456 



HISTOLOGY 



tissue layer lying outside of the follicle layer. It is probable that the 

 entire blood soaks through spaces in the connective-tissue layer, and 

 that only then do the nurse cells (follicle cells) select and elaborate the 

 proper materials and pass them as yolk food into the egg. 



The same thing is probably true of a very thick, single layer of cells 

 shown by the follicle layer of a catfish, Ameiurus nebulosus (Fig. 419). 

 While the ovum is very young, these cells are small and flat as in the 

 crayfish. During the time of greatest yolk accumulation, the cells grow to 

 the great height shown in the figure. The nuclei become smaller and 

 angular, and a space appears between the follicle cell layer and the con- 

 nective-tissue capsule. This space becomes filled with a heavy lymph 

 of some staining power. The blood supply appears as a network of cap- 

 illaries in the connective-tissue capsule instead of in sinuses outside of 

 it as in the crayfish. Figure 420 shows part of a section of the ovarian 

 egg of another teleost fish, the carp, and as this ovum was ripe and nearly 

 ready to be shed, the follicle cells, which were almost as long as those 

 of the catfish at an earlier stage, are now flat and shrunken and will soon 

 die and disintegrate. 



The cells of the single-layered follicle usually multiply by mitosis in 

 the early part of their career. Later such cells, in some forms, divide 

 by amitosis, which is a terminal process under the circumstances, as the 

 layer is destroyed at about the time the egg is laid. (See Amitosis, 

 Chapter V.) 



We wish to call attention at this point to the membrane which im- 

 mediately surrounds the ovarian egg in 

 most animals and through which the 

 food materials and other contents of the 

 egg must be passed. In the catfish (Fig. 

 419) this membrane is comparatively 

 thin, and one might think that the ma- 

 terials passed inside, in a fluid state, by 

 osmosis. In the adult fish egg as pic- 

 tured in the carp (Fig. 420), this mem- 

 brane is very clearly perforated by a 

 vast number of tiny, radial canals and 

 the cytoplasmic processes of the nurse 

 cells pass into these canals and thus 

 carry the food matter in. The mem- 

 brane is probably formed jointly by the 

 follicle cells and the ovum, the latter 

 being responsible for only the thinner 

 inner layer. In some fish eggs the outer surface of the membrane is 

 drawn out into long threads for attaching the eggs to seaweed and 



FIG. 420. Edge of ovum of another tele- 

 ost fish, Esox Americanus, to show the 

 radial canals in the thick cell wall of 

 the ovum. This cell wall is two-lay- 

 ered. The egg being ripe, the nurse 

 cells are small and degenerating, c.t., 

 connective-tissue capsule; n.c., degen- 

 erating nurse-cell layer; y., yolk in 

 ovum; z.r., two-layered zona radiata. 

 X 1300. 



