32 



PHYLUM CEPHALOCHORDA. 



the first having been formed in the ovary and rubbed ofiE during 

 the dehiscence. The segmentation is complete and almost equal, 

 the segments of the lower pole being slightly larger than those 

 of the upper. It leads to the formation of a hollow blasto- 



G 



H 



KiG. 14. — Cleavage of AmpMorns (after Salensky from Korsdielt anrl Heider). A egg before 

 cleavage, with polar body ; B two-cell stage ; C four-cell stage ; D the same seen from the 

 upper surface ; iS eight-cell stage ; F sixteen-cell stage ; 6' stage showing more rapid division 

 at the animal pole ; H the same in section ; / surface view of lilastosphere. 



sphere (Fig. 14). This becomes invaginated to form a cup- 

 shaped gastrula (Fig. 15). The blastopore, at first wide, soon 

 narrows to a small opening placed at the hind end of the future 

 dorsal surface. The embryo now elongates in the direction of 

 the antero-posterior axis (Fig.16); and the ectoderm of the dorsal 



