DR EVERETT ANDERSON / SCIENCE PHOTO LIBRARY DR EVERETT ANDERSON / SCIENCE PHOTO LIBRARY
Fertilization. Coloured transmission electron micrograph (TEM) of the fusion of a sea urchin egg (blue) and sperm (red). The arrow-shaped sperm head has penetrated the cell membrane of the egg. Entry is facilitated by the acrosome, a cap at the tip of the sperm head which contains enzymes to digest the cell wall. Eventually, the sperm releases its nucleus which fuses with the nucleus of the egg. Two large mitochondria (circular), sites of cellular respiration, can be seen at the base of the sperm head. These provide the energy required to propel the sperm.
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