Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/439

Rh ANIMAL.] REPRODUCTION 421 origin of the mesoderm, and while the different layers undoubtedly exhibit in some cases the possibility of differentiation into tissues different from those to which they usually give origin, yet the general homology of the layers is now indisputable. The ectoderm, which is indeed the primitive sensory and protective organ, forms the epidermis, sense organs, and nervous system of the adult ; the nutritive endoderm lines the enteric cavity, &c. ; and from the mesoderm are derived the muscular, vascular, and lymphatic systems and the greater part of the connective tissue, as also the excretory and generally also the generative system. Not only then is the occurrence of these three layers constant, but, with few exceptions, so is the differentiation of each into definite systems of organs. This correspondence, which is of high in most cases of final importance in determining homologies, may be termed homodermic. See MORPHOLOGY. (15) 5. Unity of Anatomical Structure. The study of development leads, however, to the recognition of a more detailed and specific iinity than the general one of homodermy. For, just as the development of the flower reveals the original resemblance of organs which become subsequently widely differentiated, so organs which in their adult modification seem hardly comparable are seen to be moulded from one pattern when compared in their embryonic simplicity. The same holds good obviously of the comparison of organs in different animals ; organogeny is the necessary foundation of comparative anatomy. Thus of late years it has been demon- strated that the nervous system and sense organs, throughout at any rate the vast majority of Metazoan forms, not only constantly arise from the ectoderm that is to say, are homodermic but their development exhibits correspondences even in detail. Recent studies of the development of the Vertebrate skeleton, and more especially of the skull, afford vivid instances of that fundamental unity of structure of which, in another department, the final unravelment of the complex problem of the structure of the urinogenital organs of Vertebrates through the researches of Balfour and Semper is one of the most recent and admirable instances. (11) 6. Unity of Type. It has been the general history of classifica- tion both of plants and animals that arrangements based on super- ficial resemblance were superseded by those founded on internal organization, while the latter have been in turn either corroborated or amended by the relationships revealed by the study of develop- ment, as from the preceding generalizations must indeed be obvious. Just as the separation of the Monocotyledons and Dicotyledons into two great alliances was anatomically recognized by Ray, and em- bryologically corroborated by De Jussieu, so the developmental studies of Von Baer led him independently to the establishment of the four great types which had been previously distinguished by the anatomical labours of Cuvier. And, apart from the recognition of the morphological unities of the greater groups, it is familiarly known how in the case of problematical forms, such, for instance, as Cirripedes or Ascidians, where the adult organization is obscured by degeneration, the detection of the true relationship is due to em- bryology. In proportion, too, as the recapitulation during develop- ment of adult structure becomes shortened and effaced, so does the determination of their real affinities become difficult ; hence the relatively slow progress of the botanist towards a knowledge of the deeper affinities of his "natural orders." (11) As the preceding outline of embryological detail is being rapidly filled up, and the resultant concrete gene- ralizations are being more and more clearly defined, and also united with those of comparative anatomy and palae- ontology, the embryologist is rapidly approximating that tolerably adequate knowledge of the morphological rela- tions of plants and animals alike which finds its graphic expression in the " family tree " of the Organisata. While, for instance, the discrepant speculations of Semper, Hubrecht, Sedgwick, and others, as to the ancestral form of the Chordata, afford illustration of how far we are from being able to construct a thoroughly definite genea- logical tree, on the other hand no better evidence alike of the rapid stages by which our knowledge has advanced and of the utility of such a method of graphic nota- tion can be obtained than by comparing the necessarily vague and hypothetical tree sketched by Haeckel only twenty years ago with any of its numerous and shortlived successors. (16) 3. The result of the study of development is not, how- ever, merely to establish the existence of such concrete structural unities as those just indicated, but leads further to the recognition of certain abstract generalizations, ex- pressive of the most comprehensive conclusions which can be drawn from the observed succession of developmental phenomena. (a) Heredity. It is an every-day observation that the offspring of any organism repeats the organization of the parent; and the very familiarity of the fact is apt to conceal the marvellousness of the process in which every egg cell develops, either directly or indirectly, into a form which not only resembles the parent in general and specific characters, but may even repeat those individual character- istics which arose by so-called spontaneous variation, or which were even impressed upon the parent by the direct influence of the external environment. The difficulty of analysing the factors which give rise to this result, that is, of understanding how the history of the developing ovum is determined by its constitution the uncertainty as to the degree in which acquired parental characteristics can be said to be transmitted, the absence in fact of any established causal explanation of the resemblance between offspring and parent, in specific and individual characters, does not of course affect the fact. Although, in analysing the popular generalization that " like begets like," it may eventually be shown how much of that likeness may be due to the hammering of the same environmental forces which formerly played upon the parent, a mysterious transmission of properties has still to be accounted for, and interpreted in terms of the physiological and morphological, the chemical and physical, composition and properties of the germinal matter of parent and offspring. To explain this mystery, various "theories of heredity" have been from time to time propounded ; from the present purely morpho- logical point of view it suffices here, however, to note the fact of heredity, leaving the discussion of its rationale to its more natural place at the outset of the article VARIA- TION AND SELECTION. (6) (6) Von Baer's Law. In comparing the degree of organization attained by different forms, we are accustomed to distinguish the general morphological ground-plan con- stant throughout the group from the detailed histological differentiation or elaboration of the various organs. Great histological simplicity may co-exist with a high general morphological plan, and vice versa. A survey of the animal and vegetable kingdom reveals a branching and ascending series of increasingly complex ground-plans, while, in any of the groups determined by these, similar branching series more and more highly differentiated in detail are to be distinguished. Von Baer (1828) was the first to discern the embryological aspect of this law of pro- gress, and to show that, in the development alike of the organism and of its component parts, there was a progress from the simple to the complex, from the general to the special. Thus, in the development of one of the higher Mammalia, those characters which are common to the low- est Vertebrates are at first alone distinguishable. Thence the embryo passes through stages resembling those of successively higher forms, till the general Mammalian type is reached, this again passing through higher and less general stages till the specific characters finally make their appearance; and this progressive differentiation from gene- ral to special holds equally of the histological differentia- tion of the organs. Von Baer guarded against the error involved in many popular versions of his generalization, by maintaining that the developing embryo resembled, not the adults, but merely the embryos of lower forms ; and, although he narrowed his proposition to the limits of the great groups, denying, for example, any resemblance between Vertebrate embryos and those of any Invertebrate type, this must be admitted a thoroughly legitimate reserve when we consider the actual state and practical possibilities of embryological research at the time. The real value and import of Von Baer's law, however, could