What is the yoke sack?

Yoke sac is the structure in which the developing embryo of a reptile, bird or mammal is enclosed, and from which it receives nourishment.

In reptiles and birds the yolk sac also contains a supply of yolk which supplies the embryo with nutrients.

In mammals, the amnion is not lined with continuous endothelium, but instead contains mesodermal cells derived from the allantois.