EDELMANN / SCIENCE PHOTO LIBRARY EDELMANN / SCIENCE PHOTO LIBRARY
False-colour transmission electron micrograph (TEM) of an egg (ovum) during fertilisation, showing a human spermatozoon penetrating the zona pellucida, the thick membrane that surrounds the oocyte, the female sex cell. The dart-shaped head (dark purple) contains the 23 chromosomes of the male sex cell. Following the successful penetration of the oocyte by a single sperm, rapid chemical changes in the membrane surrounding the oocyte cause it to thicken and prevent entry of competing sperm. Magnification: x1000 at 35mm size.
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