GASTRULA OF CHICK. 22/ 



(Plate III. Fig. 24). While, however, in the latter case the 



nutritive yelk is attached directly to the lower surface of 



tlie entoderm, filling the whole primitive intestinal cavity, 



a low germ-cavity remains between the entoderm and the 



nutritive yelk in the Disc-gastrula of the Chick ; this is a 



part of the primitive intestinal cavity (Fig. 49, d), and must 



not be confused with the cleavage-cavity (Fig. 47, s, 48, s). 



The latter lies between the nutritive yelk and the blasto- 



lerm, the former between the nutritive yelk and the ento- 



lerm. The inversion (invagination) of the Gastrula is 



complete when the primitive intestinal cavity has taken 



he place of the cleavage-cavity, the entoderm at the same 



ime attaching its inner surface to the inner surface of the 



ixoderm. 



The germ-disc (Blastodiscus), which in an unincubated, 



reshly-laid Hen's egg lies at the tread, or cicatricula, is 



hus already a complete Disc-gastrula {Discogastrula, Fig. 



9). It is plainly visible to the naked eye, and appears 



ike a small, circular, white spot, 4-5 mm. in diameter, in 



he middle of the upper surface of the yellow yelk-mass. 



t is separated from the latter by the primitive intestinal 



avity, and its thickened edges alone touch the latter. It 



3 possible to lift up the entire Gastrula. The two primary 



erra-layers are plainly visible in the perpendicular section ; 



n upper or outer layer of smaller, brighter cells forming 



he skin-layer (exoderm. Fig. 49, e) ; and a lower or inner 



lyer of larger, darker cells forming the intestinal layer 



entoderm, Fig. 49 i)!^^ 



In order to complete our survey of the important pro- 



3sses of egg-cleavage and gastrulation, we will now finally 



lance quickly at the fourth type-form of these processes 

 12 



