Yolk

Part of an egg which feeds the developing embryo

Among animals which produce eggs, the yolk (also known as the vitellus) is the nutrient-bearing portion of the egg, the primary function of which is to supply food for the development of the embryo. Some types of egg contain no yolk, for example because they are laid in situations where the food supply is sufficient (such as in the body of the host of a parasitoid) or because the embryo develops in the parent's body, which supplies the food, usually through a placenta. Reproductive systems in which the mother's body supplies the embryo directly are said to be matrotrophic; those in which the embryo is supplied by yolk are said to be lecithotrophic.

From Wikipedia, under CC BY-SA. More on occurri.