CNRI / SCIENCE PHOTO LIBRARY CNRI / SCIENCE PHOTO LIBRARY
False-colour transmission electron micrograph (TEM) of a single human ovum (egg) surrounded by sperm. The egg has been coloured orange, and the numerous sperm appear rougly spherical in this section, coloured blue/green. In fertilisation, only one sperm succeeds in penetrating through the egg's outer membrane to fuse with the female nuclei. As soon as one sperm has broken through, rapid chemical changes cause the outer membrane of the egg to thicken, so obstructing the entry of other sperm. Magnification: x700 at 35mm size.
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