The older embryologists (Pander, Baer, Remak), and, in recent times especially, His, Kölliker, and others, said that the two primary germinal layers of the hen’s ovum—the oldest and most frequent subject of observation!—arose by horizontal cleavage of a simple germinal disk. In opposition to this accepted view, I affirmed in my _Gastræa Theory_ (1873) that the discoid bird-gastrula, like that of all other vertebrates, is formed by folding (or invagination), and that this typical process is merely altered in a peculiar way and disguised by the immense accumulation of food-yelk and the flat spreading of the discoid blastula at one part of its surface. I endeavoured to establish this view by the derivation of the vertebrates from one source, and especially by proving that the birds descend from the reptiles, and these from the amphibia. If this is correct, the discoid gastrula of the amniotes must have been formed by the folding-in of a hollow blastula, as has been shown by Remak and Rusconi of the discoid gastrula of the amphibia, their direct ancestors. The accurate and extremely careful observations of the authors I have mentioned (Goette, Rauber, and Duval) have decisively proved this recently for the birds; and the same has been done for the reptiles by the fine studies of Kupffer, Beneke, Wenkebach, and others. In the shield-shaped germinal disk of the lizard (Fig. 62), the crocodile, the tortoise, and other reptiles, we find in the middle of the hind border (at the same spot as the sickle groove in the bird) a transverse furrow (_u_), which leads into a flat, pouch-like, blind sac, the primitive gut. The fore (dorsal) and hind (ventral) lips of the transverse furrow correspond exactly to the lips of the primitive mouth (or sickle-groove) in the birds.
Fig.58. Vertical section of the bastula of a hen. Fig. 59. The germinal disk of the hen’s ovum at the beginning of gastrulation. Fig. 60. Longitudinal section of the germinal disk of a siskin. Fig. 58—Vertical section of the blastula of a hen (_discoblastula_). _ fh_ segmentation-cavity, _dw_ dorsal wall of same, _ vw_ ventral wall, passing directly into the white yelk (_wd_). (From _Duval._) Fig. 59—The germinal disk of the hen’s ovum at the beginning of gastrulation; _A_ before incubation, _B_ in the first hour of incubation. (From _Koller._) _ks_ germinal-disk, _V_ its fore and _H_ its hind border; _ es_ embryonic shield, _s_ sickle-groove, _sk_ sickle knob, _d_ yelk. Fig. 60—Longitudinal section of the germinal disk of a siskin (_discogastrula_). (From _Duval._) _ud_ primitive gut, _vl, hl_ fore and hind lips of the primitive mouth (or sickle-edge); _ak_ outer germinal layer, _ik_ inner germinal layer, _dk_ yelk-nuclei, _wd_ white yelk.