Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/440

Rh REPRODUCTION [-VNIMAL. not be appreciated until the parallel between this developmental progress and the advance of anatomical type which had been exposed by Cuvier received its rational explanation at the hands of Darwin. (17) (c) HaeckeUs "Biogenetic Law" While Von Baer was the first to appreciate the value of embryology in its relation to classification and comparative anatomy, we are indebted to Haeckel for the detailed application of the Darwinian theory to the phenomena of embryology and the conse- quent restatement of Von Uaer's law in its developed form. In his Generelle Morphologie (18G6) he formulates the " fundamental law of development " (biogenetisches Grund- (jesetz). Introducing the term "ontogeny" to denote the development of the individual organism, and " phylogeny " to express the historic evolution of the " phylon" or tribe, he affirms that " ontogeny is an epitome of phylogeny," or, more explicitly, " the organism recapitulates in the short ^nd rapid course of its individual development the most important of those form-modifications undergone by the successive ancestors of the species, in the course of their long and slow historic evolution, and the causal relation of the two histories is to be explained in terms of heredity and adaptation. When these are thoroughly analysed, it will be possible to say that the phylogeny is the mechanical cause of the ontogeny." Much as Von Baer had distinguished the general morphological ground-plan from the more detailed differ- entiations of the organs, Haeckel analysed ontogeny into (a) the " palingenetic " process, in which the truly ancestral characters conserved by heredity are reproduced in de- velopment, and (6) the " kenogenetic " process, or modified evolution, to which are due those non-primitive characters which have resulted in consequence of a secondary adapta- tion of the embryo to the peculiar conditions of its own environment. The true recapitulation is constantly liable to be disguised, not only by the frequent occurrence of that abbreviated and more direct ontogeny which the need for economy tends alike to originate and to conserve, but by the action of these kenogenetic processes. Hence the corollary that "the ontogeuetic recapitulation of the phylogeny is the more perfect the more the palingenetic process is conserved by heredity, and the more imperfect in proportion as the later modified evolution (kenogenesis) is introduced by adaptation." While this distinction between adaptive characters and underlying morphological type is not only legitimate but indispensable, it must not, however, be forgotten that the difference between these is nowhere absolute, the deepest morphological characters being but the most ancient results of adaptation (cf. MORPHOLOGY, vol. xvi. p. 845). Yet it is only by the careful application of this principle that the embryologist can unravel the perplexing entanglement of primitive and adaptive characters pre- sented by so many larval forms, or solve the scarcely less difficult problems of organogeny. In this regard Balfour's dissertation on the origin and affinities of larval forms is especially valuable, while a vivid illustration of the em- ployment of the biogenetic law, in one of the most diffi- cult departments of ontogeny, may be borrowed from Prof. Parker. Reviewing the development of the skull in the Chick, he says " Whilst at work I seemed to myself to have been endeavouring to decipher a palimpsest, and that not erased and written upon again just once, but live or six times over. " Having erased, as it were, the characters of the culminating type those of the gaudy Indian Bird I seemed to be amongst the sombre Grouse, aud then, towards incubation, the characters of the Sand-Grouse and Hemipod stood out before me. Rubbing these away, in my downward walk, the form of the Tiiiamou looked me in the face ; then the aberrant Ostrich seemed to lie described in large archaic characters ; a little while and these faded into what could just be read off as pertaining to the Sea Turtle ; whilst, underlying the whole, the Fish in its simplest Myxinoid form could be traced in morphological hieroglyphics." (17) (d) Spencer on Development. The most generalized treatment of embryology is that of Spencer, who, after carefully distinguishing mere growth in bulk from de- velopment of structure, points