32 



PHYLUM CEPHALOCHORDA. 



the first having been formed in the ovary and rubbed off 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 blast o- 



FIG. 14. Cleavage of Amphioxus (after Salensky from Korschelt and Heider). A egg before 

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

 upper surface ; E eight-cell stage ; F sixteen-cell stage ; G stage showing more rapid division 

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



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

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

 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 



