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Pregnant ewes were ovariectomized 3 weeks after mating, and were injected daily with graded doses of progesterone (0·05–1·35 mg./kg.) for the next 6 weeks. A minimum daily dose of 0·15 mg./kg. was found necessary for the regular maintenance of pregnancy. On the 60th day of pregnancy, when the ewes were killed, there was little or no effect of dose of progesterone on foetal weight, weight and number of cotyledons, weight of uterus and membranes or volume of amniotic fluid. However, at the low doses of progesterone the volume of allantoic fluid was about ten times higher than normal, suggesting that progesterone plays some part in the water balance of the conceptus. In treated ewes, the cotyledons tended to be of the bovine type with foetal tissue enveloping the maternal, which suggests that growth of maternal cotyledons may be limited by the absence of ovaries.

It is suggested that the large variations usually observed in the placental size of sheep, are not due to variations in output of ovarian progesterone, and that the normal daily output of the ovine corpus luteum during the first 2 months of pregnancy is equivalent to at least 15 mg. of subcutaneously injected progesterone.

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