BILL SANDERSON / SCIENCE PHOTO LIBRARY BILL SANDERSON / SCIENCE PHOTO LIBRARY
Sperm cells. Artwork of human sperm cells swimming towards the unseen egg. Sperm cells are the male reproductive cells, which can fertilise an egg cell to produce an embryo. Each cell comprises a head, which contains the genetic material, and a tail, which propels the cell. The tip of the head has a cap called the acrosome, which contains enzymes that allow the sperm to enter the egg. Only one sperm may enter the egg. After one has done this the egg's lining becomes impermeable to other sperm.
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