CELL DIVISION AND GAMETOGENESIS 



227 



rate of mitotic division is so rapid that practically no growth 

 occurs between divisions. The growth period that follows the 

 division period is characterized by cessation of mitosis and the 

 enlargement of the oogonia by the storage of yolk in the cyto- 

 plasm. By this means oogonia are converted into primary 

 oocytes. In insects (Fig. 140) some of the oogonia are converted 

 into nurse cells and supply the growing oocytes with nutriment 

 that enters the oocyte through the nutritive string, a temporary 

 process of the oocyte. In Hydra, as mentioned in an earlier 



Fig. 140. — Longitudinal section of an ovariole showing how nutrition is sup- 

 plied to the egg of the potato beetle, Leptinotarsa signaticollis, during the growth 

 period, nc, nurse cells; ns, nutritive string; o, oocytes; fc, follicle cells. 



connection (Chap. VIII) the oocyte grows by engulfing sur- 

 rounding cells, some of which may have been potential oocytes. 

 A similar engulfment of cells by the oocyte occurs in the growth 

 period of the oocyte of flatworms. In vertebrates, the oocyte 

 derives its nutrition from the blood vessels supplying the ovary. 

 The fully developed oocyte is a cell of large size, in some cases, as 

 in birds, of macroscopic proportions. Thus the yellow or yolk 

 of the hen's eggs is the same size as the primary oocyte at the 

 end of the growth period. 



The oocyte at the end of the growth period now enters what is 

 called the maturation period, characterized by two maturation 



