94 OUTLINES OF CHORDATE DEVELOPMENT 



marks the end of the cleavage period and the formation of the 

 blastula. This arrangement is really definitely established 

 by the time thirty-two to sixty-four cells are formed, i.e., be- 

 fore many interior cells are present. The immediately sub- 



v 



FIG. 31. Sections through the blastula of the frog. A, B, from Morgan 

 (Development of the Frog's Egg). C. After O. Schultze. A, Early blastula 

 showing wall of segmentation cavity only one cell in thickness. B. Later stage 

 showing multiplication of cells in wall of segmentation cavity. C. Late blastula 

 showing the thinning of the roof of the segmentation cavity and the beginning 

 of the germ ring, a, Animal pole; gr, germ ring; p, pigment; s, segmentation 

 cavity or blastocoel; SG, Same as s; v., vegetal pole. 



sequent cleavages do not modify then- essential relations; the 

 interior cells multiply rapidly and some cells migrate inward 

 from the surface. The early blastula is spherical and about 

 the diameter of the egg (Fig. 31, B, C). The thinner roof and 

 lateral walls of the eccentric segmentation cavity, are formed 