out that development takes place primarily around a central point, as in the lowest and chiefly unicellular organisms. Central development is either uuicentral or multicentral, while, according as the insubordination to a single centre, implied in the latter case, is more or less thorough, the organism is of irregular form (e.g., many Algae), and so may readily even become discontinuous. From central we pass insensibly to n.nnl development, and this may be uniaxial or multiaxial. Here, too, development may be continuous or discontinuous, familiar instances of both being furnished by many animals and plants. The fundamental importance of these simple conceptions to the adequate treatment alike of the pro- blems of individuality (cf. MORPHOLOGY) and of the nature of the reproductive process is justly to be insisted upon, for the definition of reproduction as but a discontinuous growth and development finds here its origin and justi- fication. Spencer, moreover, expresses Von Baer's essential law in yet more general phrase : "Development is a change from an incoherent indefinite homogeneity to a coherent definite heterogeneity." The relation of ontogeny to phylogeny is not overlooked, and a yet farther parallel advance in dif- ferentiation of the organism from its environment is illus- trated alike in structure and form, in chemical composition and specific gravity, in temperature and self-mobility. The deductive interpretation of these laws is also cautiously suggested. (17) Bibliography. Without any attempt to deal with the very copious literature of the subject, it is sufficient to name some of the more important general and special works, from which full details can in turn be obtained. From the time of Haller perhaps no eminent anatomist or physiologist has omitted a more or less general treatment of the subject, and such discussions as those to be found in the well-known works of Johannes Mu'ller, Milne- Ed wards, Owen, or Carpenter are still valuable, especially as embodying the past development of the subject. More recent dis- cussions are to be found in the leading text-books, alike morpho- logical (Huxley, Gegenbauer, Glaus) and physiological (Hermann, Foster, Landois, &c.). The embryological movement can be followed by the aid of the article EMBRYOLOGY, and the valuable systematic treatise of Balfour, while the most generalized treat- ment of the subject must at first be sought in the works of Spencer and Haeckel. For almost all purposes, however, the recent careful monograph of Hensen (Physiologic d. Zeugung, forming the second part of vol. vi. of Hermann's Handbuch d. Physiologic, Leipsic, 1881), although, of course, by no means completely satisfactory, will be found not merely serviceable but indispensable to the student. The various Jalircsbcrichte must be also, of course, con- stantly appealed to, especially for progress in detail. The present state of the theory of reproduction is discussed in Heiisen, op. dt. (1) c/. Hensen, op. cil., general manuals, and pyriMcological work-. Williams, "On the Structure of the Mucous Membrane of the Uterus," Ubftrtr. Journ., 1875; Barnes, System of Midwifery, 1885; Lusk, Sci. and Art. of Mid- wifery, 1882. (2) Huxley, Anal, oj Invert. Animals, 1879; Gegenbaur, Comp. Anal., London, 1882; Wiertersheim, Lehrb. d. Comp. Anat. d. WirMtMtrf, Jena, 1883; Semper, D. Urinogenital Syst. d. PlagioBtomen, in his Arbeiten, vol. ii.; Balfour, Comp. Embryology, 1882. (3) Hensen, op. cil.; Foster's and Landois's manuals of physiology. (4) Wertli, " 1'hvMologie d. Geburt," in chap, xiii. of Hensen, op. cit.; Spiegelberg, Lehrb. d. Geburtshiilfe, Lahr, 1878. (5) Milne-Edwards, Lemons *. la /'hi/sioloyie, and later manuals. (6) Hensen, op. cit. (7) Uamgee, Pliysiol. C'hem. of Anim. Body, 1880, and Heiisen, op. cit. (8) V. La Valrite, " Ueh. d. Genese d. Sanicnkoiper," Archiv f. Mikr. Anat., xv. ; Blom- field, Quart. Journ. Micro. Set., 1880; Kenson, Arch. <l. liiol., 1882; Swn-n UM Masquelin, Arch. d. liiol., 1883 ; Gfddcs and Arthur Thomson, " On the History and Theory of Spermatogcnrsis,' 1 /'roc. Hoy. Soc. Edin., 1885-80. (9) See EMBRYOLOGY, and article "Ovum" (by Allen Thomson), in Todd-'s Cyclop, of Anat. and Physiol. (10) Gamgee, op. cit.; Hensen, op. cil. (11) Balfour, cj'. <'!., and manuals. (12) Balfour, op. cit.; Flemming, Zellstibstanz, &c., Leipsic, l?l Strasburger, Zeltbilduny, t the Mult, 1877. C 1 '- Cib ")