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In the prepubertal and fertile periods, the cortices of mammalian ovaries are crowded with follicles of different sizes (Figs. 3 and 11), from primordial follicles described in Chap. II to those showing large distended antra and which are lined with many concentric layers of granulosa cells. This shows that the population of mammalian follicles is in a continuous dynamic state and that at any given time follicles are present which progress from a state of “quiescence” to one characterized by volumetric growth and increased activity of all of its components (Chiras and Greenwald 1980). The early stages of follicular growth (Fig. 11 a–d) are believed to be independent of gonadotropic support. However, the factors which initiate the selective growth of primordial follicles, are poorly understood (Zuckerman and Baker 1977, Peters and McNatty 1980, Nicosia 1980 a). They appear to originate within the ovary. According to Greenwald (1979), a hypothesis proposed by Edwards fits the FIFO (first in, first out) model; i.e. the first formed oocytes are the first ones to be mobilized in the postnatal period. This is an interesting speculation but with no direct evidence — either pro or con. Pulse labelling of the prenatal ovary and subsequent autoradiographic studies at various times postnatally might reveal whether the maturation of oocytes is temporally programmed. Greenwald (1979) also suggested that the recruitment of primordial follicles may represent a random process which may depend upon their proximity to blood vessels, nerves, more advanced follicles or corpora lutea acting as the signal for passage out of the resting pool.