Note 17 — Chapter XVI · THE DEVELOPMENT AND EDUCATION OF THE CHICK.

The amnion originates, early in the second day, as a double fold of the yolk-surface in front of the embryo. Similar folds arise round the sides and tail, forming a low wall (Fig. 187, A); the folds gradually grow over the embryo (Fig. 187, B) until, during the fourth day, they meet (Fig. 187, C) and enclose it in a protective transparent bag, containing a watery fluid.

Fig. 187.—Diagrams to illustrate the method of development of the amnion. The embryo and the rest of the yolk are supposed to be seen in median longitudinal section; the head end is to the right. For convenience, the yolk-membrane (yk.m.) is represented at some distance from its contents. am, folds of amnion; am.c., amniotic cavity; em, embryo; yk, yolk; yk.st., yolk-stalk. (After Foster and Balfour.)