What is the role of the yolk in a chicken egg?

The yolk is the egg’s nutrient material used as food by developing embryonic animals. Eggs with a relatively small, evenly distributed yolk are called isocytal. This condition occurs in invertebrates and in all but lower mammals. Eggs with abundant yolk concentrated in one hemisphere of the egg are called telolecital. This occurs in many invertebrates and is lower in all vertebrates than marsupial mammals. In arthropods, the yolk is massaged near the center of the egg; such eggs are called centrolecytic.