Delayed implantation can be experimentally induced in the rat by various procedures postponing the completion of the progesterone-oestrogen sequence which results in the uterine state of receptivity (Psychoyos, 1967). Thus, in animals ovariectomized early in pregnancy and treated subsequently with progesterone alone, the uterus remains in a 'neutral' state which can be extended to several months, until oestrogen is added to the progesterone treatment. The present paper reports observations obtained by scanning electron stereoscopy on the ultrastructure of the surface of the luminal epithelium during this state.
The endometria used for this study were taken from Wistar-strain rats ovariectomized on the 2nd day of pregnancy and treated daily with 5 mg of progesterone. On the 10th to 12th day post coitum, laparotomies were performed under ether anaesthesia