AND DEVELOPMENT OF PYROSOMA. 225 



aud yet I could neither observe tlie 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, hy a little pains, I could open the ovisac so as to view 

 the germinal vesicle from Avithin (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 consideration. It may be 

 said, that what I have termed the germinal vesicle (represented separately in figs. 6a&S) 

 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 

 Y^g^th of an inch, while the germinal spot of the entire ovum is about xg-oo- But if this 

 corpuscle represent the germinal spot, then the only structure Avhich corresponds with 

 the wall of the germinal vesicle in fig. 8* is the structureless, oval, membranous sac, 

 wrinkled on one side, wliich 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 siotfi of an inch in 

 length, while the germinal vesicle of fig. 8* is only -5^0 ; next, it is oval and irregular on 

 one side ; and thirdly (and this is the most important difference), it contains a homoge- 

 neous yellowish deposit, which is especially accumulated around the germinal spot, but is 

 absent under the wrinkled moiety of the vesicle. 



AU 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. 4s, the unquestionable germinal vesicle is oval, and its long diameter amounts 

 to 5-aotli of an inch ; while in the ovisac represented in fig. 5, in which tlie 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 and a little longer (^^th). 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 xTT^fi 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 



VOL. XXIII. 2 H 



