Observations on the Œstrous cycle of the albino mouse

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Abstract
By observing changes in the vaginal contents a determination of an average cycle length of 5.7 days was obtained for the normal animals of this colony. The cycle as modified by copulation with vasectomized males was then observed. This was remarkably uniform in length and averaged 11 days, twice the length of the non-mated cycle. The average gestation period of mice in this colony was 19 days. The characteristic "smear sign" of pregnancy, the "placental sign" of Long and Evans, appeared between the 10th and 12th days after copulation. The average length of the first cycles following parturition, when the animals were mated with vasectomized males, was not significantly longer than other cycles. Lactation inhibited oestrus for at least 3 weeks when the litter suckled was moderately large. Nursing 1-2 young did not prevent the return of oestrus. Prolonged lactation induced by foster-mothering 1-2 additional litters did not appreciably retard the cycles after the initial 3 weeks following parturition.

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