K.H. KJELDSEN / SCIENCE PHOTO LIBRARY K.H. KJELDSEN / SCIENCE PHOTO LIBRARY
Fertilisation. Coloured Scanning Electron Micro- graph (SEM) of sperm clustered around a human egg (ovum) during fertilisation. The rounded egg (at centre) is seen on human tissue. Sperm attached to its surface appear as fine hair-like structures; each sperm has a rounded head and a long tail. They are penetrating the thick spongy surface of the zona pellucida of the egg, a surface layer that attracts sperm to the egg and enables the sperm to attach. The human female usually produces a single large egg, and only one of the millions of male sperm may penetrate the egg's wall to fuse with the egg nucleus. Once fertilised, the egg begins its process of growth by cell division.
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