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Recent work from this laboratory has shown that in the sheep there is a sharp rise in the uterine secretion of unconjugated oestrogens during the last 48 hr of pregnancy (Challis, 1971; Challis, Harrison & Heap, 1971). In addition, the production of progesterone by the placenta is significantly reduced at this time (Challis, Harrison & Heap, 1971), and any interpretation of a foetal rôle in the initiation of parturition (Liggins, 1969) must take into account such changes in the levels of steroid hormones in the mother. It was, therefore, of interest to study the pattern of oestrogen secretion in a related species such as the goat in which progesterone is exclusively derived from the ovaries throughout pregnancy (Linzell & Heap, 1968), and ovariectomy at any stage of gestation results in abortion (Drummond-Robinson & Asdell, 1926) The
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Challis et al. (1971) studied this question.
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