However, an alternative etymology references an ancient Greek goddess of childbirth, Eileithyia, worshipped in Amnisos (on the island of Crete) and nicknamed Ἀμνιάς (Amnias).

When first formed, the amnion is in contact with the body of the embryo, but about the fourth or fifth week amniotic fluid (also called liquor amnii) begins to accumulate within it.

This fluid increases in quantity and causes the amnion to expand and ultimately to adhere to the chorion's inner surface, so that the extra-embryonic part of the coelom is obliterated.

It contains less than two percent solids, consisting of urea and other extractives, inorganic salts, a small amount of protein, and frequently a trace of sugar.

That some of the liquor amnii is swallowed by the fetus is proved by the fact that epidermal debris and hairs have been found among the contents of the fetal alimentary canal.

[4] In reptiles, birds, and many mammals the amnion develops in the following manner: At the point of constriction where the primitive digestive tube of the embryo joins the yolk sac a reflection or folding upward of the somatopleure takes place.