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 

 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, s, 48, s). 

 The latter lies between the nutritive yelk and the blasto- 

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

 denn. 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. 

 49). 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 *). 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 



