Amnion, one of the foetal membranes, which, like the allantois (q.v.), is met with in the embryos of reptiles, birds, and mammals; these three groups of vertebrate animals being sometimes classed together as Amniota, as distinguished from fish and amphibia, in which no amnion is developed. The structure is formed by the growth of two folds, which arch over the embryo and finally unite in such a way that they constitute a double membrane enclosing it. The outer of these membranes is known as the false, and the inner as the true amnion, while between the latter and the embryo is left a space known as the amniotic cavity, which is filled by the amniotic fluid. The amniotic fluid is of low specific gravity and contains a small amount of albumen (q.v.) and of urea (q.v.). Sometimes rupture of the membranes does not occur in the ordinary way, and they are borne down in front of the child's head, and this constitutes what is known as a caul, around which phenomenon a perfect fabric of superstition has been woven by the imaginative.