GASTRULATION. 



65 



of the blastoderm has doubled and durini; this t;ro\vth it has lost the compact char- 

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

 in the figure, /;;/) 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 



'.'■'Si 



64" 



Fig. 64.— Sagittal section o( gastrula slightly older than the preceding. X 35. a, Archenteron ; /»/>, position of 



former blastopore; pm. 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 (he 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 



