GASTRULATION. 



65 



of the blastoderm has doubled and during this growth it has lost the compact char- 

 acter of the earlier stage. We recognize, however, in the cellular mass (at the left 

 in the figure, pi)i) the group of cells which formed the ventral lip of the blastopore, 

 and from a detail of this region, fig. 64', we conclude that the blastopore, bp, has 



64" 



■m 



Fig. 64. — Sagittal section of gastrula slightly older than the preceding. X 35. a, Archenteron ; bp, position of 



former blastopore; pni. Cells of posterior lip of blastopore; sc\ Segmentation cavity. 

 Fig. 64'. — Detail of preceding section showing the region of the blastopore. 

 Fig. 64". — Lateral section from the series from which fig. 64 was drawn. 



become closed, owing probably to stress arising from the rapid extension backward 

 of the entire blastoderm; and we note in this connection the greatly compressed 

 character of the cells. Parenthetically, we may also call attention in another 



