The variation in the length of the follicular phase in many mammals may be related to the cellular origin of oestradiol secreted during the luteal phase. In all species the time taken for a small developing follicle (4-5 layers of granulosa cells) to mature to a preovulatory follicle may be the same as that which has been found experimentally in the mouse (10-17 days). In animals such as the sheep, in which there is no source of oestradiol other than the Graafian follicle, follicular development proceeds unimpaired throughout the luteal phase, and the 'follicular phse' which involves only the final stages of maturation of the Graafian follicle is relatively short. In primates, however, in which there is an extrafollicular source of oestrogen from the CL, the secretion of gonadotrophins is suppressed during each luteal phase to a level too low to initiate and maintain follicular development. At the end of each luteal phase and the beginning of the subsequent follicular phase, therefore, it is necessary to initiate the growth of a new crop of small follicles. The time taken for these follicles to develop inot preovulatory follicles determines the length of the follicular phase.

The response of mouse ovaries maintained in organ culture to prostaglandin E-2 (PGE2) and prostaglandin F-2alpha (F2alpha) was assessed using quantitative histological and radioimmunoassay procedures. Prostaglandin E2 induced histological changes in the cultured follicles comparable to those induced by human chorionic gonadotrophin (HCG) and the increase in the number of oocytes undergoing preovulatory maturation over the control value was the same irrespective of the treatment (PGE2 alone, HCG alone, or PGE2+HCG). The amount of progesterone/ml of culture medium was also significantly higher with these preparations than in control cultures (about 125 ng/ml compared with 57 ng/ml). By contrast, 5 mug PGF2alpha/ml medium increased neither the number of oocytes undergoing maturation nor the concentration of progesterone in the culture medium. The latter increased when the dose of PGF2alpha was increased to 30 mug/ml, although the proportion of oocytes beyond the dictyate stage remained at the control level. There was no augmentation in the response (above the level for HCG alone) when HCG and PGF2alpha were added to the explant medium simultaneously. These results are discussed in terms of the possible mechanism of action of the various preparations.

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SUMMARYOvaries derived from fetal and neonatal mice of the Schofield strain (aged x¢ days p. c. to 14 days p. p.) were maintained in organ culture for periods ranging from 2 to 14 days. Histological sections of these gonads were compared with controls that were fixed immediately after removal from coeval animals.It was found that germ cells in fetal ovaries progress through meiotic prophase at the same rate in culture as they do in vivo, although the number of germ cells was reduced if the ovary was cultured through a critical period (days 14 -1 8 !).e.

Foetal human testes (12-22 weeks gestation), maintained in organ culture, were treated with human chorionic gonadotrophin (HCG) and the amount of testosterone produced compared with control cultures. In all cases the testes produced testosterone, but from the 13th to 18th week of gestation significantly more testosterone was produced by, and the Leydig cell hyperplasia was maintained in, the HCG stimulated organ cultures. It is suggested that HCG is ultimately responsible for differentiation of the human male external genitalia.