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PHYSIOLOGY OF DEVELOPMENT] Biologie, 4) of the reduction of the number of chromosomes in the nucleus of both male and female gametes, and of the fact that the male and female pronuclei contribute the same number of chromosomes to the zygote-nucleus. He also showed that the gametogenesis in the male is a similar process to that in the female, and paved the way for the acceptation of the view (due to Bütschli) that polar bodies are aborted female gametes. These discoveries were extended and completed by subsequent workers, among whom may be mentioned E. van Beneden, J. B. Carnoy, G. Platner, T. Boveri, O. Hertwig, A. Brauer. The subject is still being actively pursued, and hopes are entertained that some relation may be found between the behaviour of the chromosomes and the facts of heredity.

Since 1874 (W. His, Unsere Körperform und das physiologische Problem ihrer Entstehung) a new branch of embryology, which concerns itself with the physiology of development, has arisen (experimental embryology). The principal workers in this field have been W. Roux, who in 1894 founded the Archiv für Entwickelungsmechanik der Organismen, T. Boveri and Y. Delage who discovered and elucidated the phenomenon of merogony, J. Loeb who discovered artificial parthenogenesis, O. and R. Hertwig, H. Driesch, C. Herbst, E. Maupas, A. Weismann, T. H. Morgan, C. B. Davenport (Experimental Morphology, 2 vols., 1899) and many others.

In the elucidation of remarkable life-histories we may point in the first place to the work of A. Kowalewsky on the development of the Tunicata (“Entwickelungsgeschichte d. einfachen Ascidien,” Mém. Acad. Pétersbourg (7), 10, 1866, and Arch. f. Mic. Anatomie, 7, 1871), in which was demonstrated for the first time the vertebrate relationship of the Tunicata (possession of a notochord, method of development of the central nervous system) and which led to the establishment of the group Chordata. We may also mention the work of Y. Delage in the metamorphosis of Sacculina (Arch. zool. exp. (2) 2, 1884), A. Giard (Comptes rendus, 123, 1896, p. 836) and of A. Malaquin on Monstrilla (Arch. zool. exp. (3), 9, p. 81, 1901), of Delage (Comptes rendus, 103, 1886, p. 698) and Grassi and Calandruccio (Rend. Acc. Lincei (5), 6, 1897, p. 43), on the development of the eels, and of P. Pergande on the life-history of the Aphidae (Bull. U.S. Dep. Agric. Ent., technical series, 9, 1901). The work of C. Grobben (Arbeiten zool. Inst. Wien, 4, 1882) and of B. Uljanin (“Die Arten der Gattung Doliolum,” Fauna u. Flora des Golfes von Neapel, 1884) on the extraordinary life-history and migration of the buds in Doliolum must also be mentioned. In pure embryological morphology we have had Heymons’ elucidation of the Arthropod head, the work of Hatschek on Annelid and other larvae, the works of H. Bury and of E. W. MacBride which have marked a distinct advance in our knowledge of the development of Echinodermata, of K. Mitsukuri, who has founded since 1882 an important school of embryology in Japan, on the early development of Chelonia and Aves, of A. Brauer and G. C. Price on the development of vertebrate excretory organs, of Th. W. Bischoff, E. van Beneden, E. Selenka, A. A. W. Hubrecht, R. Bonnet, F. Keibel and R. Assheton on the development of mammals, of A. A. W. Hubrecht and E. Selenka on the early development and placentation of the Primates, of J. Graham Kerr and of J. S. Budgett on the development of Dipnoan and Ganoid fishes, of A. Kowalewsky, B. Hatschek, A. Willey and E. W. MacBride on the development of Amphioxus, of B. Dean on the development of Bdellostoma, of A. Götte on the development of Amphibia, of H. Strahl and L. Will on the early development of reptiles, of T. H. Huxley, C. Gegenbaur and W. K. Parker on the development of the vertebrate skeleton, of van Wijhe on the segmentation of the vertebrate head, by which the modern theory of head-segmentation, previously adumbrated by Balfour, was first established, of Leche and Röse on the development of mammalian dentitions. We may also specially notice W. Bateson’s work on the development of Balanoglossus and his inclusion of this genus among the Chordata (1884), the discovery by J. P. Hill of a placenta in the marsupial genus Perameles (1895), the work of P. Marchal (1904) on the asexual increase by fission of the early embryos of certain parasitic Hymenoptera (so called germinogony), a phenomenon which had been long ago shown to occur in Lumbricus trapezoides by N. Kleinenberg (1879) and by S. F. Harmer in Polyzoa (1893). The work on cell-lineage which has been so actively pursued in America may be mentioned here. It has consisted mainly of an extension of the early work of A. Kowalewsky and B. Hatschek on the formation of the layers, being a more minute and detailed examination of the origin of the embryonic tissues.

