D. PHILLIPS / SCIENCE PHOTO LIBRARY D. PHILLIPS / SCIENCE PHOTO LIBRARY
Fertilisation. Coloured Scanning Electron Micro- graph (SEM) of sperm clustered on the surface of a human egg (ovum) during fertilisation. The surface of the egg is blue at lower frame. Sperm (brown) attached to it are hair-like structures; each sperm has a head and a long tail. Here, head first they are penetrating this thick surface of the egg that attracts sperm and enables them to attach. The human female usually produces a single large egg, and only one of the millions of male sperm released may penetrate the egg to fuse with the egg nucleus. Once fertilised, the egg begins its process of growth into a human embryo. Magnification: x2,000 at 5x7cm size.
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