378 ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
The evidence by which this conclusion is supported, however, 
will be found, if closely sifted, to be, for the most part, not only 
negative, as by the nature of the case it must be, but weakly negative. 
That is to say, not only is the conclusion based upon the circum- 
stance that, at a given period, the observer was unable to find the 
germinal vesicle or to identify its contents,—but, in most cases, the 
circumstances are such that he might very well have missed them 
had they existed. Even in Pyrosoma it is no easy matter, until 
one has had some practice, to find the germinal vesicle when it is 
passing into the blastoderm, although, in all the earlier stages, nothing 
can be more obvious; and had the ovisac of Pyrosoma been filled 
with even a very slightly granular yelk, I believe the discovery of 
the germinal vesicle, at this period, would be almost impracticable. 
What wonder, then, that it should be impossible to identify the 
germinal vesicle or its contents in the midst of the more or less 
opaque and coarsely granular substance of which the yelk of ninety- 
nine ova out of a hundred is composed? The only case to which 
this reasoning does not apply is that described by Kolliker in the 
paper already referred to (2 « p. 76) :-— 
“As regards the internal changes undergone by the eggs [of 
Ascaris dentata], the most striking fact is that, immediately after 
fecundation, the germinal spot and the germinal vesicle have disap- 
peared and the clear and transparent yelk contains nothing but 
scanty elementary granules. This is a point of great importance ; 
and to show that there is no possibility of being deceived about 
it, I add, that the ovum of Ascaris dentata, including its chorion and 
vitelline membrane, is so transparent that all the outlines of a body 
which may happen to lie beneath it are quite sharply and distinctly 
recognizable, and its contents are so clear and patent that hardly the 
smallest elementary granule of the yelk can remain hidden. Which 
of the two parts first disappears, the germinal spot or the germinal 
vesicle, I cannot as yet say with certainty ; but, in one individual, I 
saw two ova which had hardly traversed the seminal cells in- the 
fundus uteri, and though they still exhibited a germinal vesicle, had 
no germinal spot. In another individual, I observed the same thing 
in an ovum imbedded in the midst of the seminal cells; so that 
I have some ground for the opinion that it is the germinal spot which 
disappears first. Further and repeated observations must decide 
whether this is the rule or whether, in other cases, it is not the 
germinal vesicle which disappears first. But I must observe, that 
this first stage of the development of the ova appears to be of very 
short duration; for, while no fecundated female Ascaris which I 
