THE BLASTOPORE 



241 



wait for the blastopore to close. Indeed, this takes a long 

 time, for the anterior edge of the blastoderm has to grow a 

 long way down and back under the yolk before it comes up 

 underneath and opposite the dorsal lip to form the ventral 

 lip of the blastopore. 



The edge of the blastoderm in the dogfish corresponds to 



oe^v!. 



Fig. 113. — Views of a developing embryo of a dogfish. (After Jenkinson.) 



A from above ; B, C, and D from the left side. The lips of the blasto- 

 pore are formed from the edge of the blastoderm, ae, anterior edge of the 

 blastoderm ; dl, dorsal lip of the blastopore ; e, embryo ; vl, ventral lip of 

 the blastopore ; y, yolk. 



the edge of the pigmented cells of the animal hemisphere in 

 the frog, and this is the place where the overgrowing lip which 

 is the rim of the blastopore arises in the frog also. All round 

 the rim of the blastopore, the ectoderm, mesoderm, and 

 endoderm are in contact. In the case of the frog, the blasto- 

 pore starts a little below the equator of the spherical embryo, 



R 



