440 Wyman's Observations on - 
to be analogous to a Graafian vesicle; that is, the egg of 
the fish floats free in a sac much larger than itself, just as 
the mammiferous egg does in the vesicle of De Graaff. "There 
were no intermediate conditions between this and the im- 
pregnated condition to enable me to determine whether or 
not it is this sac which forms the external covering of the 
ctus. Valenciennes seems to adopt the idea that it does, 
and compares it toa chorion.* - If this view of its nature be 
true, then there seems no alternative, since development 
advances so far before the sac ruptures, but to suppose that 
impregnation must take place through its parietes and that the 
spermatozüon cannot enter bodily into the substance, or even 
come in direct contact with the vitelline membrane of the 
egg, except through the walls of this outer covering, which is 
not probable. 1t would seem that it must act simply by its 
presence on the surface of the egg-sac, or by an endosmosis 
of its fluid contents through the membranes by which the 
ovum is invested. 
A microscopic examination of the egg-sacs in the advanced 
- foetuses proves conclusively, that they do not consist of loose 
areolar tissue only, as stated by Valenciennes, but that while 
the tissue in question forms the basis of them, they are in 
reality very highly vascular, large trunks and minute ramifi- 
cations of vessels being easily traced by the aid of the coagu- 
lated blodd which they contain. 
In comparing fcetuses of different stages of development 
together, a very interesting question is “presented to us in 
connection with their growth. In the smallest specimen 
examined, the yelk was no longer visible, it had been wholly 
consumed in supplying materials for the formation of the 
embryo; and yet subsequent to this disappearance of the 
yelk, the embryo, while still in its ovarian sac and cut off 
from all external communication, continues to increase in size, 
Cai » farmer ane 
I 
* “ La cellule qui contie: ffi é s'aggrandit 
sorte de Chorion.” On dt T. Vill. p. 261. 
t Op. cit. p. 261. 
