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 

 foetus. Valenciennes seems to adopt the idea that it does, 

 and compares it to a 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 

 spermatozoon 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. It 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 blood which they contain. 



In comparing foetuses 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 wasjio 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, 



* "La cellule qui contient un ceuf f^conde s ; aggrandit etfinit par former uric 

 sorte de Chorion." Op. cit. T. xviii. p. 261. 

 t Op. cit. p. 261. 



