DETELOPMENT OF THE OVAETES IN THE BLOWFLY. 429 



nated eggs whilst they still lie in the ovary ; this layer was 

 described by Weismann, and called by him the Blastoderm plasma 

 (Keimhauthlastem) . He supposed that it is this layer which 

 forms the blastoderm. In my sections it projects in places as 

 if it possessed the power of amoeboid movement, more especially 

 at the anterior egg-pole * (fig. 10, a) ; these may, however, be the 

 result of post-mortem contraction. The central granular yelk- 

 substance consists of small granules, 2 to 3 ju in diameter 

 (fig. 11), imbedded in an apparently structureless, possibly in the 

 living egg semifluid, matrix. These granules are spheroidal, 

 stain deeply, and exhibit either a dark or light centre with 

 alterations of the focus of the microscope. In the ripe 

 unimpregnated ovum I have entirely failed to find any nuclei or 

 cellular elements of any kind, and I feel sure that if any such 

 elements were present they would be readily distinguished in my 

 sections. 



It is not necessary for my purpose to enter into any details in 

 regard to the structure of the chorion and the nature of the 

 vitelline membrane; there are, however, some controverted points 

 upon which I would say a few words. 



It is generally held that the epithelium of the egg forms the 

 chorion as an exudation from its inner surface (E. Korschelt, 14 ; 

 Weismann, 26). Whether this is so or whether the chorion is 

 formed from the cells themselves (Leuckart, 16), the manner in 

 which the ova leave the oviducts is entirely in favour of the 

 latter view. This is eflected by the rupture of the remaining 

 rudiment of the egg-string between the ripe egg and the imper- 

 fect ovum immediately in front of it. Thus the thin tunica 

 propria and the epithelium of the egg descend in the ovarian 

 follicle and enter the oviduct together. The remains of the egg- 

 string attached to the unripe ovum in the ovarian follicle have 

 been seen and described by several observers, notably by 

 Miiller, Landois (15), and Leuckart (16). I hold therefore that 

 whether the cellular epithelium is shed with the thin cuticular 

 egg-sheath in the oviduct, or whether it remains as the chorion 



* The polar globules of Eobin, which he described as formed by budding 

 and fission, are possibly only mobile processes of this layer in a contracting 

 yelk. 



32* 



