1222 PHYSIOLOGY 



OVULATION. At birth the ovary consists of a stroma of spindle-shaped 

 cells, and is covered by a layer of cubical epithelium (the germ- epithelium) 

 continuous with the endothelium lining the general peritoneal cavity. Em- 

 bedded in the stroma, but especially numerous just underneath the epithe- 

 lium, are a vast number of ' primordial follicles.' These are formed during 

 foetal life by down- growths of the germinal epithelium. Of the cells pro- 

 longed in this way from the germinal epithelium, some undergo enlargement 

 to form the primordial ova, while the others are arranged as a single layer 

 of flattened nucleated cells, the ' follicular epithelium/ as a sort of capsule 

 to the ovum. Of the primordial follicles, about 70,000 are to be found in 

 the ovary of the new-born child. During the first twelve to fourteen years of 

 life they remain in a quiescent condition. With the onset of puberty one or 

 more of the primordial follicles begin to develop. Indeed, this development 

 may be regarded as the causative factor in the various phenomena which 

 are characteristic of puberty in the female (v. p. 1217). The first stage in 

 the growth of the follicle is a proliferation of the follicular epithelium, the 

 cells of which become cubical and are arranged in several layers round the 

 ovum. At one point in the mass of cells surrounding the ovum a cavity 

 appears filled with fluid, the liquor folliculi. The epithelium thus becomes 

 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. 561). 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 25/x, 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 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 condi- 

 tions, 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. 



