PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY
Sperm fertilising egg. Coloured scanning electron micrograph (SEM) of two sperm (orange) attempting to fertilise a human egg (blue). The rounded head of each sperm is seen and their long tail. Women usually produce one large egg (ovum) for fertili- sation, whereas men release some 300 million much smaller sperm (spermatozoa) which travel through the uterus to reach the egg. A few hundred sperm survive this journey and encounter the egg in the fallopian tube. However, only one sperm succeeds in penetrating the egg's wall and fertilises the egg by fusing with its nucleus. When this occurs, the egg membrane forms a barrier to other sperm. Magnification: unknown.
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