Fig.61. Longitudinal section of the discoid gastrula of the nightingale. Fig. 61—Longitudinal section of the discoid gastrula of the nightingale. (From _Duval._) _ud_ primitive gut, _ vl, hl_ fore and hind lips of the primitive mouth; _ak, ik_ outer and inner germinal layers; _vr_ fore-border of the discogastrula. Fig.62. Germinal disk of the lizard. Fig. 62—Germinal disk of the lizard (_Lacerta agilis_). (From _Kupffer._) _u_ primitive mouth, _ s_ sickle, _es_ embryonic shield, _hf_ and _df_ light and dark germinative area. The gastrulation of the mammals must be derived from this special embryonic development of the reptiles and birds. This latest and most advanced class of the vertebrates has, as we shall see afterwards, evolved at a comparatively recent date from an older group of reptiles; and all these amniotes must have come originally from a common stem-form. Hence the distinctive embryonic process of the mammal must have arisen by cenogenetic modifications from the older form of gastrulation of the reptiles and birds. Until we admit this thesis we cannot understand the formation of the germinal layers in the mammal, and therefore in man. I first advanced this fundamental principle in my essay _On the Gastrulation of Mammals_ (1877), and sought to show in this way that I assumed a gradual degeneration of the food-yelk and the yelk-sac on the way from the proreptiles to the mammals. “The cenogenetic process of adaptation,” I said, “which has occasioned the atrophy of the rudimentary yelk-sac of the mammal, is perfectly clear. It is due to the fact that the young of the mammal, whose ancestors were certainly oviparous, now remain a long time in the womb. As the great store of food-yelk, which the oviparous ancestors gave to the egg, became superfluous in their descendants owing to the long carrying in the womb, and the maternal blood in the wall of the uterus made itself the chief source of nourishment, the now useless yelk-sac was bound to atrophy by embryonic adaptation.” My opinion met with little approval at the time; it was vehemently attacked by Kölliker, Hensen, and His in particular. However, it has been gradually accepted, and has recently been firmly established by a large number of excellent studies of mammal gastrulation, especially by Edward Van Beneden’s studies of the rabbit and bat, Selenka’s on the marsupials and rodents, Heape’s and Lieberkühn’s on the mole, Kupffer and Keibel’s on the rodents, Bonnet’s on the ruminants, etc. From the general comparative point of view, Carl Rabl in his theory of the mesoderm, Oscar Hertwig in the latest edition of his Manual (1902), and Hubrecht in his _Studies in Mammalian Embryology_ (1891), have supported the opinion, and sought to derive the peculiarly modified gastrulation of the mammal from that of the reptile.