Of the enormous number of ova existing in each woman, relatively few mature and it is apparent that still fewer are fertilized, since each impregnation results in an abortion, a premature labor or a full term child. Nature’s lavish provision of something more than 100,000 ova for each woman, who uses only about 500 in the course of her life, excites no little wonder. But whatever the purpose of this enormous supply, its existence makes possible the removal of all but a small fragment of ovarian tissue in cases of disease, without interference with the process of ovulation, which in turn permits reproduction. =Menstruation=, which is the evidence of sexual maturity, is a monthly hemorrhage from the uterus which escapes through the vagina, normally recurring throughout the entire child-bearing period, except during pregnancy and lactation. The duration of this child-bearing period, or sexual activity, is about thirty years and continues from puberty to the menopause. The frequency of the menstrual periods varies in different women from twenty-one to thirty days, but the normal interval between periods is twenty-eight days, which corresponds in point of time to the menstrual cycle. Thus it is usually four weeks, or a lunar month, from the beginning of one period to the beginning of the period following, making thirteen menstrual periods during each calendar year. Just why menstruation occurs about every twenty-eight days is not known, but the belief is that, although menstruation is in some way dependent upon ovulation, its periodicity is regulated by the corpus luteum. It is also believed that the corpus luteum of pregnancy holds menstruation in check during the nine months of gestation. The menstrual cycle is divided into four stages, and though there is not entire unanimity of opinion concerning the changes which take place during these four stages, the preponderance of evidence is in favor of the following processes. =The first or constructive stage= lasts about seven days. It is during this stage that the preparative changes, which have been described, are made for the reception of the matured ovum. The uterus becomes engorged with blood and is somewhat enlarged and softened as a result. The endometrium grows deep red, thick and velvety, partly because of the greatly augmented blood supply, and partly because of an actual increase of connective tissue in its structure. There is also an increase in the size and activity of the uterine glands and in the amount of their secretions. If the ovum remains unfertilized, which is usually the case, it does not attach itself to this elaborately prepared lining, but passes out with the uterine discharges, and all of this preparation and increased vascularity not only go for naught, but must be undone.