How Is the Egg Formed in the Chicken?
An egg starts in a chicken’s body during ovulation inside the hen’s ovary as a yolk. At this point in development, the yolk is called an oocyte. After the oocyte has formed, the hen’s ovary releases it into the oviduct.
When the yolk has grown as large as that seen in a fresh-laid egg, it passes slowly through a special passage in the chicken’s body. Along the way, the yolk is surrounded by egg white and then by two very thin skins, or membranes.
As the egg nears the end of the egg passage, it is covered with a coating made up mostly of calcium, which hardens into the eggshell. The egg is now ready to be laid. An egg-laying chicken usually lays more than 240 eggs a year, which is often more than eight times her own weight.
The newly laid egg may or may not contain everything it needs to develop into a baby chick, depending on whether or not it has been fertilized. If the egg is fertilized, it can be incubated in a specially designed incubator or cared for by a broody hen until it hatches into a chick.
While the embryo inside the egg is developing, the unhatched chick will use the yolk inside the egg to provide the nutrition the chick needs to grow and develop.