=Ovulation=, which probably occurs about midway between the menstrual periods, is simply the name which has been given to the principal function of the ovary and may be defined as the development of the ovum, or egg, and its expulsion, when mature, from the ovary. As the entire human body has its origin in this tiny ovum, its career and course of development are of momentous importance to us, and at the same time furnish a tale of absorbing interest. The ovaries are packed full of these tiny egg-like cells, which probably lie dormant, as stated before, until the girl reaches puberty. Then they begin to develop and grow and push their way from the inside of the ovary to the surface where they look more or less like blisters. When an ovum reaches the surface of the ovary, a thin membrane which contains it, bursts, and it is suddenly expelled into the abdominal cavity. You will remember that the ovary is very near the funnel-like end of the tube, so, when the little cell is shot out of the ovary, it finds itself floating around quite close to this wide opening. Some of the ova that are projected into the abdominal cavity are doubtless lost, but others find their way into the near-by mouth of the tube, and if not fertilized by uniting with a male cell, which we shall explain presently, they pass down the tube into the uterus and are finally carried out in the menstrual flow. It is probable that as a rule only one ovum ripens and escapes from the ovary each month from puberty until the menopause or change of life. The interesting thing about all of this is that each time an ovum does mature and is discharged from the ovary, the lining of the uterus becomes thicker and softer in order to facilitate the attachment of the ovum, if it is fertilized, this attachment being necessary if a baby is to develop. This preparation of the uterine lining is often, and very appropriately, referred to as “nest-building.” =Menstruation=, which is the evidence of sexual maturity, is a monthly hemorrhage from the uterus, escaping through the vagina, and it normally recurs regularly throughout the childbearing period, except during pregnancy and while the young mother nurses her baby. The length of this childbearing period is about thirty years and continues from puberty until the menopause. The frequency of the menstrual periods varies from twenty-one to thirty days but the normal interval between periods is twenty-eight days, which is the length of what is called the “menstrual cycle.” Thus it is usually a lunar month from the beginning of one period to the next one, making thirteen menstrual periods during each calendar year. The complete course of a menstrual cycle consists of four stages, which, it is believed, occur somewhat as follows: