356 ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
and a little longer (z15th). Its contents are quite clear, and its wall is 
but very slightly corrugated. But no one can question the identity 
of this body with that represented in place in fig. 6, and separately 
magnified in fig. 6a, which has a long diameter of ;1,th of an inch, 
whose walls are much wrinkled, and which contains a dense yellow 
deposit. 
I have no hesitation then in regarding the body, fig. 8, which 
agrees in all essential respects with that represented in fig. 6a, 
as the germinal vesicle of the primitive ovum, stripped of its 
vitellus, 
Though devoid of any vitelline investment, however, the germinal 
vesicle has been neither free nor bare, in any ovisac which I have 
examined. It is always seen to occupy one spot of the inner face of 
the ovisac, a little behind and to the right of the upper aperture of 
the duct; and when the ovisac is opened, the germinal vesicle is 
found to adhere to this point with considerable tenacity. It is, in 
fact, held in place by a continuation of the epithelial lining, which 
lies between it and the cavity of the ovisac—the germinal vesicle 
being now situated between the epithelium and the membrana propria, 
so that while its outer face is covered by the latter its inner face is 
invested by the former. All this will be rendered easily intelligible 
by examining the profile views (fig. 6 and 6a), and the view from 
within (fig. 7), of the germinal vesicle 2 sztz. 
But it has been seen that the ovum, containing the germinal 
vesicle, originally lay inside the wall of the ovisac, which has become 
metamorphosed into the epithelium, and hence it follows that the 
germinal vesicle, after losing its yelk, must pass through the epithe- 
lium of the ovisac. It will be recollected that the mammalian ovum 
becomes similarly related to the epithelium of the Graafian follicle, 
and that the germinal vesicle of the bird’s egg in like manner passes 
into and through the peripheral layer of its yelk. 
Throughout the present stage there is not the least difficulty in 
observing the germinal vesicle and its spot in the uninjured ovisac. 
The spot, in fact, is particularly well-defined, and immediately strikes 
the eye when even a low magnifying power is used. But, with such 
a power (say 200 diameters), it is easy to fall into error as to the 
shape of the germinal vesicle. It constantly appears to be hemi- 
spherical, the truncated side being that which is turned away from the 
upper aperture of the duct. This appearance arises from the fact 
that, with such a power, one sees the contents and not the wall of the 
germinal vesicle, and as the yellow deposit fills only that moiety 
which lies nearer the upper aperture of the duct, it appears like a 
