Yolk sac
Definition: A sac attached to a developing embryo that holds yolk, a store of fats and proteins that nourishes the embryo — and, in many fish, the newly hatched larva until it can find food on its own.
In simple words: A built-in packed lunch that feeds a baby animal before it can feed itself.
More about yolk sac
Animals that lay eggs, such as birds, reptiles, amphibians and fish, depend on the yolk while they develop. In mammals a yolk sac also forms early in development, although the placenta soon takes over as the main source of nourishment.