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Fertilization is the process by which the haploid nuclei of the sperm and egg cell(gametes) join to form a diploid zygote. The process of meiosis, a reduction division, produces haploid gametes or sex cells. Meiosis occurs in the gonads--testes in males and ovaries in females. Mammals like humans have internal fertilization. This insures a greater chance of the egg becoming fertilized by a sperm cell. If fertilized, the egg develops internally. This provides added protection for the developing embryo inside the womb. At ovulation, an egg is released from the ovary and it will wait for about one day to join with a sperm in the oviduct. If fertilization occurs, the egg will travel through the oviduct to the womb or uterus where it will implant. During this time, cleavage of the fertilized egg begins and this stage takes about 10 days. By the time it implants, it has many cells and gives rise to the placenta which allows materials to pass between mother and embryo necessary to help it develop during pregnancy. An umbilical cord attaches the fetus to the placenta and an amniotic sac surrounds the fetus with amniotic fluid providing a cushioned and protected environment. If a female's egg is not fertilized, it will disintegrate and the uterine lining will be shed during menstruation. The following month, another egg is released and the cycle continues from puberty until menopause.
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