An Introduction to Zoology: Chapter 6

The Amnion:

The amnion is a more or less waterproof membrane that surrounds the developing embryo and prevents it from drying out. A layer of fluid known as the amniotic fluid fills the space between the amnion and the embryo. Not only does the amniotic fluid prevent the embryo from dying of dehydration, it also helps to cushion and protect the embryo.

In placental mammals, uterine contractions during labor cause the amnion to rupture, releasing the amniotic fluid. When this happens in a human, we say that her “water breaks.”