DR YORGOS NIKAS / SCIENCE PHOTO LIBRARY DR YORGOS NIKAS / SCIENCE PHOTO LIBRARY
Egg fertilisation. Coloured scanning electron micrograph (SEM) of sperm penetrating the layers around a human egg during fertilisation. Each sperm (green) has a head and a long tail. Here, many sperm are penetrating cells of the corona radiata (brown), an outer layer of cells which surrounds the healthy egg. Beneath the corona radiata is the zona pellucida (pink), a thick inner spongy layer around the egg. The egg itself is not seen here. The sperm must pass through both of these layers before they can reach the egg. Only one sperm can penetrate the egg, after which the egg begins to divide into an embryo. Magnification: x780 at 5x7cm size.
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