=Number of Ova.= When the female infant is born, her ovaries contain as many ova or eggs as they ever will contain. In fact, they contain more than they will at puberty. For it is estimated that at birth each ovary contains about 100,000 ova; the majority of these, however, disappear so that at the age of puberty each ovary contains only about 30,000 ova. As only one ovum ripens each month from the time of puberty to the time of the menopause (i.e., about 300 to 400 ova at the utmost during a lifetime), and as only a dozen or two ova would be necessary for the propagation of the race, it seems a superabundance of ova, an unnecessary lavishness. But nature _is_ lavish where the propagation of the species is concerned. A portion of an ovary or of both ovaries might become diseased, and thousands of ova might become unfit for fertilization; nature therefore puts in an extra reserve supply. We see a still more striking example of this extreme extravagant lavishness in man; only one spermatozooen is necessary to impregnate the ovum, and only one spermatozooen can penetrate the ovum; nevertheless each normal ejaculation of semen contains between a quarter and half a million spermatozoa. =The Graafian Follicles.= Each primitive or primordial ovum[3] is imbedded in a little vesicle or follicle, which is generally known as _Graafian follicle_, and there are as many Graafian follicles as there are ova. (The Graafian follicles were first described about 250 years ago--in 1672--by a Delft physician named De Graaf, hence the name.) Until puberty, that is the commencement of menstruation, the Graafian follicles with the ooecytes or primitive ova are in a more or less dormant condition. But with the onset of puberty there commences a period of intense activity in the ovaries. This period of activity is repeated regularly once a month, and it constitutes the process of _ovulation_ and _menstruation_. The two processes are closely though not causally connected. Ovulation consists in the monthly maturation and extrusion of a ripe ovum; menstruation, which will be further discussed in a separate chapter, consists in the monthly discharge of blood, mixed with mucus from the inside lining of the uterus. Every twenty-eight days, from the time of puberty to the time of the menopause, a Graafian follicle bursts and an ovum is extruded from the ovary. Before the follicle bursts, it swells and enlarges and reaches the surface of the ovary; the whole follicle is congested with blood, but at one point near the surface of the ovary it is pale and thin, and here the rupture takes place. [Illustration: SECTION OF OVARY. 1. Graafian follicle in the earliest stage. 2, 3, 4. Follicles in more advanced stages. 5, 7. Almost mature follicle. 6. Follicle from which the ovum has escaped. 8. Corpus luteum.]