DETELOPMENT OP THE OVAUTES IN THE BLOWFLY. 425 



the ovarian follicle (<?/.), 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, but to be closed by a cellular mass (^), 

 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 {psy) are found in 

 abundance. These consist of globules {Kornclienhugehi) 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 diflerentiation 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 diff'ers but little from the 

 stage last described, except in the form of the egg-follicles, which 

 now exliibit 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, i.e.). 



The peritoneal coat (q/*), or ovarian folKcle, 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 tliese only diff*er 

 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 w^ith 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. 

 XXYIII. 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. 



