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Illustration of an egg (pink) being released from an ovary (bottom left), a process known as ovulation. The egg is being released into a fallopian tube, which will carry the egg to the womb (uterus, not seen), where it may or may not be fertilised by a sperm cell. After sexual maturity females release one egg during each menstrual cycle. (KEYSTONE/SCIENCE PHOTO LIBRARY/NICOLLE R. FULLER)