CHRISTIAN DARKIN / SCIENCE PHOTO LIBRARY CHRISTIAN DARKIN / SCIENCE PHOTO LIBRARY
Fertilization. Image 1 of 2. Computer artwork of a sperm cell (centre left) about to penetrate an egg cell (round). Each cell contains one half of the genetic material needed to form a new human. The sperm is from a male and the egg from a female. Only one of some 300 million sperm released during ejaculation will fertilise the single egg. Once it has penetrated, rapid chemical changes make the egg's outer layer (zona pellucida) thicken, which prevents any other sperm entering the egg. The head of the successful sperm releases its genetic material to mix with the egg's genetic material. For a sequence showing a sperm cell penetrating an egg cell, see images P648/178-179.
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