118 BKITISH APHIDES. 



be found also amongst some Coleoptera and Hymen- 

 optera. 



Dr. A. Brandt has devoted some thought to the 

 import of this element ; but the function it performs 

 may still be considered yet to want complete elucida- 

 tion. Dr. Brandt states that the basis of the yolk- 

 passage (Dottergange) is sharply bounded by the 

 neighbouring epithelium of the ovary where it appears 

 conical ; whilst towards the apex its outline is termi- 

 nated by the adjoining germinal vesicle of the yolk- 

 forming cells. Its designation of a yolk-cord (employed 

 previously by Huxley and translated Dotterstrang) 

 perhaps best accords with what is really known, than the 

 term yolk-passage, for it has not yet been proved that 

 the vesicles pass through it as a tube. Huxley has not 

 drawn the vesicle as situated within such; and Brandt, 

 in 1878, had not been able there to trace it. This cord 

 occurs both in the parthenogenetic and the fully sexed 

 female organs, though some differences of condition 

 connected with these forms may be pointed out. 



This yolk-cord passes from the end chamber of the 

 ovary, through one or more chambers, to any other 

 particular one ; that is, in such cases where the ovaria 

 are many-chambered. This disposition is remarkable ; 

 and Brandt offers the following as a possible explana- 

 tion of its action : 



" Let us imagine that, instead of a single ovarian 

 chamber, one or more (connected with the terminal 

 cyst by its yolk-cord) form at the basis of the same. 

 Then those formed earlier will, together with their yolk- 

 cords, be pushed aside by those formed later. After 

 the whole stock of egg-rudiments (Eianlagen), together 

 with their yolk-passages, has been differentiated, then, 

 by the splitting up of the terminal cyst into yolk-forming 

 cells, the change of its remaining undifferentiated 

 elements will take place." 



Further on he says, " Every break in the epithelial 

 partition which separates the yolk-forming elements 

 from the egg-rudiments (Eianlagen) can be morpho- 





