No. I.] POLYCHOERUS CAUDATUS. 171 



dinal section through the ten-celled stage. As will be seen in 

 these figures, the ovum is much arched, and the cells abutting 

 on A, A' appear to be pressing them into the cavity. 



" Eine deutliche Urdarmhohle " doubtless refers to the 

 blastopore-like opening left when A, A' finally pass into the 

 segmentation cavity. 



Very shortly after this takes place the cells ^3, ^3, ^'3, and 

 B' T^ close over this depression, and it is completely obliterated 

 except for the pigment which remains for some time. This, 

 however, disappears, and it is impossible to determine whether 

 the mouth of the future embryo is to be formed at this point 

 or elsewhere. 



Marine Biological Laboratory, Woods Holl, Mass., 

 September 12, 1894. 



