FEMALE REPRODUCTIVE CELLS 



459 



of food material being elaborated in the cytoplasm distal from their 

 nucleus. 



The extreme outer layer is distinguished by its lack of food materials 

 and is evidently not so 

 actively engaged as the 

 middle layer in which the 

 largest cells lie. The in- 

 nermost layer also con- 

 tains smaller cells. 



These cells multiply by 

 mitosis during the earlier 

 development of the ovum. 

 A mere increase in the 

 size of the individual cells 

 appears to give the follicle 

 sufficient capacity when 

 the maximum growth is 

 attained. In the mam- 

 mals the multiple follicle 

 layer continues to increase 

 the number of its cells by 

 mitosis up to the full 

 maturity. 



The ovum of a mam- 

 mal begins its growth 

 period with a single layer 

 of nurse cells. As the 

 growth proceeds, these in- 

 crease by mitotic divisions 

 to a double or many lay- 

 ered covering. Later, the 

 outer part of the covering 

 becomes separated from 

 that immediately covering 

 the ovum, except at a ' 

 single point called the hilum. The space between the two layers 

 becomes filled with a fluid that must act as an intermediate carrier for 

 nearly all the exchanged materials. 



The double or triple layer of nurse cells, covering the egg at this time, 

 each sends a cytoplasmic process down to the egg membrane. This egg 

 membrane is moderately thick, but seems to be of a soft consistency. 



No radial canals are shown by which the processes might pass 

 in, or through which the yolk substance might pass (Fig. 425). 



FIG. 423. Part of nearly matured ovum of Scolia dubia 

 with the attendant yolk follicle used up and degenerat- 

 ing. All communication between ovum (ov.) and yolk 

 follicle (./.) is cut off. X 350. 



