REPRODUCTION IN MAN 1275 



separated into two parts, i. e. the membrana granulosa, several layers thick, 

 lining the whole follicle, and the discus proligerus, a mass of cells attached to 

 one side of the follicle, in which is embedded the ovum (Fig. 574). Round 

 the growing follicle the stroma assumes a concentric arrangement and forms 

 a capsule, of which the internal layer consists chiefly of spindle-shaped cells 

 richly supplied "with blood vessels, while the outer layer the theca externa 

 is made up of a tough fibrous tissue. With the growth of the follicle 

 the ovum also becomes larger and surrounds itself with a distinct membrane, 

 known as the zona pellucida. This membrane 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 trans- 

 parent 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 25jU, to 

 200ja. Before the ovum can undergo fertilisation, the double division of 

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

 tion 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 con- 

 ditions, the membrane ruptures, and the ovum is discharged in the liquor 

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

 proligerus. In some animals this process of ovulation occurs at definite 

 periods of the year. In others such as the rabbit, the occurrence of ovula- 

 tion 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 ovulation, and 

 then gradually undergoes regressive changes. If however the ovum, 



