118 LOGY: [Vorsix 
Zieglers, but it is a distinct, stage which is not figured nor 
described by them. 
This early appearance of the basis of the eye, before the 
formation of the medullary canal, has for a long time been 
known to occur in the mammals, but it was regarded as pre- 
cocious development in that class of animals. It has been 
described and figured in the mole, by Heape,! and, more recently 
has been noted in Birds by Duval and other observers, in 
Necturus, by Whitman,? and in Necturus and other amphibia 
by Eycleshymer.? So far as I am aware the early condition in 
Elasmobranchs was described for the first time in my paper # 
before referred to. 
The optic vesicles are clearly outlined in Squalus acanthias, 
before the medullary groove is fairly established, and long before 
the medullary canal is formed in any part of the embryo. 
But, more interesting than the fact of their very early 
appearance in Elasmobranchs, is their apparent relationship to 
other depressions that are formed upon the cephalic plate 
behind the already established optic vesicles. 
The new involutions referred to make their appearance upon 
the cephalic plate just back of the optic vesicles. Two of 
them (Figs. 3 and 51,2) take precedence of all others in 
development, and they are so distinctly formed as to afford 
a good basis for comparison with the optic vesicles. They are 
circular depressions formed in precisely the same manner as the 
optic vesicles in front of them, and they produce upon the 
exterior corresponding rounded elevations. The optic vesicles 
are formed first, and when, at a very little later stage, the others 
arise behind them, it appears as if the process of eye formation 
were repeating itself serially. 
The anterior one is larger, and makes the nearest approach 
to the eye vesicle in size and structure ; the second is smaller, 
and is faintly bilobed. 
In Fig. 3, these structures are shown as they first appear, 
and in Figs. 4 and 5, they are shown, both from within and 
1 Heape, Quar. Journ. Mic. Sci., Vol. 8, Oct., 1886. 
2 Journ. Morph., Vol. 2, April, 1889. 
8 Journ. Morph., Vol. 8, March, 1893. 
4 Journ. Morph., Vol. 8, May, 1893. 
