RUSSELL KIGHTLEY / SCIENCE PHOTO LIBRARY RUSSELL KIGHTLEY / SCIENCE PHOTO LIBRARY
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Fertilization. Computer artwork of sperm cell about to penetrate an egg cell. Each cell contains one half of the genetic material needed to form a new human. Only one of some 300 million sperm released during ejaculation will fertilise the single egg. Once it has penetrated, rapid chemical changes make the egg's outer layer (zona pellucida) thicken, which prevents any other sperm entering the egg. The nucleus of the successful sperm fuses with the nucleus of the egg to form a zygote, which will undergo repeated mitosis (cell division) and develop into an embryo.
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