It was indeed a gigantic step to demonstrate such an identity in the material basis of the development of all animals, when their anatomical structure was already known to exhibit such radically different forms. The universal presence of eggs in all animals, and the unity of their structure, which was soon after- wards fully ascertained, constitute in my opinion the greatest discovery in the natural science of modern times." The ovum has long ceased to be regarded as a mysterious microorganism, and, while in some cases a number of cells unite to nourish and perfect it, the generalization remains unshaken, that the ovum of every organism is a simple cell. The minute structure of the ovum, its differentiations and variations, its fertilization and development, must be expressible in terms of the morphology and physiology of the ordinary cell. The elucidation will be obviously mutual, for the analysis of the phenomena observnl in the case of the highly differentiated reproductive cells, with their consequent greater physiological simplicity, will shed light on analogous phenomena in ordinary cellular and Protistau life lu-iv, with less differentiation, there is greater physiological complexity, while such problems as the origin and import of polar globule extrusion, fertilization, &c., receive some elucidation from sugges- tive analogies among the Protozoa. See SEX. (11) 2. Segmentation. The segmentation of the ovum of the Metazoa varies in regularity and completeness throughout the groups, but the different types are not sharply defined from one another and are, as has been already noted, partially at least explicable as mechanically conditioned by the quantity of nutritive as opposed to formative material. Segmentation being only a special case of ordinary cell-multiplication (to which in behaviour of nuclei, &c., it fully corresponds), it awaits whatever elucidation may arise out of the present conflict of speculation as to the physical and physio- logical causes of division in general. It has further to be compared on the one hand with such cases of multiple division as are observed in spermatogenesis, and on the other hand, and more especially, with the earliest attempts to form cell aggregates and so elf'ect the passage from Protozoa to Metazoa. (11) 3. Gastraea Theory. We have seen in the development of the various groups how the result of segmentation is not a mere aggregate of cells but an integrated two-layered individual with a distinct enteric cavity. In 1872 Haeckel emphasized the import- ance of this form, and in 1875 he elaborated his "gastrcea" theory, according to which all animals pass through a gastrula stage which, in its typical form, resembles the ancestral Metazoon. He showed how the variously modified quasi-gastrula forms of the higher groups might be derived from the normal type, and corroborated his theory by reference to the persistence of the gastrula stage in certain Porifera, as well as in those Godentera and Vermes where the adult animal is not far above the level of the gastrula organi- zation. The occurrence of the planula and the delamination pro- cess in some cases alongside of the invaginate gastrula is a diffi- culty in the way of his theory so far as it defines the primitive Metazoan ancestor, and this has given rise to Lankester's rival planula theory. The case on either side is fairly stated in Balfour's Embryology (vol. ii. chap. xiii. ). (Recent researches on Hydrozoa seem to show that delamination is an extreme form of immigra- tion, which is itself a modified invagination.) The passage from Protozoa to Metazoa was, according to Biitschli, effected neither by planulan nor gastrula but by a disk-like "placula." The ingenious 'coelom theory" of the Hertwigs is an attempt to divide all the Metazoa with three germinal layers into two distinct groups characterized by different modes of formation of body-cavity and mesoderm, and by other minor differences. In the Enteroccela, which include the Chsetopoda, Gephyrea, Brachiopoda, Nematoda, Arthropoda,Echinodermata,, Enter opneusta, and Chordata, the body- cavity is formed in the fashion noted in the case of Sagitta by two ingrowths from the archenteron, the bulk of the mesoderm arising from the differentiation of the epithelium of these diverticula, though sometimes partially also from amoeboid cells budded off into the gelatinous tissue between ectoderm and endoderm. In the smaller group of Pseudoccela, including the Mollusca, Polyzoa, Rotifera, and Platyhelminthcs, the mesoderm originates in the second of these ways, and the body-cavity is formed by a split in this "mesen- chyme." This theory has been criticized by Balfour (Emir. , ii. 13), and more recently ably defended by Haeckel and others ; further detailed studies are at least needed before it can claim full con- firmation. (15) 4. Unity of Tissues and Homology of Layers. Since it is impos- sible that all the cells resulting from segmentation can continue to occupy the same position, a certain lamination or layer formation with consequent division of labour and structural modification invariably occurs, finding its first expression in the two-layered gastrula stage to which we have referred. Just as in the embryo of the higher plants there arise an outer dermatogcn, an inner plerome, and a middle layer or pcriblem, so in the animal embryo there constantly occur three germ layers, the ectoderm, the endo- derm, and the intermediate layer formed from them the meso- derm. As early as 1768 Wolff seems to have had a definite presentiment of the formation of the different systems from distinct germ layers ; in 1817 Pander distinguished in the embryo chick the outer or "serous" from the inner or "mucous" layer, and You Baer in 1828 yet more definitely distinguished them as animal and vegetative respectively. A great step towards the recognition of the full significance of these germ layers was due to the brilliant insight of Huxley (1859) in comparing them to the ectoderm and endoderm which he had demonstrated in Ccelentera. Tin- researches of Remak, Rathke, Kowalevsky, and others led to their general recognition throughout the Metazoa. Modern progress is marked by the demonstration of the general homology of all the three germ layers and consequently of the systems of organs which arise from them. While much uncertainty still obtains as to the.