CLOUDS HILL IMAGING LTD / SCIENCE PHOTO LIBRARY CLOUDS HILL IMAGING LTD / SCIENCE PHOTO LIBRARY
Fertilisation. Light microscope footage of numerous sperm cells (spermatozoa) attempting to fertilise a human egg (ovum). Each sperm has a rounded head and a tail which it uses to both swim and burrow into the egg. Women usually release one egg per month, whereas men release millions of sperm in each ejaculation. Only one of these sperm will penetrate the egg's thick outer layer (zona pellucida) and fertilise it. Fertilisation occurs when the genetic material (deoxyribonucleic acid, DNA) in the sperm's head fuses with the egg's DNA to form a zygote. The egg then forms a barrier to other sperm. Magnification unknown.
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