458 



HISTOLOGY 



n. b 



ey.p 



ure 423 shows the destruction of the contents almost completed. In the 

 earwig, Forficula, but one follicle cell becomes a nurse cell. In other 



insects, as Vanessa, sev- 

 eral follicle cells are so 

 \ n. f. modified, but remain an 

 ^ ( integral part of the follicle 



and are not separated 

 from it as in Scolia. 



Multiple layers of fol- 

 licle cells are confined to 

 the higher and most 

 highly specialized animals 

 and are instructive in 

 that they show the ne- 

 cessity of bodily contact 

 between the ovum and 

 the yolk-supplying cells. 

 Figure 424 shows the 

 layer of yolk cells sur- 

 rounding the growing 

 ovum of a water snake, 

 Natrix sipedon. When 

 the ovum first begins 

 to grow, this layer is 

 single. The cells increase 

 in size and form a strati- 

 fied layer by amotitic pro- 

 liferation. This layer is 

 thickest just before the 

 ovum has attained its full 

 size, and when this size 

 has been secured, the fol- 

 licle cells degenerate into 

 a layer of dead cells, 

 which form a mucous 

 covering to permit the ovum to slide out. The figure shows the 

 most actively secreting stage. Study the middle layer and note that 

 although the layer is stratified, yet every cell secreting has a strand 

 of its cytoplasm drawn out into a process, the yolk process, which 

 comes into contact with the ovum by passing through the egg mem- 

 brane. This provides a necessary pathway for the yolk material, 

 in solution or in fine granules, to be carried into the egg without being 

 passed from cell to cell. Note also that the larger cells show a mass 



FIG. 422. Older ovum of Scolia dubia. Lettering same 

 as in Figure 421. At n.b. the chromatin has not yet be- 

 come distributed as it is in the very actively secreting 

 cells, cy.p., cytoplasmic process of ovum through which 

 the nourishing matter is drawn as a streaming vacuole 

 (vac.). X 350. 



