No. 1.] OPTIC VESICLES OF ELASMOBRANCHS. II9Q 
without, after they have become distinctly circular in outline 
and cup-like in character. 
These circular pockets not only arise in a similar way, but 
structurally resemble the optic vesicles. In cross and longi- 
tudinal sections, the cells of the sunken patches are similar to 
those in the eye pockets. They may be properly designated 
accessory optic vesicles, 
Other circular depressions arise later, upon the cephalic 
plate, behind the two just described, but they are less obvious 
Fic. 1.— Embryo with 3 mesoblastic somites, just after formation of the 
cephalic plate, showing beginning of optic vesicles. 
Fic. 2.— Embryo with 9 somites, showing optic vesicle established. 
Fic. 3.— Somewhat older embryo, showing optic vesicles, and the first appear- 
ance of the two homologous depressions. 
Fic. 4.— Embryo with 17 somites, after the rise of the medullary folds, showing 
the optic vesicles, the two homologous depressions, and two supplementary 
depressions. 
Fic. 5.—Slightly older embryo, placed so as to give a view into the front of 
the medullary groove. 
Fic. 6.— Embryo of about the same age, seen from the side, showing (1, 2) 
structures homologous with the optic vesicle. 
All the figures are from photographs X about 20 diameters. 
7, central tongue-like elevation on cephalic plate ; of., optic vesicles ; 1, 2, depres- 
sions homologous with the optic vesicles ; 3, 4, depressions appearing later, possibly 
homologous with 1 and 2. 
