PROFESSOR P.M. MOTTA ET AL / SCIENCE PHOTO LIBRARY PROFESSOR P.M. MOTTA ET AL / SCIENCE PHOTO LIBRARY
Fertilisation. Coloured scanning electron micrograph (SEM) of a human egg (ovum) being penetrated by sperm during fertilisation. Sperm are coloured green, each with a rounded head and a long tail. They are seen on the thick spongy surface of the zona pellucida which surrounds the egg. The zona pellucida attracts sperm to the egg and enables the sperm to attach. The human female usually produces a single large egg, and only one male sperm can penetrate the egg's wall to fuse with the egg nucleus. When this happens, the egg membrane forms a barrier to any other sperm. Magnification: x1000 at 6x7cm size.
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