Although considerable information is accumulating concerning the endocrinology of parturition, the nature of the underlying stimulus controlling the onset of parturition remains obscure (for review see Marshall & Moir, 1952; Zarrow, 1961). The present study was undertaken to determine whether or not parabiosis would reveal the existence of a humoral factor which controls the onset of parturition. Earlier studies by Sauerbruch & Heyde (1910), Fels (1929) and Hill (1932) demonstrated that parturition is unaffected in a pregnant rat joined in parabiosis with a non-pregnant rat. Kross (1926) and Munemitsu & Segal (1959) reported no evidence of a disturbance in length of gestation or in parturition when two pregnant rats were placed in parabiosis. We reasoned that a humoral factor released as a stimulus to parturition in the rat might not cause the onset of parturition in its parabiotic
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