The characteristic changes in the ovary at the time of the menarche originate in the changes undergone by the ovarian follicles. A large number of small separate follicles is to be found already in the ovary of the new-born infant. These structures, known as primitive follicles, are formed by detachment from the egg-tubes that grow down into the stroma from the superficial germinal epithelium; they are spheroidal vesicles, enveloped by a single layer of cubical cells, and their interior is entirely filled by the primitive ovum or egg-cell. This latter consists of very finely granulated protoplasm with spherical nucleus and distinct nucleolus, but no trace of an investing membrane can as yet be discerned. The further development of the ovarian follicles takes according to _Toldt_ the following course: A rapid multiplication of the cubical cells that form the wall of the follicle occurs, so that the ovum is surrounded by two, three, or several layers of cubical or rounded cells, and the whole follicle gradually increases in size. At the same time the ovum assumes an eccentric position in the interior of the follicle. At or near the middle of the follicle a slit-shaped space now appears, filled with a clear colorless fluid. As this space gradually enlarges, the follicle[19] becomes converted into a vesicle filled with fluid, the wall of which is composed of small cubical cells. Simultaneously with the growth of the follicle a lamination of the elements of the surrounding stroma takes place, so that a somewhat sharply defined capsule is formed. In this condition these glandular structures of the ovary are known as graafian follicles.[19] 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. [Illustration: FIG. 15.—Primitive follicles. ]