454 THE HUMAJi EMBRYO. 



becomes several cells thick. By splitting apart of these cells, 

 accompanied by rapid growth of the outer layer, a cavity is 

 formed in the thickness of the wall of the follicle ; and this 

 cavity, which is filled with fluid, rapidly increases in size, dividing 

 the follicle into an outer wall, the tunica granulosa, and an 

 inner one, or discus proligerus, which immediately invests the 

 ovum (cf. p. 349). 



The fully formed Graafian follicle (cf. Fig. 133, GK, p. 347) 

 is ovoid or ellipsoidal in shape: its walls consist of: (i) an 

 outer investment of vascular connective tissue, derived from the 

 stroma of the ovary, and divisible into a rather ill-defined outer 

 layer, the tunica fibrosa folliculi ; and an inner well-marked layer 

 of fine connective tissue, abundantly supplied with capillary 

 blood-vessels, the tunica propria folliculi. (ii) Within this 

 latter is the tunica granulosa (Fig. 133, GB), a thick layer 

 of granular, spherical or polygonal cells. At one part, the 

 tunica granulosa is much thickened, forming a roundish mass 

 projecting into the cavity of the follicle ; and embedded in the 

 middle of this roundish mass, or discus proligerus, is the ovum, 

 OW. The cells immediately surrounding the ovum are distinctly 

 columnar in shape, while the remaining cells of the follicle 

 are spherical or polygonal. The cavity of the follicle is filled by 

 the watery liquor folliculi. 



In the early stages of their formation (Fig. 133) the more 

 mature Graafian follicles lie in the deepest parts of the ovary ; 

 but, as they increase in size, their growth takes place in all 

 directions ; and ultimately the outer walls of the follicles approach 

 very close to the surface of the ovary, or actually push the super- 

 ficial layer of epithelium and connective tissue of the ovary 

 before them, and so form rounded external projections on its 

 surface. 



At the most prominent part of the ripe Graafian follicle is a 

 small spot, the hilum folliculi, distinguished from the rest of the 

 follicle by being devoid of blood-vessels : at this place, shortly 

 after the follicle has reached its full dimensions, i.e. a diameter 

 of from 1*25 to 4 mm., rupture of the follicular wall occurs, and 

 the ovum, together with the liquor folliculi, is discharged on the 

 surface of the ovary. 



This rupture of the wall of the Graafian follicle is due in part 

 to fatty degeneration of the cells composing the wall ; and in 



