The process by which male and female gametes (spermatozoa and oöcytes respectively) fuse to form a zygote which develops, by a complex process of cell division and diﬀerentiation, into a new individual of the species. In humans, fertilisation occurs in the FALLOPIAN TUBES. Sperm deposited in the upper vagina traverse the cervix and uterus to enter the Fallopian tube. Many sperm attempt to penetrate the zona pellucida surrounding the oöcyte, but only one is able to penetrate the oöcyte proper and this prevents any other sperm from entering. Once the sperm has entered the oöcyte, the two nuclei fuse before the zygote begins to divide.
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