DEVELOPMENT OF THE OV ABIES IN THE BLOWFLY. 
425 
the ovarian follicle (q/!), formed of fusiform mesoblastic elements. 
This is the so-called peritoneal coat of Stein. The ovarian follicles 
at this stage do not appear to open directly into the channels in 
the calyx of the ovary, hut to he closed by a cellular mass (x), 
with which the stalks of the egg-follicles are continuous. This 
cellular mass appears to me to be formed by the cells which filled 
the posterior part of the ovarian capsule at the earlier stage of 
development above described. 
Between the ovarian follicles, which no longer lie close together, 
the elements of the pseudo-yelk of the pupa ( psi /) are found in 
abundance. These consist of globules (. Kornchenkugeln ) and 
leucocytes ; they are derived from the breaking up of the fat- 
bodies and the tissues of the larva. The pseudo-yelk, at this 
period, forms the greater part of the bulk of the pupa. Indeed, 
if such a pupa be opened it appears to contain nothing but a 
milky fluid, in which all the tender half-developed imaginal tissues 
are concealed. 
There is as yet no differentiation of the contents of the egg- 
follicles into epithelium, yelk, or germ-cells. 
Even at the time when the fly is ready to escape from the pupa 
the ovary remains in a condition which differs but little from the 
stage last described, except in the form of the egg-follicles, which 
now exhibit a narrow stalk and an ovoid middle portion constric- 
ted above so as to form a small, but distinct nipple-like terminal 
chamber (fig. 7, t.c.). 
The peritoneal coat (of), or ovarian follicle, is also more 
developed, is much thicker, and exhibits numerous tracheal 
vessels ( tr .). There is still, however, no trace of differentiation in 
the cells contained within the egg-chamber, and these only differ 
from those in the terminal chamber in being slightly larger. 
In the observation of the further developmental changes we 
must have recourse to flies which have been on the wing for 
some time, and therefore the age of these insects is unknown. 
Development progresses very slowly in captive insects, and as 
these never unite with the males, there is no certainty that 
development progresses at the same rate or in the same manner 
as in free insects. 
The next stage which I shall describe is represented in PI. 
XXVIII. fig. 6. The ovary now measures ‘35 m. in thickness and 
is discoid. The calyx is very thin, as the ducts which form it 
appear to be flattened by the lateral growth of the organ. 
