BLASTULA, GASTRULA, AND GERM LAYERS 345 



peripJiery of tJie blastodisc. The process of gastrulation concerns 

 only the blastodisc, for the yolk mass takes no more share in 

 this than in the later processes of development; the gastrula 

 forms separately from the yolk, which is left outside the embryo 

 and as we shall see, comes into relation with it only indirectly. 

 The blastula or blastodisc, once formed as a flat plate, many 

 cells in thickness, begins to extend over the surface of the yolk 

 mass. Its central part becomes quite thin in consequence, but 



pm 





FIG. 159. Diagrammatic drawings of sagittal sections through embryos of 

 Sauropsids. After Greil, after Will and Schauinsland. A, B. Two stages of 

 Reptile. C. Bird, a, archenteron; ch, rudiment of notochord; en, gut endo- 

 derm; pm, peristomial mesoderm; s, blastocoel or sub-germinal cavity; y, yolk. 



the margin of the disc, or germ ring, remains thickened, and as 

 it advances over the yolk its margin becomes slightly involuted, 

 forming a narrow shelf of cells on its inner surface, toward the 

 yolk (Fig. 157). This inner layer is the rudiment of the primary 

 inner layer. Gastrulation is thus primarily accomplished by 

 involution. The margin of the germ disc is clearly the germ 

 ring or rim of the blast opore, although its form and relation to 

 the yolk are quite unlike what we have seen heretofore. 



The Elasmobranchs and Reptiles afford important transi- 

 tional conditions here, in that a definite process of invagination 

 is indicated (Figs. 158, 159). Invagination is here limited to 



