A. Early proliferative endometrium (days 3–6). Surface epithelium is intact. Glands are straight and tubular without mitotic figures or pseudostratification. This normal endometrium was exposed only to estrogen stimulation at the time of biopsy. B. Late secretory endometrium (days 25–26) in a normal menstrual cycle. Tissue has been predominantly stimulated by progesterone for 11 to 12 days. Glands are convoluted and have expended most of their secretory products. The stroma has undergone an extensive decidual reaction.