Amnion

Formed as the body wall of embryo of higher vertebrates grows up and envelops the back of the embryo by the union of the folds as they meet. The true or inner amnion forms a bag, filled with amniotic liquid in which the embryo floats. It is popularly known as the "bag of waters." The outer amnion is at first a perfectly spherical shell surrounding the true amnion and the yolk sac. From the latter it ultimately splits entirely away and becomes attached to the eggshell or chorion. That of insects is formed in the same way as in vertebrates, and similarly overlies the neural plate, which becomes the ventral part of the insect and the dorsal part of the vertebrate.