MEHAU KULYK / SCIENCE PHOTO LIBRARY MEHAU KULYK / SCIENCE PHOTO LIBRARY
Fertilisation. Computer artwork of human sperm approaching an egg (ovum, at centre) during the process of fertilisation. Each sperm has an oval head and a hair-like tail which it beats with a whiplash motion to swim. The human female usually produces a single large egg from the ovary, while the male releases some 300 million much smaller sperm. The sperm travel through the uterus (womb) and up the fallopian tubes to reach the egg. The sperm must penetrate a thick layer around the egg; this penetration is aided by enzymes contained in the sperm's head. Only one sperm can fuse with the egg nucleus. Fertilisation enables male and female genetic material to be shared.
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