MEHAU KULYK / SCIENCE PHOTO LIBRARY MEHAU KULYK / SCIENCE PHOTO LIBRARY
Fertilisation. Computer artwork showing human sperm propelling themselves towards an egg. Sperm (spermatozoon) and egg (ovum) are sex cells containing genetic material in the form of 23 chromosomes. Fertilisation occurs when a sperm penetrates the egg, forming a zygote cell by the fusion of its genetic material with that of the egg. Only one sperm can successfully penetrate the egg and fertilise it. Sperm are formed in the testes. Their flexible tails allow them to move along the female reproductive tract. Eggs develop in the ovaries. Both sperm and eggs are formed by a special type of cell division known as meiosis.
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