ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA = 357 
semicircular cake. Under a higher power (500 diameters) the 
wrinkled! membrane of the other moiety of the vesicle is always 
readily discernible. 
Fifth Stage. Ovesacs between pth and p5th of an inch in diameter, in 
which the gerininal spot disappears, and a number of minute 
granules take its place. 
The germinal vesicle, held in place on the neural wall of the ovisac, 
immediately behind and rather to the right of the upper aperture of 
the duct, in the manner which has just been described, next under- 
goes changes of very great interest and importance. I have devoted 
a very great deal of time and patience to the analysis of these changes, 
but it is only recently that I have felt satisfied with the results of my 
investigations ; and I must warn any one who is disposed to repeat 
these observations, that while everything which I have described up 
to the present moment may be demonstrated with the utmost readi- 
ness in almost any thin vertical section of the ascidiarium, the condi- 
tions of the germinal vesicle which I am about to describe occur but 
very rarely, and require the aid of high powers of a thoroughly good 
microscope for their complete elucidation. Out of a vast number of 
preparations which I have made, at intervals, during the last twelve 
months, not more than eight or nine have exhibited the exact features 
of which I am about to give an account. 
The germinal vesicle represented in fig. 8a, belongs, strictly 
speaking, to the preceding stage. But it differs from the character- 
istic germinal vesicles of that stage, in that its spot has lost its solid, 
opaque 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 yj-gth of an inch. The vesicle itself measures 
ith, 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, 
1 The fact that the Pyrosoma observed by me had been preserved in spirit should always 
be recollected. It is highly possible that the wrinklings are artificial. 
