No. I.] OPTIC VESICLES OF ELASMOBRANCHS. i 
early stages of development of Torpedo was published as 
recently as last year, failed to note the optic vesicles which 
are so well developed (in Acanthias at least) at the stage 
which has been designated by them ‘Stage D.’ As I shall 
presently show, the optic vesicles appear at a considerably 
earlier period. JI have examined Professor Ziegler’s figures, 
and, also, the models he has made from them, but I do not 
find traces in either, of the optic vesicles nor of the other 
related sensory depressions of which I shall soon speak. The 
Zieglers worked upon Torpedo, and my own observations were 
made upon Squalus acanthias, Linn., of the western Atlantic 
coast. 
The involutions that are to give rise to the optic vesicles 
appear at a very early stage. As soon as the cephalic plate 
has been formed, by an expansion of the anterior end of the 
embryo, two faint circular depressions are to be seen upon 
its extreme anterior surface (Fig. 1, of.). These depressions 
grow deeper, and run together in the middle line, and the 
continuous infolding produced in this way divides the cephalic 
plate into an anterior depressed region, and a posterior elevated 
region. 
The infolding gives rise, also, to the infundibulum. 
The optic vesicles, which are started near the median line, 
grow outwards laterally, and come to occupy the lateral parts 
of the depressed region, but as the infolding forming it does 
not extend completely across the cephalic plate, the optic 
vesicles do not reach the margin of the medullary folds. 
The latter are broadly expanded beyond the optic vesicles 
(Figs. 2 and 3). 
When distinctly formed, the optic vesicles are circular in 
outline, concave from within, and they form rounded elevations 
on the outside (Figs. 3 and 4, of.) where the cups come in 
contact with the outer layer of the epiblast. 
The earliest stage in which I have noted the circular areas 
shows three mesoblastic somites, and the medullary folds of 
both head and body are not only broadly open, but they are 
even ventrally curved. It is a stage occurring intermediate 
between the stages designated C and J, respectively, by the 
