THE GEAAFIAN FOLLICLE AND CORPUS LUTEUM. 455 



part to increased pressure on the follicle, caused by a sudden 

 accession of blood to the ovary. 



The ripe ovarian ovum. The ripe human ovum is a spherical 

 cell, about 0'2 mm. in diameter. It consists of a granular mass 

 of protoplasm ; within which is a nucleus, or germinal vesicle, 

 about 0-045 mm. in diameter, containing a nuclear reticulum 

 and a conspicuous nucleolus or germinal spot. The ovum is 

 invested by a transparent elastic membrane, the zona pellucida, 

 which is about O'Ol mm. thick. 



Each Graafian follicle, as a rule, contains only a single ovum ; 

 in exceptional cases two ova, and in a few instances three, have 

 been seen in the same follicle. 



2. The Corpus Luteum. 



After the discharge of the ovum, important changes occur in 

 the Graafian follicle, leading to the formation of the body known 

 as the corpus luteum, which occupies and fills up the cavity of 

 the follicle. 



The corpus luteum is formed by rapid growth of the wall of 

 the empty follicle, which becomes thrown into radial folds, pro- 

 jecting into the cavity of the follicle, and blocking this up almost 

 completely. The folding involves both the follicular epithelium 

 and the connective-tissue wall of the follicle, but the latter 

 takes the most active share in the process. The characteristic 

 yellow colour of the folded wall, which has given rise to the 

 name corpus luteum, is due to large numbers of yellowish cells, 

 derived apparently from the connective tissue stroma of the 

 ovary. Between the two layers of each of the folds, blood- 

 vessels pass in freely; and the central cavity of the follicle, 

 which, by ingrowth of the radial folds, is reduced to an irre- 

 gularly stellate space, becomes occupied by a cicatricial fibrous 

 tissue, which is red in the early stages, but in the later ones 

 becomes grey. 



The subsequent changes in the corpus luteum differ con- 

 siderably according to whether the ovum, which has been dis- 

 charged from the follicle, (i) is fertilised and develops into an 

 embryo ; or (ii) is not fertilised, but dies without undergoing 

 any further development. 



In the latter case, i.e. if the ovum is not fertilised, the corpus 



