=Ovulation. The corpus luteum.=--The ovaries of woman (Fig. 18) contain a considerable number of cells or ovules, although infinitely less than the number of spermatozoids contained in the testicles. From time to time some of these ovules enlarge and are surrounded by a vesicle with liquid contents, which is called the Graafian follicle. At the time of the monthly periods an egg (sometimes two) is discharged from its Graafian follicle, from one or other ovary. This phenomenon is called _ovulation_. The empty follicle becomes cicatrized in the ovary and is called the _corpus luteum_ (yellow body). The egg after its discharge arrives at the abdominal orifice of the Fallopian tube, which communicates directly with the abdominal cavity. Some authors state that the end of the tube becomes applied against the ovary by the aid of muscular movement and, so to speak, sucks in the discharged ovule, while others hold that the movements of the vibratile cilia, with which the epithelium of the tubes is furnished, suffice to draw the ovule into its cavity. Figure 18 explains this phenomenon. Having arrived in the tube, the ovule moves very slowly in the almost capillary tube by means of the vibratile cilia and arrives in the cavity of the womb. Fecundation probably takes place most often at the entrance to the tube or in its canal; sometimes possibly in the womb. On some occasions a squad of spermatozoids advances to meet the descending egg, and numerous spermatozoids are often found in the tubes, even as far as the abdominal cavity. =Fixation of the egg. Formation of the Decidua.=--After fecundation, the egg becomes attached to the mucous membrane of the cavity of the womb. This mucous membrane proliferates and becomes gradually detached from the womb to form the _membrana decidua_ which envelops the egg or ovule. An egg fecundated and fixed in this way may keep its position and grow during the first weeks of pregnancy, by the aid of villosities covering its envelope which penetrate the wall of the womb. [Illustration: FIG. 18. Diagrammatic section in median plane of the female genital organs. It shows the position of an ovule which has just been discharged lying in the opening of the right tube, and that of another ovary fecundated and surrounded by the decidual membrane. In reality this could hardly coexist with the other ovule freely discharged. In the right ovary are seen ovules in various degrees of maturity in their Graafian follicles: also a corpus luteum--an empty Graafian follicle after expulsion of the ovule. The figure also shows the end of the penis in the vagina at the moment of ejaculation of semen, and the position of a preventive to avoid fecundation.] [Illustration: FIG. 19. The mouth of the tube applied to the ovary at the moment of expulsion of the ovule.]