A STUDY ON THE CONTROL OF LENGTH OF GESTATION IN THE RAT WITH NOTES ON MAINTENANCE AND TERMINATION OF GESTATION

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Abstract
All embryos were surgically excised from the uteri of rats pregnant for varying lengths of time; the remaining placentae were retained in utero until the day of expected parturition and were then delivered. These placentae responded to the normal birth mechanism, demonstrating that the fetus is neither the essential regulator of the length of pregnancy nor the agent responsible for the onset of labor. When paraffin pellets were inserted into the empty uterine gestation chambers after Cesarean section in the last half of pregnancy they were aborted in 48 hrs.; if, however, placentae were allowed to remain in utero, the pellets were retained and delivered at full term. The presence of the placenta is thus essential for the maintenance of the uterine quiescence characteristic of pregnancy. Evidence was adduced indicating that various maternal factors are subordinate to the placenta in maintaining and controlling length of gestation. However, since in no expt. was the possible endocrine role of the uterus excluded, the "placento-uterine complex" is probably the essential factor in maintaining gestation and controlling its length. These functions of the placento-uterine complex are shown to be on an endocrine basis, and the importance of the placento-uterine complex in the birth mechanism and the possible mechanisms of its action are discussed.