Before puberty, these graafian follicles are small vesicles of a diameter of one to two millimetres, containing the large unicellular ova. Each of these consists of an envelope, the zona pellucida (also known as the zona radiata, or striated membrane of the ovum); an external granular mass of protoplasm, the vitellus or yolk; a vesicular, spherical nucleus, the germinal vesicle; and a nucleolus, which if single is large and prominent, the macula germinativa or germinal spot. As early as the second year of infancy every imaginable intermediate stage between the primitive follicle and the fully-developed vesicular graafian follicle can be observed.
At the time of puberty certain larger follicles are always to be distinguished, which have moved inward toward the interior layers of the ovary, whereas the smaller follicles have a more peripheral situation; thus, according to _Waldeyer_, we observe at this time in a section of the ovary, proceeding from without inward, first the epithelium, next the fibrous tunic, next the zone of younger follicles, and finally the zone of older follicles. According to _Henle_ and _Waldeyer_, at the commencement of puberty, there are in each ovary about 36,000 ova, giving a total for the two of 72,000.
In the further course of development of the graafian follicles at this period, the most advanced now reapproach the surface of the ovary, so that a fully-matured follicle comes to occupy almost the entire thickness of the cortical substance, and may even give rise to a localized bulging of the surface of the organ. In such a mature follicle, which has attained nearly the size of a pea, we recognize an outermost connective-tissue investment (_theca folliculi_), consisting of condensed ovarian stroma, in which two layers are distinguished, sometimes called simply _outer tunic_ and _inner tunic_, sometimes known by the names of _tunica fibrosa_ (outer) and _tunica propria_ (inner), respectively; within this is the cellular layer known as the _membrana granulosa_ (or _stratum granulosum_), the portion of which, now greatly enlarged, immediately surrounding the ovum is known as the _discus proligerus_ (or _cumulus oöphorus_); the interspace between the _discus proligerus_ and the membrana granulosa is filled with a clear fluid, the _liquor folliculi_. In consequence of the continued increase in its fluid contents, the graafian follicle ultimately bursts along the most prominent portion of the superficial wall, and the ovum passes out through the rupture, finding its way under normal conditions into the Fallopian tube and through this into the uterus. The follicle itself then undergoes a regressive metamorphosis, forming the _corpus luteum_, the rent in the envelope of which, after the absorption of the yellowish semi-fluid contents, undergoes cicatrization. Contemporaneously with this development at puberty of the process of ovulation, menstruation also for the first time makes its appearance, recurring thenceforward at four-weekly intervals as the regular catamenial discharge.