An egg yolk is a part of an egg which feeds the developing embryo. The egg yolk is suspended in the egg white (known more formally as albumen or ovalbumin) by one or two spiral bands of tissue called the chalazae. Prior to fertilization, the yolk together with the germinal disc is a single cell; one of the few single cells that can be seen by the naked eye.
As a food, yolks are a major source of vitamins and minerals. They contain all of the egg’s fat and cholesterol, and almost half of the protein.
If left intact while cooking fried eggs, the yellow yolk surrounded by a flat blob of whites creates the distinctive sunny-side up form of the food. Mixing the two components together before frying results in the pale yellow form found in omelettes and scrambled eggs.