The distribution of binding sites for human chorionic gonadotropin (hCG) in the preovulatory follicle was studied by autoradiography. An ovulatory dose (10 IU/rat) of [125I]hCG (1.4 muCi/IU) was administered intravenously, and large Graafian follicles were isolated 3 h later by microdissection. Injection of excess unlabeled hCG (500 IU/rat) prevented uptake of radioactivity by the follicle, indicating that binding of iodinated hormone was confined to specific and saturable receptor sites. The density of bound hormone molecules was highest in the theca interna and in three to four layers of mural granulosa cells adjacent to the basement membrane; labeling was chiefly associated with the cell borders. No significant binding could be detected either on the oocyte or on the cumulus cells surrounding the oocyte. We therefore suggest that the induction of ovum maturation does not require attachment of the hormone to the oocyte itself or to follicle cells in its immediate vicinity.

It is proposed that the action of lh on oocyte maturation involves the mediation of the adenyl cyclase/cyclic AMP system and possibly of the prostaglandins. An action of steroids could not thus far be implicated. The experimental model described permits study ofthe mechanism of the meiosis-inducing action of lh under controlled conditions in vitro.

Morphological features of the interaction between the im¬ planting blastocyst and the uterine endometrium were studied by electron microscopy, using material fixed in situ. The observations extended from the pre-attachment stage to the completion of tropho¬ blastic erosion of the uterine epithelium and early decidual transforma¬ tion of the stromal cells.After shedding of the zona pellucida, the blastocyst establishes contact with the tips of microvilli and with bleb-like cytoplasmic protrusions of the epithelial cells. Attachment occurred during the afternoon of the 5th day post coitum (p.c.). Patches of'bristle-coated' membrane appeared on the trophoblast membrane at that time. A lattice-like substructure was discernible in the cytoplasmic plaques, which are abundant in the early blastocyst. Sperm-tail inclusions were observed in both trophoblast and inner cell mass as late as Day 5 p.c.By the 6th day p.c., the epithelial microvilli were usually lost and the trophoblast membrane interlocked with that of the epithelial cells, with frequent formation of tight junctions. Leucocytes were occasionally seen in the epithelial layer at the attachment site. Unusually electron-dense cells were often found wedged between the trophoblast and epithelial cells; their possible relation to Wilson bodies is discussed.The earliest morphological response of the stromal cells to blastocyst attachment was the appearance of changes in nuclear shape, chromatin distribution and nucleolar structure in the subepithelial layer. The deeper subepithelial stroma became oedematous, and diapedesis, erythro-phagocytosis and later the formation of sinusoids could be observed.

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SUMMARY Testosterone and androstenedione were determined chemically in testes of bulls ranging in age from 28 days to 17½ years. The earliest age at which it was possible to detect both steroids was 39 days. Their combined contents in the testes varied from 4·7 to 61 μg at 39–90 days, from 31 to 475 μg at 3½–5½ months, and from 155 to 3450 μg at 10½ months-17½ years of age. The ratio androstenedione/testosterone decreased with age. In immature bull calves, less than 4 months old, it exceeded 1:1, in animals above 9 months of age it was less than 1:10. The appearance of chemically demonstrable amounts of testosterone and androstenedione in the testes of young bull calves coincided with the onset of an active process of fructose and citric-acid production by the seminal vesicles. Between the 2nd and 6th month of life, the weight, fructose level and citric-acid level of the seminal vesicles increased at an exponential rate, and this sharp increase in the activity of the seminal vesicles was accompanied by a similarly sharp increase in the testosterone content of the testes. In adult bulls, differing widely in age and breed, the testicular content of testosterone was significantly correlated with the weight, fructose content and citric-acid content of the seminal vesicles.

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Progesterone pretreatment (5 mg/day) of ovariectomized\p=m-\ adrenalectomized rats for at least 12 hr abolished the response of the uterine luminal epithelium to oestradiol (0\m=.\2\g=m\g) with respect to (i) mitotic activity, (ii) nucleolar enlargement, and (iii) [3H]uridine uptake, and suppressed the uptake of [3H]oestradiol by the epithelial cells, as judged by dry-mount autoradiography. Progesterone pretreatment for periods exceeding 36 hr redirected the

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