From Egg To Chick:

Formation and Parts of the Egg

Incubation is not the only project a student may undertake with eggs, and in the remainder of this circular we shall look into some of the more complicated and technical projects that are of interest. Let us start by paying some close attention to the egg itself.

The avian egg is one of nature's finest foods. The yolk, white, and shell of the egg contain all the proteins, fats, carbohydrates, minerals, and vitamins necessary to nourish the embryo during the 21-day incubation period.

Since the embryo actually begins with the formation of the egg within the hen's body, some understanding of the formation of the egg will lead to a better comprehension of embryonic development. The major reproductive organs of the hen are the ovary and the oviduct (Figs. 5 and 6).

Position of the ovary in a hen. (From Egg Grading Manual, USDA Agricultural Handbook No. 75, July, 1961.) (Fig. 5)	A hen's reproductive system. Illustration Copyright © 1998, Janet Hanlon and the University of Illinois. Not the same illustration as appears in the original text, but close and more colorful. (Fig. 6)

The females of most animals have two functional ovaries, but the hen has only one. The right ovary stops developing when the female chick is hatched, but the left one continues to develop. The left ovary will contain up to about 4,000 tiny ova from which the full-sized yolks will develop when the hen matures.

Each yolk or ovum is contained in a thin membranous follicle. Blood vessels in the follicle carry nutrients to the developing yolks. When a yolk matures, the follicle ruptures along a line relatively free from blood vessels known as the stigma, and the yolk is released. The yolk is kept intact by the vitelline membrane surrounding it.

Upon release from the follicle, the yolk drops into the body cavity. There the infundibulum, or funnel, engulfs the yolk and starts it on its way down the oviduct. The oviduct is more than two feet long and is lined with glands which secrete the materials for the albumen, shell membranes, and shell. Twenty-four hours or more are required from the time the yolk is released until the completed egg is laid.

The fertilized egg is a highly complex reproductive cell and is potentially a small center of new life (Fig. 7). The germinal disc from which new life develops is attached to the yolk of the egg. Surrounding and protecting the germ cell and yolk is the white or albumen consisting of several layers.

The albumen is somewhat elastic, and it is a shock-absorbing, semi-solid material with a high water content. The albumen and the yolk of the egg serve as food for the growing embryo during the incubation period. On opposite sides of the yolk are two twisted, whitish cord-like objects known as chalazae. They anchor the yolk and keep it from rising and touching the shell. Nature provided them also to serve as a rotating axis to keep the germ cell on the top side of the yolk and, therefore, next to the heat of the hen's body. So, the yolk and albumen work together to protect and sustain the life of the growing embryo.

Around the albumen and yolk are two shell membranes and the shell itself. These give protection to both the yolk and albumen, and the shell provides for an exchange of gases and serves as a method of conserving the food and water supply encased within the shell for the benefit of the developing embryo.

The parts of an egg. Illustration Copyright © 1998, Janet Hanlon and the University of Illinois. Not the same illustration as appears in the original text, but close and more colorful. (Fig. 7)

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