428 PROF. B. T. IiOWNE ON THE STRUCTURE AND 
undergo the some changes, and soon become fused with each 
other and with the yelk formed from the lowest cell. 
The nuclei during these changes present a very variable 
appearance; but all the changes of the nucleus are similar to 
those which characterize the nuclei of the degenerating cells of 
the larva, during the formation of the pseudo-yelk of the pupa — 
a phenomenon well seen in the nuclei of the cells of the salivary 
glands and fat-bodies of the larva during their histolysis. 
I conclude therefore that the several cells from which the yelk 
of the Dipterous egg is formed are of equal morphological 
significance, that these all undergo histolytic changes, and so 
form the yelk of the mature ovarian ovum. 
So far as my observations go, there is no reason for supposing 
one nucleus rather than another is the germinal vesicle. 
When I first began this investigation, more than two years 
ago, I looked for days in vain for some character by which I 
might recognize the germinal vesicle. Sometimes one nucleus, 
sometimes another presents a clearer contents and smaller 
diameter, and frequently several nuclei appear to possess equal 
claims in this respect to be considered the nucleus of the germ- 
cell. 
As the young ova approach the condition of maturity, the cell- 
substance becomes more and more distinctly granular, the nuclei 
lose their sharp contour, and exhibit what Stuhlmann describes 
as an extrusion or outstreaming of nuclear particles, whilst 
these are lost to view in the granular surrounding protoplasm, 
and the cells themselves become fused into a single yelk-mass. 
These changes commence in the lowest and largest cell of the egg; 
but precisely the same changes afterwards occur in the remaining 
cells as each attains its full growth. 
The mature ova consist of a yelk surrounded by two mem- 
branes, the vitelline membrane and the chorion. Such ova are 
closely embraced by the structureless cuticular membrana 
propria, and lie loosely in the distended ovarian follicle, which is 
now a very thin-walled tube surrounded by a dense network of 
tracheal vessels. 
The yelk consists of an outer clearer layer (PL XXVIII. 
fig. 11, a) and an inner granular substance (fig. 11, b), but neither 
contain any nuclei or cellular elements of any kind. 
The clear peripheral layer of the yelk exists in the unimpreg- 
