360 ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
In another ovisac 4th of an inch in diameter, there is the same 
entire absence of the germinal vesicle and the same presence of 
a delicate membrane of precisely the same characters, but not more 
than ;}s:nd of an inch in long diameter and 49th of an inch wide, 
In this specimen the edge of the membrane which is turned towards 
the duct is still more distinctly semicircular, and it is almost as well- 
defined as the edge of the germinal vesicle in its latest condition, 
though no distinct membrane is discernible. The irregular part of 
the membranous disk bears a smaller proportion to the semicircular 
part, than in the preceding case. 
In each instance the membranous disk, which has been described, 
lies between the modified epithelium and the membrana propria. In 
position, therefore, it exactly corresponds with the germinal vesicle ; 
its colour, when the light passes through a thickness of it sufficient 
to give colour, is exactly that of the contents of the germinal vesicle ; 
the diameter of the semicircular portion is but very slightly greater 
than that of the germinal vesicle in its later stages ; and finally, the 
minute bodies which cccupy the centre of each component corpuscle 
of the membrane are not a little similar in character to the small 
spheroidal particles which appear upon the contents of the germinal 
vesicle during the latest stages of its existence. 
Putting all these circumstances together, I venture to express the 
belief that this membrane, which the further progress of development 
proves to be the blastoderm out of which all the parts of this embryo. 
take their rise, results from the metamorphosis of the contents of the 
germinal vesicle; and that the curved contour which lies towards 
the upper end of the duct is, in fact, the contour of that side 
of the germinal vesicle which first becomes filled with the yellow 
deposit. 
Thus far, I feel little difficulty in interpreting the appearances 
presented ; but if the surface and the immediate edges of the blas- 
toderm are examined with great care, minute rod-like bodies will 
be seen scattered about, so similar in form and size to the heads of 
the spermatozoa, that I have been frequently tempted to regard them 
as such, and the more so, as in this stage the duct looks shrunken 
and shrivelled, and contains but very few, if any, remains of the plug 
of spermatozoa so conspicuous previously. 
In this stage, each of the blastoderms which I have examined has. 
presented these appearances ; but as, in spite of long search, the total 
number which I have found in this state does not exceed four, I do. 
not feel myself in a condition to pronounce positively upon the nature 
of the bodies in question. 
