AND DEVELOPMENT OF PYROSOMA. 
227 
stage. But it differs from the 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, but so pale that under a low power it would readily escape notice. 
Its diameter is roVoth of an inch. The vesicle itself measures Troth, 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 -fond 
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 indistinctness 
of the germinal vesicle does not arise from want of size or clear definition ; for, if I put 01 1 
a high power, I find it to have a diameter of Tooth 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 sioth 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 
rofeoth 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 ^th 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 ^-th of an 
inch in diameter, whose germinal vesicle, aioth of an inch in diameter, is represented in 
%. Sb. Here the contents can be searched through and through with the greatest ease ; 
but not the least trace of a germinal spot is discoverable, while the minute clear corpuscles 
Toboth to Wb-oth 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 germmal vesicle increases in size until it attains as 
much as ^^th of an inch in Ions diameter ; and that the germinal spot, as such, entirely 
240" u «■»• «" JXivn A" ^"n 
2 
