1368 PHYSIOLOGY 



presents a fine radial striation, which is supposed to indicate the 

 existence of canals through which the ovum can obtain sustenance 

 from the surrounding cells of the follicular epithelium. The nucleus 

 also becomes larger, and forms the germinal vesicle containing one or 

 two well-marked nucleoli the germinal spot. The mature Graafian 

 follicle projects from the surface of the ovary as a transparent vesicle 

 about the size of a pea. (Its diameter is about 15 mm.) In the 

 process of growth the ovum has increased from a diameter of 25^ to 

 200 /x. Before the ovum can undergo fertilisation the double division 

 of the nucleus, or germinal vesicle, has to take place, which leads to 

 the formation and extrusion of the two polar bodies. This process 

 probably occurs just before or just after the discharge of the ovum, 

 from the ovary. 



With increasing size of the Graafian follicle the membrane covering 

 it becomes progressively thinner. At certain periods, or under certain 

 conditions, the membrane ruptures, and the ovum is discharged in 

 the liquor folliculi, still surrounded by an adherent mass of the cells 

 of the discus proliferus. In some animals this process of ovulation 

 occurs at definite periods of the year. In others, such as the rabbit, 

 the occurrence of ovulation depends upon coitus taking place during 

 the period of sexual activity. We shall have later to discuss the 

 relation of ovulation in the human female to the periodic changes 

 occurring in the other parts of the reproductive apparatus. 



After the discharge of the ovum the remaining portions of the 

 follicle undergo a characteristic series of changes, which result in the 

 production of the corpus luteum. Immediately after the rupture 

 the follicle becomes filled with blood, apparently resulting from the 

 sudden release of the pressure on the capillaries in the walls of the 

 follicle. The cells of the membrana granulosa rapidly increase in size, a 

 few of them undergoing mitotic division, so that a dense mass of cells is 

 formed, nearly filling the original follicle. At the same time the cells of 

 the internal theca proliferate, with the formation of connective tissue, 

 which grows in among the cells filling the Graafian follicle. These cells 

 finally attain a size four or five times that of the cells of the membrana 

 granulosa in the mature follicle. Blood-vessels grow from the external 

 theca towards the centre of the follicle. The cells within the follicle 

 then undergo fatty degeneration and present a yellow colour due 

 to a fatty pigment known as lutein. The corpus luteum, as the body 

 so formed is called, attains its greatest size about a week after ovula- 

 tion, and then gradually undergoes regressive changes. If, however, 

 the ovum, which has been discharged, undergoes fertilisation, and 

 pregnancy results, the corpus luteum continues to grow for a con- 

 siderable time and attains its largest size at about the third month 

 of pregnancy. It does not entirely disappear until after the end of 



