A layer of glycoproteins and other stuff which surrounds an egg (ovum), just outside of the egg's cell membrane. The coat protects the egg, and for organisms which practice external fertilization (fertilization outside of the parents' bodies) it can help prevent the sperm of other species from entering the egg. For the eggs of mammals, the egg coat is called the zona pellucida and for sea urchins (a popular study organism for developmental biologists) it is called the vitelline layer.