; 22 SCIENCE PROGRESS 



and numerous blood vessels. It is surrounded by a single layer 

 of columnar cells, the germinal epithelium, and has embedded 

 in it a large number of ova in all stages of growth, each sur- 

 rounded by an epithelium. The ovum, together with its 

 enveloping epithelium, is termed a Graafian follicle, and such 

 follicles are present in all stages of development from the early 

 " primordial " follicles up to those that are mature. Here only 

 the latter need be considered. 



The ripe follicle is a vesicular structure containing a large 

 central cavity filled with a fluid, the liquor folliculi, which 

 appears in section as a lightly staining coagulum. The ovum 

 is situated within the cavity and usually towards one side. The 

 wall of the follicle is formed by an epithelium several cells deep, 

 known as the membrana granulosa, and a special part of the 

 membrana, the discus proligerus, surrounds the ovule. In 

 fairly small follicles mitotic figures are by no means uncommon 

 in the cells of the membrana granulosa, but they appear to be 

 entirely absent in the ripe follicle. 



The thickness of the membrana granulosa varies in different 

 parts of the follicle. It may be six to ten or even more cells 

 thick on the inner side or at the point where it meets the discus 

 proligerus, while at its outer edge it is thinner, and at the point 

 where it will rupture, i.e. the stigma, it is not more than two or 

 three cells thick. The outer limit of the membrana granulosa 

 is marked by a clear homogeneous basal membrane. Outside 

 this again the follicle is surrounded by a fibrous structure, the 

 theca folliculi, derived from the ovarian stroma. This is 

 divisible in the Eutheria into two parts, an inner coat of more 

 or less granular cells, the theca interna, which is very vascular, 

 and an outer coat of more fibrous nature, the theca externa. 

 In one of the Marsupials (Dasyurus), however, as Sandes(i9) 

 pointed out, the two layers of the theca folliculi cannot be 

 distinguished one from the other. 



Usually several of these follicles attain maturity at the same 

 time and burst, discharging their contained ova, together with a 

 certain amount of the liquor folliculi and some blood. The 

 cause of this rupture is obscure, and although it has been 

 suggested that it is due to a rise of blood pressure or to the 

 stimulation of erectile tissue in the ovary, a satisfactory 

 explanation is not yet forthcoming. 



The mammals fall into two classes, according to the manner 



