No. 3-] THE GASTRULATION OF AMPHIOXUS. 



579 



In one of these the ectoderm cells on the dorsal side and in 

 the other on the ventral side are dividing more rapidly. Other 

 preparations of the same age were found in which the cell divi- 

 sion is not more common around the blastopore than elsewhere. 

 At 4.30 A.M. the embryo has closed in further (Figs. XXV 

 a, b and XXVI a, b). In the first of these figures it is 



XXYU a 



xxyn 5 zzyiU'tt 



Figs. XXI-XXVIII. 



XZYIE S 



noticeable that cell division is very frequent in the ectoderm 

 of the ventral side. In the other very little cell division is 

 present, and that mostly around the ventral lip of the blastopore. 

 Figs. XXVII a, b and XXVIII a, b 2X 6 a.m. show, in one 

 case, several endoderm cells and few ectoderm cells in process 

 of division. In the other embryo the reverse is true. 



