Egg. The reproductive cell of a female organism is called an ovum, or egg. When a sperm, the reproductive cell of a male organism, unites with an ovum (a process called fertilization), a cell is formed that develops into a new organism. For a discussion of fertilization, In common usage, the term ``egg'' is also used to refer to a reproductive body that is laid (passed outside the body) by certain kinds of animals. Animals that lay eggs include birds, and most insects, fish, and reptiles. This article is mainly about the egg laid by the domestic chicken. Chicken eggs are an important part of the diet for many people.

A chicken's egg is, on the average, 2 1/4 inches (57 mm) long and 1 3/4 inches (44.5 mm) in diameter through its widest part, and weighs about two ounces (57 g). In comparison, a human ovum is only about 1/200 of an inch (.13 mm) in diameter. The largest eggs produced by animals still in existence are those of the ostrich, which are up to nine inches (23 cm) long.

Parts of An Egg

A bird's egg has five major parts—a shell, shell membrane, albumen, yolk, and germinal spot. The first four parts protect and nourish the embryo, which develops from the germinal spot when the egg is fertilized.

The outer layer, or shell, makes up about 11 per cent of the egg's total weight and is composed mainly of calcium carbonate. The next layer is the double-walled shell membrane, which consists of a thick-walled outer membrane and a thin inner membrane. The two membranes separate at the blunt end of the egg, forming an air space that allows for the diffusion of oxygen into the egg. The next layer is the albumen, or ``white,'' which is composed of water and albumin, a protein. A pair of twisted cord-like structures, called chalazae, pass through the albumen, anchoring the yolk to the inner membrane at either end.

The yolk is a round, yellow structure that lies in the middle of the albumen. It consists of light and dark bands of proteins. The yolk contains nutrients that keep the young bird alive before it emerges from the egg and for up to 36 hours after it hatches.

Eggs As Food

The edible part of a chicken's egg is approximately 74 per cent water, 12 per cent protein, and 11 per cent fat. The remaining 3 per cent consists of carbohydrates, vitamins, and minerals. One egg provides about 80 calories of energy. The yolk is more nutritious than the white; it contains vitamins A, B6, B12, and D, and riboflavin. However, the yolk also contains a large amount of cholesterol, a substance that can contribute to arteriosclerosis (hardening of the arteries). Egg white contains considerable amounts of protein and vitamin B2. Shell color depends on the breed of hen and has no effect on nutritional value.

Egg substitutesare eggs, processed into liquid or powder form, from which the yolks have been removed. Egg substitutes typically contain no cholesterol.

Commercial Production of Eggs

Eggs are commercially produced by poultry farms and egg farms. The leading egg-producing states are California, Pennsylvania, Indiana, Ohio, Georgia, Arkansas, Texas, Iowa, and North Carolina.

Eggs are prepared for shipment at the farms or in nearby processing centers. They are usually washed in a detergent, and then sorted for size before being packed in cartons or cases. An egg is graded by freshness and size. The grades established by the U.S. Department of Agriculture are AA (the freshest), A, B, and C (lowest edible quality). Sizes range from peewee (1 1/4 ounces [35 g] to jumbo (2 1/2 ounces [70 g]).

The freshness of eggs is determined by a method called candling—placing the egg above a bright light that shines through the shell. If the egg is fresh, it will appear clear—without a spot. If it is rotten, the egg will look clouded. And if the embryo has begun to develop, a well-defined dark spot will show. Only when the egg appears clear is it fit for consumption. Unfertilized eggs are preferred.

Eggs are shipped by rail or truck to major marketing areas. Most eggs are marketed for immediate consumption. However, large quantities are placed in cold storage. Eggs for cold storage are first dipped in a thin, tasteless, and odorless mineral oil. Sealing the pores of the shells in this way helps preserve quality. Eggs may also be broken out of their shells for sale in liquid, frozen, or dried form to bakers, confectioners, and producers of dry cake-mix.