STRUCTURE OF THE OVUM IN INSECTS. 191 



of the laterally attached and axial cells become conspicuous on 

 account partly of their size, but especially their large and 

 lustrous nuclei. These gradually come to occupy more and more 

 the axis of the tube, and are enclosed in groups of from six 

 to eight in moniliform dilatations of the tubular wall. We now 

 also find segments of the ovarian tubes, as at 6, in the centre of 

 which are numbers of primordial egg cells (ova), whilst the 

 parietal cells distinctly assume the characters of an epithelial 

 lining to the follicles ; for as follicles the several segments of 

 the moniliform tube must now be regarded. Still lower down, 

 however, a structural change occurs; for one of the egg cells, and 

 always that which lies the lowest, becomes considerably en- 

 larged, preserves it nucleus, which also undergoes great increase, 

 and at the same time its protoplasm becomes darker, whilst 

 the nuclei of the other primordial egg cells dimmish in size, 

 and ultimately, with their cell-protoplasm, undergo granular 

 degeneration. Coincidentally with this the larger egg cells 

 are separated from each other by an inflection of the epithelium, 

 The two portions of the follicles originating in this manner 

 are commonly distinguished as the vitelline cavity and the 

 germ cavity ; whilst, after Lubbock, with whom most recent 

 writers are in accordance, the upper confused mass of egg 

 cells have been called yolk-forming cells, because it has been 

 supposed that these cells furnish the yolk of the completely 

 formed egg cells. The existence of a cord-like structure that, 

 in many Arthropoda, as, for example, in the Coccidse (Glaus, 30), 

 connects the yolk-forming cells with the egg cells supports this 

 view. I cannot, however, accept it; for I have constantly 

 found evidences of fatty degeneration and of disintegration in 

 the yolk-forming cells, whilst the yolk-mass of the definitive 

 egg still continues to augment, long after those cells have 

 disappeared. I therefore regard the yolk-mass to be, in great 

 part at least, only a product of the follicular epithelium, to 

 which also the vitelline membrane, that in Insects often exhibits 

 very delicate sculpture, must owe its origin. The micropyle is 

 formed at that pole of the egg that is turned towards the yolk 

 cavity (see fig. 195). The structure of the ovaries in the 

 majority of Insects, so far as we have learnt from the investiga- 

 tions of Stein (137), H. Meyer (77), Weismann (125), Glaus (30), 