The most important text-books and summaries which have appeared in this period have been Korschelt and Heider’s Lehrbuch der vergleichenden Entwickelungsgeschichte der wirbellosen Tiere (1890–1902), C. S. Minot’s Human Embryology (1892), and the Handbuch der vergleichenden und experimentellen Entwickelungslehre der Wirbeltiere, edited by O. Hertwig (1901, et seq.). See also K. E. von Baer, Über Entwicklungsgeschichte der Tiere (Königsberg, 1828, 1837); F. M. Balfour, A Monograph on the Development of Elasmobranch Fishes (London, 1878); A Treatise on Comparative Embryology, vols. i. and ii. (London, 1885) (still the most important work on Vertebrate Embryology); M. Duval, Atlas d’Embryologie (Paris, 1889); M. Foster and F. M. Balfour, Elements of Embryology (London, 1883); O. Hertwig, Lehrbuch der Entwicklungsgeschichte des Menschen u. der Wirbeltiere (6th ed., Jena, 1898); A. Kölliker, Entwicklungsgeschichte des Menschen u. der höheren Tiere (Leipzig, 1879); A. M. Marshall, Vertebrate Embryology (London, 1893).

Physiology of Development [in German, Entwicklungsmechanik (W. Roux), Entwicklungsphysiologie (H. Driesch), physiologische Morphologie (J. Loeb)] is, in the broadest meaning of the word, the experimental science of morphogenesis, i.e. of the laws that govern morphological differentiation. In this sense it embraces the study of regeneration and variation, and would, as a whole, best be called rational morphology. Here we shall treat of the Physiology of Development in a narrower sense, as the study of the laws that govern the development of the adult organism from the egg, and forming the subjects of special articles.

After the work done by W. His, A. Goette and E. F. W. Pflüger, who gave a sort of general outline and orientation of the subject, the first to study developmental problems properly in a systematical way, and with full conviction of their great importance, was Wilhelm Roux. This observer, having found by a full analysis of the facts of “development” that the first special problem to be worked out was the question when and where the first differentiation appeared, got as his main result that, when one of the two first blastomeres (cleavage cells) of the frog’s egg was killed, the living one developed into a typical half-embryo, i.e. an embryo that was either the right or the left part of a whole one. From that Roux concluded that the first cleavage plane determined already the median plane of the adult; and that the basis of all differentiation was given by an unequal division of the nuclear substances during karyokinesis, a result that was also attained on a purely theoretical basis by A. Weismann. Hans Driesch repeated Roux’s fundamental experiment with a different method on the sea-urchin’s egg, with a result that was absolutely contrary to that of Roux: the isolated blastomere cleaved like half the egg, but it resulted in a whole blastula and a whole embryo, which differed from a normal one only in its small size. Driesch’s result was obtained in somewhat the same manner by E. B. Wilson with the egg of Amphioxus, by Zoja with the egg of Medusae, &c. It thus became very probable that an inequality of nuclear division could not be the basis of differentiation. The following experiments were still more fatal to the theories of Roux and of Weismann. Driesch found that even when the first eight or sixteen cells of the cleaving egg of the sea-urchin were brought into quite abnormal positions with regard to one another, still a quite normal embryo was developed; Driesch and T. H. Morgan discovered jointly that in the Ctenophore egg one isolated blastomere developed into a half-embryo, but that the same was the case if a portion of protoplasm was cut off from the fertilized egg not yet in cleavage; last, but not of least importance, in the case of the frog’s egg which had been Roux’s actual subject of experiment, conditions were discovered by O. Schultze and O. Hertwig