40 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



accompanied, no doubt, by a certain amount of involution though 

 this is difficult to determine with certainty, has proceeded much 



yp 



ent 



-9-i 



Fig. 25. — Sagittal, sections through the egg of liana lemporaria illustrating the process of 

 gastrulation. (After Jenkinson, 1913.) 



ect, ectoderm ; ent, archeuteric cavity ; g.l, gastrular lip ; s.c', segmentation cavity ; y.'p, yolk-plug. 

 The arrow indicates the original apical pole. 



further so that the archenteron is much deeper. The spreading of 

 the yolk-cells along the roof of the segmentation cavity, already 



