700 THE GENERATIVE ORGANS. 
In the latest stages of the pupa numerous large tracheal 
vessels are seen covering the convex surface of the ovary and 
extending through the ovarian cavity into the spaces between 
the ovarian follicles. 
Some of my sections from pupz about the eighth day 
(Pl. XLIX., Fig. 1) exhibit the section of one or more convo- 
luted epithelial tubes lying in the ovarian cavity at the inner 
limit of this cavity. I believe that these are sections of the 
czecal end of the parovarium. 
In the investigation of the further changes which occur in 
the ovary it is necessary to have recourse to flies which have 
been on the wing for some time, and therefore the age of these 
insects is unknown. Development of the ovary progresses 
very slowly in these insects when bred and kept in captivity. 
As such insects never unite with the males, there is no cer- 
tainty that development progresses in the same manner as in 
impregnated females, and, so far as my experience goes, the 
ovaries of captive insects are always arrested, and no mature 
egg-tubes are ever formed. 
In the stage represented in Pl. XLIX., Fig. 2, the egg-stocks 
are divided by superficial constrictions into several segments, 
usually five or six. The stalk is greatly increased in length; in 
some cases it measures as much as ‘15 mm. in length. The 
anterior extremities of the egg-stocks are closely surrounded 
by ovarian stroma, whilst the posterior end and the egg-stalk 
lie in a well developed ovarian follicle. The ovarian follicles 
are now lined by a well-marked epithelial layer, apparently 
derived from the epithelial tissue which covers the convex 
surface of the ovary. 
At this period the oviducts spread over the inner margin of 
the convex surface of the ovary, and are seen in section as a 
cavity behind the ovary. There is no evidence that the follicles 
open into this cavity, but at a somewhat later period the con- 
tinuity between these and the cavity of the oviduct becomes 
established by the shedding of the epithelial layer of the follicles. 
In the adult ovaries I have found no traces of the follicular 
epithelium. The blind end of the tuba of the oviduct appears 
