BULLFROG AS TYPICAL VERTEBRATE ANIMAL 



99 



proceed the cells at the animal pole become smaller and more numer- 

 ous than those at the other pole. The final result of the following 

 divisions is to form a hollow sphere, the hlastula, whose cavity is 

 known as the hlastocoele. The blastula is essentially one cell layer 

 thick, although in reality some cells have been crowded from the sur- 

 face, giving the appearance of additional layers. 



Ectodernn , 



Endoderm-- 



Archenteron - 



or 



primitive 

 Intestine 



Dorsa] lip of blastopore 



Yolk pluqi -Ventral l\p of blastopore 



Fig. 34. — Section through gastrula stage of developing frog. 



Gastrulation. — Gastrulation begins with the appearance of a small 

 groove slightly below the equatof of the egg. The upper edge of 

 this groove is known as the dorsal lip of the blastopore. Coincident 

 with the appearance of this groove, the pigmented animal cells begin 

 to grow down over the white vegetal, or yolk, cells; and the dorsal 

 lip moves downward as the line advances. It also extends its edges 

 in a crescent shape laterally around the egg until they finally meet to 

 form a circle. The area enclosed by this circle shrinks as the cells 

 grow down from all sides and its rim moves downward. Cells on the 

 side where the groove began advance more rapidly than the others; 



