Day 1: The Journey Begins

The Yolk

The formation of the egg starts in a hen's ovary. (Watch for more on the formation of an egg on Day 5.) 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. (Watch for more on egg nutrition on Day 8.) There are two kinds of yolk: Yellow yolk is produced during the day and contains more fat. White yolk is produced at night and contains more protein. Most of the white yolk is located directly below the nucleus (the position of the possible future embryo) in the latebra and the nucleus of Pander. 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.

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