Abstract
SUMMARY The pituitaries of three species of cartilaginous fish, 11 species of bony fish and two species of amphibian were subjected to pigeon-crop and rabbit mammary-gland tests for prolactin-like activity. Both pituitary homogenates and partially purified pituitary extracts were investigated. Positive reactions were observed with pituitaries from all species tested, though there were invariably marked differences from the normal response produced by injections of purified sheep prolactin. These differences suggest that a principle is present in lower vertebrate pituitaries which has some, but not all, of the characteristics of higher vertebrate prolactin. The secretory stimulatory element responsible for the onset of milk secretion in the target organ is absent, but the proliferative element responsible for growth of the gland during a response is already demonstrable.
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