172 



,<* 



3*1*1 



.oV 



Ticg. Z 



Fig. 1. Embryo of Squalus acanthias , 3 mm long, with 3 somites, showing the 

 first appearance of the optic vesicles. 



Fig. 2. Slightly older embryo, showing at op primary optic vesicles, and, at A. op 1 

 and A. op 2 , the beginnings of the accessory optic vesicles. 



A-elP 



A 6\>' 



5\q\ 3 



Fig. 3. Embryo with open neural groove, placed in such a position as to give 

 an external view of the vesicles on one side and an internal view on the opposite side 

 of the neural folds. 



