Every ovum originates in the ovaries. These are organs peculiar to women, having the size and shape of large almonds, and placed in the lower part of the abdominal cavity. Though the ovaries are two in number, one alone is sufficient for every requirement of health. It has been estimated that the ovaries together contain at the time of birth about 40,000 ova, distributed equally between them. Since less than 500 ova are required to insure regularity in the menstrual function, it is clear that, if the surgeon finds it necessary to remove one of the ovaries, the other will provide abundantly for menstruation and for the bearing of children. Although every ovum that will be produced as long as a woman lives has already sprung into existence by the time she is born, not a single one ripens for from twelve to fifteen years. The ripening process begins about the time of puberty, and, unless suspended through the occurrence of pregnancy, continues until the menopause. During this period, which is also characterized by the periodical appearance of menstruation, one ovum ripens each month; sometimes, though rarely, several ripen at once, and this tendency is partly responsible for twins. The human ovum is a tiny structure, measuring about 1/125 of an inch in diameter. With the naked eye it can barely be seen; magnified by the microscope it appears as a little round bag made of a transparent membrane. Briefly described, the ovum is a single cell. That is, it belongs to the simplest class of anatomical structures, and is one of the millions upon millions of units that make up the body. It contains a nucleus surrounded by nutritive material, the yolk. Yet the quantity of yolk is exceedingly small. In this particular the human ovum differs notably from the egg of birds, as it does also in that it lacks a shell. Obviously, a shell would not only be useless to an embryo developing within the body of its parent, but would shut off the nourishment, which, since the ovum contains so little, must necessarily be provided by the mother. When the ovum has ripened, it becomes detached from the ovary, and enters a fleshy tube about the size of a lead pencil, known as the oviduct. There are two of these tubes, one running from the neighborhood of each ovary; both enter the uterus, but on opposite sides. The ovum travels down the tube which corresponds to the ovary where it originated. The journey is fraught with momentous consequences, for it is during this passage through the oviduct that the fate of the ovum is determined. If it is to develop into a living creature, a great many conditions must sooner or later be fulfilled; but there is one which must be promptly satisfied. Shortly after leaving the ovary the ovum must receive the stimulus to live and grow; otherwise it will quickly wither and die. This vital stimulus can be imparted only by the spermatozoon.