340 GENERAL EMBRYOLOGY / 



above, a region of more actively dividing cells can be distin- 

 guished extending around the fully formed blastula just above 

 its equator, that is, between animal and vegetal poles (Fig. 154) ; 

 this is termed the germ ring, although it is frequently not a 

 complete ring, being interrupted on the anterior side of the 

 blastula. Gastrulation commences by the true invagination of 

 cells just below the germ ring on the posterior side. The inert- 

 ness of the large yolk-filled cells of the vegetal hemisphere 

 greatly impedes the process of invagination and it is never 

 carried very far. Involution occurs extensively in the dorsal 



FIG. 154. Section through late blastula of frog, showing location of germ ring. 

 Later the germ ring is thickened and contracted, forcing the yolk cells upward 

 into the segmentation cavity. After O. Schultze. a, animal pole; gr, germ ring; 

 p, pigment; s, blastoccel; v, vegetal pole. 



region and is accompanied by very pronounced epiboly (Fig. 

 155); this latter process is very marked in the lateral and 

 ventral regions where little or no invagination occurs. All of 

 these processes are relatively less extensive than in the ccelo- 

 gastrula of Amphioxus however, probably on account of the 

 larger amount of yolk contained in all of the cells. Their place 

 in development is taken, in a way, by a new process, namely, 

 delamination. This is a process of splitting, whereby an 

 extended mass or sheet of cells becomes cleaved apart by the 

 rearrangement of the cells into two more or less distinct layers, 

 separated by a definite space. This process of delamination 



