Usually for some strange reason, one, and only one, ovum ripens regularly each month during the years from puberty to the menopause, excepting during pregnancy, when this function is suspended. Occasionally, however, several ova mature at once, a condition which may be one factor in the development of twins. After puberty the ovary contains ova in all stages of development, from the primordial follicle to the Graafian follicle just described. When a Graafian follicle containing a matured ovum reaches the ovarian surface, its membrane becomes thinner and finally ruptures because of increased tension in the ovary, due to certain circulatory changes. The ovum surrounded by the discus proligerus is thus discharged into the peritoneal cavity near the fimbriated end of the tube. Some ova enter the tube and others float about in the peritoneal cavity, finally disintegrate and are lost. The torn envelope of the follicle which remains in the cortex of the ovary becomes filled with blood, which forms into a clot. This clot is first surrounded, and then invaded, by cells containing bright yellow pigment called _lutein_. The membrane formed from these cells compresses the clot and brings about other changes which speedily transform it into the _corpus luteum_. If the discharged ovum becomes fertilized, the corpus luteum remains practically unchanged for months and is termed the _corpus verum_ or corpus luteum of pregnancy. Its secretion is believed to influence the implantation of the ovum and to promote the woman’s general well-being during the period of gestation. It continues to exist throughout pregnancy, and until after delivery, when it is soon absorbed and replaced by normal ovarian tissue, without the formation of scar tissue. If fertilization does not occur, the body in the ovarian cortex, which is then termed the corpus luteum of menstruation, or _false corpus_, undergoes rapid degenerative changes and is almost wholly absorbed within a few weeks. By means of this rather complicated procedure the ovary is saved from becoming a steadily enlarging mass of scar tissue, and consequently devoid of reproductive powers, which would be the case if the wound made by the rupturing of each Graafian follicle were to heal by the usual formation of cicatricial tissue. Ordinarily the ovum remains unfertilized and is propelled down the Fallopian tube, by the cilia in its lining, to the uterine cavity, where it is lost in the uterine secretions and ultimately carried out in the menstrual flow. Each time that an ovum matures, however, and is discharged from the ovary the lining of the uterine cavity increases in vascularity and becomes thicker and more velvety; a condition which facilitates an attachment of the ovum in case of fertilization. This preparation of the endometrium is termed “pre-menstrual swelling,” or in popular language, nest-building.