EDELMANN / SCIENCE PHOTO LIBRARY EDELMANN / SCIENCE PHOTO LIBRARY
False-colour transmission electron micrograph (TEM) of an oocyte (a female sex cell) following its penetration by a single human spermatozoon during fertilisation. The sperm has entered the oocyte and its head begins to expand and de- condense. The 23 male chromosomes that were packed into the arrow-shaped head start to relax prior to being united with their counterparts in the female pronuclei. The oocyte now contains two pronuclei, each of 23 chromosomes. Magnification: x1000 at 35mm size.
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