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H. M. BRUCE, R. B. LAND and D. S. FALCONER

Summary.

Pregnancy-block after exposure of recently inseminated females to strange males was studied in mice of the Q strain. In one series of experiments the incidence of blocked pregnancy was very low. This strain displayed a normal increase in the frequency of oestrus when the females were exposed to males, hence the low pregnancy-block rate could not be attributed to anosmia. In other experiments, Q females displayed a high incidence of pregnancy-block after a period of adaptation. The initial low rate of pregnancy-block was evidently due to disturbance resulting from unaccustomed daily handling during the test.

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J. Falconer, M. D. Mitchell, L. A. Mountford and J. S. Robinson

Summary. Plasma samples were obtained from 8 conscious rhesus monkeys at 3–4 day intervals throughout the menstrual cycle. Oxytocin concentrations were significantly higher in mid-cycle (Days 10–11) than at Days 4–5 (P < 0·01) and Days 28–29 (P < 0·02, Wilcoxon signed rank test). No significant correlations between oxytocin and oestradiol or progesterone were found.

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D. S. FALCONER, R. G. EDWARDS, R. E. FOWLER and R. C. ROBERTS

Summary.

The variation in the number of eggs shed by the two ovaries of mice has been examined by statistical analyses of 697 egg counts and 390 corpora lutea counts, made on mice from a variety of outbred strains, both after natural oestrus and after oestrus induced in adults by pregnant mares' serum (pms) and human chorionic gonadotrophin (HCG). The numbers of eggs or corpora lutea were distributed between sides approximately at random, the variation conforming fairly closely to a binomial distribution. This was true even after superovulation. There was, however, a slight but significant excess of variation between sides over the random amount in the egg counts, particularly after natural ovulation. Corpora lutea counts differed from egg counts in showing a slight but significant reduction of the variation below the random amount. Several possible reasons for these small deviations from a random distribution are discussed.

The correlation between the numbers of eggs shed by the two ovaries was negative after natural ovulation but positive after superovulation. This difference can be fully accounted for by the random distribution between sides together with the differences of mean and variance between natural ovulation and superovulation. The variation of total egg number was proportional to the mean egg number after natural ovulation. The variation after superovulation was much higher than after natural ovulation, even when the difference of mean was taken into account, and the greater variation of total egg number caused the correlation between sides to be positive after superovulation.