ON Tilk ANATOMY AND DEVELOPMENT OF PYROSOMA 355 
and consistent as to retain its form (fig. 8*) ; and yet I could neither 
observe the smallest trace of the yelk in entire ovisacs in this stage, 
nor, however carefully I opened them, discover any trace of yelk 
within them. I found, furthermore, not only that, by a little pains, I 
could open the ovisac so as to view the germinal vesicle from within 
(figs. 6 and 7), but that I could evert it, turn it in all directions, and 
even detach it entirely : and when I discovered, by these means, not 
merely that no vitellus surrounds the germinal vesicle in this stage, 
but that it is enclosed and held in place by something which is 
assuredly not vitellus, I was forced back into my original conclusion, 
that in this stage the vitellus, as such, has disappeared. 
There is, however, one suggestion which deserves careful con- 
sideration. It may be said, that what I have termed the germinal 
vesicle (represented separately in figs. 6a & 8) is in fact the ovum. 
To meet this objection, I would beg the reader to compare figs. 8 and 
8*; the former of which represents the body whose nature is in 
dispute, and the latter an ovum which has not reached its full size, 
the two figures preserving the true relative proportions of the 
originals. It is at once obvious that the circular solid-looking cor- 
puscle, situated towards the upper end of fig. 8, is identical in all 
essential respects with the germinal spot of fig. 8*, the only difference 
being that it is slightly larger, measuring ;,,th of an inch, while the 
germinal spot of the entire ovum is about ys45. But if this corpuscle 
represent the germinal spot, then the only structure which corresponds 
with the wall of the germinal vesicle in fig. 8* is the structureless oval, 
membranous sac, wrinkled on one side, which encloses the germinal 
spot in fig. 8. This sac, it must be admitted, differs a good deal from 
the germinal vesicle of fig. 8*, not only in size, but in form and in 
contents. In the first place, it is much larger, measuring ;45th of an 
inch in length, while the germinal vesicle of fig. 8*, is cnly 33, ; next, 
it is oval and irregular on one side ; and thirdly (and this is the most 
important difference), it contains a homogeneous yellowish deposit, 
which is especially accumulated around the germinal spot, but is 
absent under the wrinkled moiety of the vesicle. 
All doubts as to the identification of the body (fig. 8) with the 
germinal vesicle and spot of fig. 8*, however, vanish when a series of 
ovisacs, intermediate in size between that which yielded the ovum, fig. 
8*, and that represented in fig. 6, are studied. Thus in fig. 4, the 
unquestionable germinal vesicle is oval, and its long diameter amounts 
to =45th of an inch; while in the ovisac represented in fig. 5, in which 
the yelk has disappeared, the body in dispute is precisely similar to 
the germinal vesicle of fig. 4, except that it is a little more flattened 
ba, 2 
