=Ovulation.=—The ovaries produce ova that are retained in cavities termed ovisacs (Graafian follicles), which are scattered throughout the ovary. Sometimes an ovisac contains more than one ovum, but usually each ovisac contains but one. When the ovary becomes functionally active, some of the ovisacs in the deeper part of the organ increase in size and develop toward the surface. From time to time one of these ovisacs, having continued to increase in size so as to form a projection on the surface of the ovary, actually bursts and sets free an ovum. This frequently occurs in association with menstruation, but may take place at other periods. While this development of the ovisac has been taking place there is a special flow of blood to the pelvic contents, and the fringe-like processes of the Fallopian tube become more closely applied, so that a cup is formed by the fimbriated extremity of the Fallopian tube, into which the ovum drops. Eventually the ovum reaches the cavity of the uterus, and, unless it becomes fertilized or impregnated, it is in due course cast off from the uterus in the menstrual discharge and perishes. When the uterus has become the seat of a fertilized ovum there is no further ripening of ovisacs. After the escape of an ovum from the ovisac the follicle is filled with blood, and this takes part in the formation of a structure known as the corpus luteum, which is formed within the burst ovisac. The corpus luteum varies in its development according as the ovum becomes fertilized or not. If pregnancy takes place it remains to its close, but if the ovum is not fertilized it disappears in about three months. The ruptured ovisac always contains the elements of a corpus luteum, but its development depends on the fate of the ovum. Ovulation has been said to commence before menstruation; ovisacs may sometimes ripen and burst in childhood. Conception may take place before menstruation has set in. =Menstruation.=—The blood is derived from the mucous membrane of the uterus (endometrium) and not from that of the cervix. The endometrium becomes swollen and thickened prior to the commencement of the flow, and the blood-vessels are distended. With the beginning of the flow this swelling gradually diminishes. A few superficial cells of the endometrium are usually cast off during menstruation, and this is regenerated during the interval between the periods. The mucous membrane of the uterus is thus never at rest until menstruation ceases for good. =Conception.=—Fertilization of the ovum may occur anywhere in its course from the ovisac to the uterus, but ordinarily it takes place in the uterus. Certain changes take place in the uterus preparatory to the arrival of the fertilized ovum. The whole organ enlarges, it contains more blood than before; the mucous membrane becomes soft, spongy, thickens greatly, and is thrown into folds which are called the decidua vera.