The occurrence of puberty marks the establishment of _ovulation_ and _menstruation_. These two functions are usually performed once a month, ovulation probably occurring about midway during the intermenstrual period. =Ovulation=, which is the prime function of the ovary, may be defined as the formation and development of the ovum, and its expulsion, when mature, from the ovary. The formation of each woman’s full quota of ova is probably complete at birth, though the process may continue until about the second year. At this time it is variously estimated that each of the two ovaries contains from 50,000 to 70,000 ova, but they remain unmatured until puberty, the period at which ovulation is most active. [Illustration: FIG. 16.—Diagram of human ovum. ] As the entire complex human body has its origin in this tiny ovum, its course of development is of momentous importance to us, and at the same time it provides a tale of intense interest. In its unmatured state, the ovum, termed a _primordial follicle_, or _oöcyte_, is a single cell, 1/125 inch in diameter, consisting of clear protoplasm, the _vitellus_, and a surrounding _vitelline membrane_ composed of small, spindle-shaped epithelial cells. The protoplasm contains a fairly large _nucleus_, or _germinal vesicle_, within which lies a _nucleolus_ known as the _germinal spot_. (Fig. 16.) The primordial follicle probably lies dormant in this state until puberty, when developmental changes take place, though it is the belief of some authorities that follicles are in the process of development from birth until the end of sexual life, though none fully mature until puberty. With the advent of puberty the cells composing the vitelline membrane change in character and proliferate rapidly, with the result that the ovum is surrounded by several layers of epithelial cells. Some of the inner cells degenerate and liquify, thus surrounding the ovum with fluid which is contained in a membrane of vascular connective tissue, the _theca folliculi_; this in turn is lined with epithelial cells, the _membrana granulosa_. This structure constitutes a _Graafian follicle_, named for Dr. de Graaf who first described it, and in the course of its maturation is pushed toward the surface of the ovary, where it presents more or less the appearance of a clear blister. At one point in the enveloping membrana granulosa, the cells proliferate into a mass in which the floating ovum becomes embedded. This mass is termed the _discus proligerus_ and the fluid which surrounds it is the _liquor folliculi_.