The yolk of a chicken egg. The yolk is a part of an egg (or just of the egg cell in non-egg-laying animals) that feeds the developing embryo in animals. In whole eggs, 43% of the protein comes from the yolk.Also known as deutoplasm, this food material is accumulated during oogenesis, together with RNA molecules and other substances.he yolk is a part of an egg (or just of the egg cell in non-egg-laying animals) that feeds the developing embryo in animals.