DEVELOPMENT OF THE GENERATIVE ORGANS. 713 



a distinct membrane." This phenomenon was regarded by 

 Weismann as exceptional, and probably due to the accidental 

 non-fertilization of the eggs. 



Metschnikow [341] says that he failed to discover a germinal 

 vesicle in Insects' eggs, and Ganin [343] states that in all the 

 Ichneumonidae which he examined the germinal vesicle dis- 

 appears at a very early period of egg-formation, before the 

 mother organism escapes from the pupa. 



Stuhlmann [363], like Brandt, believes that the ovarian 

 egg in the Blow-fly is developed from a single cell, and that 

 the so-called nutrient cells undergo atrophy and remain outside 

 the chorion. He regards the nucleus of this cell as the 

 germinal vesicle, and says, ' I have been enabled, by a large 

 number of observations on Insects' eggs, to establish the extru- 

 sion of nuclear particles from the germinal vesicle, which are 

 afterwards lost in the egg-plasma. Later the germinal vesicle 

 disappears, until at last we find it again at the upper egg-pole 

 as a segmentation nucleus.' The ' outstreaming of nuclear 

 particles,' described by Stuhlmann, is quite unlike the extrusion 

 of the polar globules in other ova. It undoubtedly occurs from 

 all the nuclei of the yelk-cells and also from those of the various 

 larval organs during their degeneration in the pupa. It is a 

 phenomenon characteristic of the histolysis of the tissues and 

 the breaking up of the nuclei of effete cells. 



Lastly, the identity of the first segmentation nucleus with the 

 nucleus of the egg-cell is assumed by Stuhlmann, and not proved, 

 as the disappearance of one structure and the subsequent 

 appearance of another cannot establish a genetic connection 

 between them. 



Henking [350], like his predecessors, has attempted the 

 identification of a germinal vesicle in the egg of the Blow-fly. 

 He says, ' The unripe egg contains a germinal vesicle and 

 germinal spot. The former is clear and does not stain deeply 

 with borax carmine.' In some of my sections I have found 

 one or even two yelk nuclei, which resemble Henking's germinal 

 vesicle, but the same nuclei in the next section in the series 

 exhibit characters which are indistinguishable from those of 



