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 Myzostoma, 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 



* Z.s. 



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- 



