GASTRULA OF CHICK. 227 



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

 nutritive yelk is attached directly to the lower surface of 

 the 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, 8, 48, s). 

 The latter lies between the nutritive yelk and the blasto- 

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

 derm. The inversion (invagination) of the Gastrula is 

 complete when the primitive intestinal cavity has taken 

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

 time attaching its inner surface to the inner surface of the 

 exoderm. 



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

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

 thus already a complete Disc-gastrula (Discogastrula, Fig. 

 4i9). It is plainly visible to the naked eye, and appears 

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

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

 It is separated from the latter by the primitive intestinal 

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

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

 germ-layers are plainly visible in the perpendicular section ; 

 an upper or outer layer of smaller, brighter cells forming 

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

 layer of larger, darker cells forming the intestinal layer 

 (entoderm, Fig. 49 f). 74 



In order to complete our survey of the important pro- 

 cesses of egg-cleavage and gastrulation, we will now finally 

 glance quickly at the fourth type-form of these processes 



