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Apparently the mammary gland grows in response to sustained high serum concentrations of estrogens and progesterone which are incurred during pregnancy in ruminants. Quantities of prolactin, growth hormone, and glucocorticoid in serum are probably sufficient to support mammary growth during pregnancy. In sheep and goats mammary growth may be stimulated by a lactogenic material of placental origin.
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E. Μ. Convey (1974) studied this question.
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