THE REPRODUCTIVE SYSTEM 



365 



An escape of blood into the follicle from the torn vessels of the 

 theca always accompanies the discharge of the ovum. The follicle 

 again becomes a closed cavity, while the contained blood clot becomes 

 organized by the ingrowth of vessels from the theca, to form the corpus 

 hcemorrhagiciim (Fig. 260), which represents the earliest stage in the 

 development of the corpus luteum. 



Fig. 257. — Graafian Follicle and Contained Ovum of Cat; directly reproduced 

 from a photograph of a preparation by Dahlgren. X 235. (From "The Cell in Devel- 

 opment and Inheritance," Prof. E. B. Wilson; The Macmillan Company, publishers.) 

 The ovum is seen lying in the Graafian follicle within the germ hill, the cells of the 

 latter immediately surrounding the ovum forming the corona radiata. The clear zone 

 within the corona is the zona pellucida, within which are the egg protoplasm, 

 nucleus, and nucleolus. EncircHng the follicle is the connective tissue of the theca 

 folliculi. 



The corpus luteum (Fig. 261), which replaces the corpus haemor- 

 rhagicum, consists of large yellow cells — lutein cells — and of connec- 

 tive tissue. The latter with its blood-vessels is derived from the 

 inner layer of the theca. The origin of the lutein cells is not clear. 

 They are described by some as derived from the connective-tissue 

 cells of the theca; by others as the result of proliferation of the cells 

 of the stratum granulosum. The cells have a yellow color from the 

 presence of fatty (lutein) granules in their protoplasm, and it is to 



