Abstract
Synopsis The changes in the vascular pattern of the pregnant rat uterus at placental sites and the intervening areas, have been studied radiologically and by injection techniques during the period of pregnancy. A description is given of the development and final pattern of the maternal placental blood channels. From the 15th day onwards wide variations are found in the vascular filling of placentæ after arterial injection. Possible mechanisms for the control of blood flow to placental sites are discussed.