The yolk sac is an organ containing a supply of nutrients. It is used to feed the fish larva at the beginning of its independent life and to breathe. The blood vessels formed in the walls of the yolk sac ensure the embryo's respiration and delivery of nutrients that cause the larvae to grow until they move to external food. Depending on the size of the yolk sac, the duration of feeding by its contents varies.

The yolk sac (Saccus vitellinus) is the embryonic or larval organ of most vertebrate groups. In a typical case, the yolk sac is an outgrowth of the intestine, inside which there is a supply of yolk that is used by the embryo or larva for nutrition. The original evolutionary role of the yolk sac is to digest the yolk and assimilate the digestive products and transport them to the bloodstream of the embryo (the yolk sac has a branched network of blood capillaries).