Reproductive process in which a male sex cell (sperm) unites with a female sex cell (egg). During the process, the chromosomes of the egg and sperm will merge to form a zygote, which will divide to form an embryo. In humans, sperm travel from the vagina through the uterus to a fallopian tube, where they surround an egg released from an ovary usually two or three days earlier. Once one sperm has fused with the egg cell membrane, the outer layer becomes impenetrable to other sperm. See alsocross-fertilization, self-fertilization.
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