THE EARLY DEVELOPMENT OF THE FROG 105 



of the blastoccel is added to the archenteron and the endodermal 

 wall is completed by growth of the portion already formed. 



The formation of the archenteric cavity in a region formerly 

 occupied by yolk cells, and the gradual enlargement and shift- 

 ing of this cavity as well as of the blastoccel, obviously alter the 

 position of the center of gravity of the gastrula as a whole, and 

 the extensive changes in the relative positions of yolk and 

 protoplasmic cells, whose specific gravities are unlike, contrib- 

 ute to the same alteration. This all results in a rotation of 



gastrula about a horizontal transverse axis (Fig. 34). 

 During the early phases of gastrulation, as just described, the 

 more rapid growth of the dorsal (posterior) lip of the blasto- 



A 



FIG. 35. Diagrams of the frog's gastrula showing the position of the blasto- 

 pore at various ages. A. Posterior view. B. Lateral view. 1-5 indicate the 

 successive positions and forms of the blastopore. The change in position is due 

 both to the actual growth movements of the blastopore, and to the rotation of 

 the entire gastrula. Compare Figs. 32, 34. 



pore carries this to or even past the lower pole of the gastrula, 

 even past the lower gravitational pole. Then as the blastopore 

 continues to narrow, the whole gastrula rotates slowly in the 

 opposite direction, carrying the blastopore back to the region 

 where the dorsal lip first appeared and then on dorsally some- 

 what above the equator, into a postero-dorsal position, where 

 it remains stationary for a time. Thus the shifting of the blas- 

 topore is the combined result of changes due to growth and to 

 rotation. On account of the localization of the growth processes 

 in the gastrula, which will be described presently, there is not 

 a complete correspondence between the chief axis of the bias- 



