348 THE RABBIT. 



2. The Graafian Follicles. 



Each follicle consists, at first, of a single layer of cells 

 surrounding an ovum, the cells being derived directly from 

 the germinal epithelium, and being therefore morphologically 

 comparable with the ova themselves. Very shortly, each follicle 

 becomes two-layered (Fig. 133, ga), the second layer appearing 

 within the first one, between this and the ovum. As to the 

 origin of this second layer of follicular cells opinions differ ; it 

 is generally held to be formed by division of the cells of the 

 originally single layer ; but some investigators maintain that it 

 is derived from the ovum itself. 



Immediately around the ovum, between it and the inner 

 layer of follicle cells, a thick non-cellular layer, with faint radial 

 striations, the zona radiata, is formed, apparently as a cuticular 

 secretion from either the ovum or the follicle cells. Yolk- 

 granules, elaborated by the follicle cells, accumulate within the 

 ovum, which consequently increases in size. 



The follicle cells increase rapidly by cell-division, so that the 

 follicle soon becomes several cells thick. The outer layer of 

 cells now grows much more rapidly than the inner layer, so that 

 a space, somewhat crescentic in section, and filled with fluid, 

 appears between the two layers (Fig. 183). 



By further growth of the two layers, the fully-formed 

 Graafian follicle (Fig. 133, gk) is established. This consists of, 

 (i) an outer layer of follicular cells, gb, arranged three or four 

 cells deep, and invested by the vascular connective tissue of the 

 ovary ; and (ii) an inner layer of similar cells, GC, which closely 

 invests the ovum, ow. This inner layer is attached to the inner 

 surface of the follicle, with which it is also connected by 

 irregular radiating strands of follicular cells, well shown in 

 Fig. 133. The cavity of the Graafian follicle is filled with fluid. 



The riper egg-follicles lie at first, as noticed above, in the 

 deeper parts of the ovary ; but as the Graafian follicles enlarge 

 they gradually extend nearer and nearer to the surface, and 

 when fully formed cause rounded projections on the surface of 

 the ovary (Fig. 133). This enlargement of the Graafian follicles, 

 with accompanying ripening of the ova, occurs successively in 

 different parts of the ovary, so that there are never more than a 

 limited number, half a dozen or so, of ripe Graafian follicles at 

 any one time in a given ovary. 



