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Endometrium, a membrane that lines the uterus, showing developing glands during the early proliferative phase. Cyclical changes in the endometrium prepare the uterus for implantation and also for menstruation, in the event that fertilization does not occur. The proliferative phase spans the time period from the end of menstruation to ovulation. (KEYSTONE/SCIENCE SOURCE/BIOPHOTO ASSOC.)