FEMALE REPRODUCTIVE CELLS 



455 



FIG. 418. Two nurse cells of the single layer which feeds yolk into 

 the growing ovum of the crayfish, Cambarus. c.t., part of the con- 

 nective-tissue follicle; n.c., two nurse cells of the ovarian follicle; 

 y., yolk granules in the ovum. X 1300. 



In Myzosloma, Wheeler has described the ovum as growing and 

 securing its yolk supply through the agency of two nurse cells, one of 

 which is attached 

 to either end. Fig- 

 ure 417 shows five 

 stages in this pro- 

 cess and it can be 

 seen that the nurse 

 cells, unlike that of 

 Ophryotrocha, grow 

 in size as the ovum 

 does and finally 

 fuse with the cyto- 

 plasm of the full- 

 grown egg. There appears to be a slight differentiation in the yolk 

 which the two nurse cells produce, this differentiation resulting in a 

 polarity of the ovum, which is retained through its further development. 

 In by far the largest number of animals the yolk is prepared and 

 loaded into the egg by a single, epithelial-like layer of nurse cells. This 

 layer varies in thickness from the very thin layer found in most echino- 

 derms through many grades of thickness and probably increased effi- 

 ciency, to the long columnar 

 cells that make up the nurse- 

 cell layer of a siluroid fish. 



Omitting the case of an 

 echinoderm because it is 

 mentioned later, we shall 

 demonstrate such a series 

 beginning with the yoke- 

 forming nurse cells of a 

 crustacean, Cambarus, part 

 of a section of whose half- 

 grown ovarian egg is shown 

 in Figure 418. The nurse 

 cells form a flat, thin layer 

 here of wide cells, with 

 nuclei that are not much 

 different from those of the 

 surrounding tissues. 



The food materials, from 

 the blood supply which is 



FIG. 419. Part of the single layer of follicle cells which 

 surround the growing ovum of the catfish, Ameiurus 

 nebulosus. c.t., connective-tissue follicle; n.c., nurse 

 cells of the ovarian follicle; z.r., transparent zona ra- 

 diata or cell wall of ovum ; y., part of ovum, showing 

 two large yolk spheres; l.s., lymph space (artificial 

 or pathological?). X 870. 



constantly circulating through the surrounding space indicated by l.s., 

 must go through two layers of cells to get into the egg, a connective- 



