N a hormonally controlled cycle of activity of the reproductive organs in many female mammals. The follicular stage (growth of the Graafian follicles, thickening of the lining of the uterus, secretion of oestrogen, and ovulation, is succeeded by the luteal phase (formation of the corpus luteum and secretion of progesterone), followed by regression and a return to the first stage 发情周期 →see oestrus
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