GERM LAYERS. 



63 



k — Area apaca 

 Area pellucida 



Blastopore 

 — (crescentic 

 groove) 



Fig. 43. — Surface view of blai 

 derm of unincubated hen's 

 Hertwig. 



It will be remembered that in the discoidal cleavage of Birds the blastula 

 consists of a cleft-like segmentation cavity, the roof of which is formed by the 

 proliferating micromeres constituting the germ disk, while the floor is formed by 

 the partially segmenting yolk (Fig. 29). The former corresponds to the micro- 

 meres of the blastula roof in Amphioxus and Amphibians, the latter to the 

 underlying yolk cells. (Compare Figs. 26, 6, 30 and 29.) 



In Reptiles the beginning of gastrulation is 

 evidenced by the appearance of an opacity just in 

 front of what may now be designated the posterior 

 margin of the disk (Fig. 40). This is due to more 

 rapid proliferation of cells at this point. The 

 opacity soon shows a depression or groove which 

 more or less sharply defines the posterior margin 

 of the disk. It varies in shape in different Rep- 

 tiles. It is frequently crescent-shaped and has 

 been called the crescentic groove (Fig. 41). This 

 groove is the blastopore, and corresponds to the blastoporic invagina- 

 tions of Amphioxus, Triton and the frog. Soon after the formation of the 

 crescentic groove, there appears in front of it an oval opacity which extends 

 forward in the medial line (Fig. 41). This opacity is due to growth of cells 

 forward from the blastopore under the surface cells as seen in Fig. 42 which 

 shows the progress of the invagination in the lizard. These figures should be 



compared with Figs. 34, 36 and 37, 

 showing the stages of gastrulation in 

 Amphioxus and Triton, and especially 

 with Figs. 38 and 39 showing gastrula- 

 tion in the frog. 



In Fig. 42, 1, the blastopore is seen 

 as a distinct invagination. As in the 

 frog (Fig. 39) the invagination pushes 

 in front of it a wedge-shaped mass of 

 cells which extends forward under the 

 outer layer. These cells are the pro- 

 tentoderm. They form the roof and, 

 with the underlying yolk entoderm, the 

 floor of the new invagination cavity 

 (Fig. 42, 2). As they extend forward they meet with a thickened part of 

 the yolk entoderm, the " Erganzungsplatte " or "completion plate" (Fig. 42, 

 2, 3, 4 and 5; compare Fig. 39). There are thus present at this stage, just 

 as in the frog, three cavities, (1) the slit-like remains of the segmentation 

 cavity, (2) the invagination cavity and (3) the "completion cavity." Also 

 S 



Fig. 44. — From vertical longitudinal section 

 through germ disk of siskin, showing beginning 

 of gastrulation. Duval. 



a.b., Anterior lip of blastopore; arc, archen- 

 teron; ec, ectoderm; en., entoderm; p.b., posterior 

 lip of blastopore; y., white yolk; y.c, yolk cells 

 (merocytes). 



