The formation of the egg starts in a hen's ovary. What appears to be the yolk of an egg is actually a single cell also known as the ovum. The cell membrane of the ovum is surrounded by a non-cellular vitelline membrane produced by the ovum. The ovum stores nutrients in its cytoplasm as yolk granules for use by the embryo if the ovum is fertilized.
White yolk is produced at night and contains more protein.
But as egg laying time grows closer,yolk is produced continually and begins to accumulate rapidly. For the last seven or eight days before the ovum is ovulated, yellow and white yolk are laid down in rings like those of a tree. When the ovum has accumulated enough yolk to grow a chick, it is released from the ovary.