AND DEVELOPMENT OF PYROSOMA. 227 



stage. But it differs from tlie characteristic germinal vesicles of that stage, in that its 

 spot has lost its solid, opake aspect and has apparently become a vesicle with a thin, 

 sharply- defined wall, hut so pale that under a low power it would readily escape notice. 

 Its diameter is ^ 9^0 ^^^ of an inch. The vesicle itself measures srotlij it is much flattened, 

 and its contents are somewhat paler than before. I have described this germinal vesicle 

 here, because I believe that it is in that condition which constitutes the transition from 

 the typical form of the last to the typical form of the present stage. As this last is of very 

 great importance, I will note down the appearances presented to me by several germinal 

 vesicles which exhibit it. 



If I examine a slide, at present under my microscope, I observe, under a low power, in 

 one place, an ovisac belonging to the fourth stage. The germinal vesicle, with its yellow 

 contents, is very obvious, and the round, sharply-defined germinal spot strikes the eye at 

 once. If I now move the slide a little way, I bring into view a large ovisac about ^^aiif^ 

 of an inch in diameter. In this, it is only with difficulty that I can trace the outline of 

 the germinal vesicle, and nothing is to be seen of the germinal spot. This intUstinctness 

 of the germinal vesicle docs not arise from want of size or clear definition ; for, if I put on 

 a high power, I find it to have a diameter of Tg^th of an inch, and its contour is perfectly 

 well marked. The yellow deposit occupies about half its cavity as before, but it is paler ; 

 and partly on this account, and partly by reason of a further change in the structure of 

 the epithelium of the ovisac, the vesicle is less obvious than previously. Of the germinal 

 spot not a trace is to be seen anywhere, although the vesicle and its contents are quite 

 transparent. Whether the contents exhibit any new structure or not cannot certainly be 

 made out, on account of the interference of the wall of the ovisac, through which the 

 germinal vesicle is seen; In another ovisac in this stage, also about ^nd of an inch in 

 diameter, the germinal vesicle, very similar to that first described, measures -g^th of an 

 inch in length, and is half filled with the yellow deposit. No vestige of the germinal spot 

 is to be seen, but, on that side of the contents which in earlier stages is occupied by the 

 germinal spot, there are a number of minute, spheroidal clear granules, none exceeding 

 jTo^o-g-th of an inch in diameter and arranged so as to form an elongated patch on the 

 surface of the contents, the rest of which is quite free from such bodies. In another 

 ovisac of about the same size the germinal vesicle is axo^b of an inch in diameter with 

 pretty nearly half that thickness, and similar granules are observable upon the face of its 

 contents, while there is nothing to be seen of a germinal spot. 



But the best example of this stage is that afforded by yet another ovisac a^th of an 

 inch in diameter, whose germinal vesicle, -aioth of an inch in diameter, is represented in 

 fig. 85. Here the contents can be searched through and through with the greatest ease ; 

 but not the least trace of a germinal spot is discoverable, whUe the minute clear corpuscles 

 loooo^h to g^cToth of an inch in diameter, scattered over the face of the contents, are 

 exceedingly distinct. Whether they are free, or whether they are imbedded in any 

 clear substance, I cannot say certainly, but I suspect the latter to be the case. 



Putting the facts observed in this stage together, we find, that in ovisacs between ^th 

 and -^th of an inch in diameter, the germinal vesicle increases in size until it attains as 

 much as ^ j-Q-th of an inch in long diameter ; and that the germinal spot, as such, entirely 



2 u 2 



