104 OUTLINES OF CHORDATE DEVELOPMENT 



their elevation in the dorsal or postero-dorsal region, draws 

 away the yolk cells from beneath the original segmentation 

 cavity so that as the archenteron advances the segmentation 

 cavity recedes. Meanwhile the archenteron enlarges over the 

 whole animal region and actually encroaches upon the blasto- 



FIG. 34. Diagrams of median sagittal sections through the frog's cleavage 

 and gastrula stages, showing the changes in position during gastrulation. From 

 Ziegler (Lehrbuch, etc.}, after Kopsch. The arrow marks the vertical. (Accord- 

 ing to Morgan and others the figures of the first two stages should be rotated 

 about 15 in the counter-clockwise direction.) 



coel, so that as the latter moves toward the anterior side of the 

 gastrula it diminishes in size and soon disappears (Figs. 32, 33). 

 In some cases the wall of yolk cells separating the archenteron 

 and blastocoel becomes thin and breaks through, before the 

 blastoccel has completely disappeared; in this case the remnant 



