The first membrane (of three, see Amniote) around an amniote embryo, counting from the embryo to the outside world, is the amnion. This membrane surrounds the embryo and creates a fluid-filled cavity in which the embryo develops.

This membrane is generally seen as one of the most important factors in allowing animals to colonize dry land, as this membrane allowed animals to become completely free from their dependence of water. Without this membrane, eggs would soon dry out on land, forcing animals to return to water to reproduce (basically what amphibians do)

The Amnion is thought to have developed about 310 million years ago, although fossil records are incomplete and difficult to read. This is because the first Amniote eggs had a leathery shell instead of the calcified birds eggs we know now. But from other characteristics that we speculate must have developed at the same time, a pretty good estimate can be made.

It has been suggested that the original function of the unique extra-embryonic membranes of the amniotic egg was to facilitate easy interactions between the mother and the embryo. This hypothesis however would only have a distinct advantage if the eggs are held within the mother for a long period. This is not seen in the fossile records or in most proposed phylogenies. We are therefore left with the original idea that the original function of the amnion is to reduce the loss of water from the egg.

In some groups the function and structure of the amnion has since been extensively modified. Mammals (except marsupials) for example, have elaborated the amniotic membranes to enable nutrients and waste to pass directly between mother and embryo. Here the advantage of retaining the embryo is used fully. Waste is efficiently removed and nutrients are supplied for a longer time, so that when the baby is born, it is more fully developed, and therefore has a higher survival chance.