Incubation and Embryology - University of Illinois

Structure of the Egg

The egg is a biological structure intended by nature for reproduction. It protects and provides a complete diet for the developing embryo, and serves as the principal source of food for the first few days of the chick's life. The egg is also one of the most nutritious and versatile of human foods.

When the egg is freshly laid, the shell is completely filled. The air cell is formed by contraction of the contents during cooling and by the loss of moisture. A high-quality egg has only a small air cell.

The yolk is well-centered in the albumen and is surrounded by the vitelline membrane, which is colorless. The germinal disc, where fertilization takes place, is attached to the yolk. On opposite sides of the yolk are two, twisted, whitish cord-like objects known as chalazae. Their function is to support the yolk in the center of the albumen. Chalazae may vary in size and density, but do not affect either cooking performance or nutritional value.

A large portion of the albumen is thick. Surrounding the albumen are two shell membranes and the shell itself. The shell contains several thousand pores that permit the egg to "breathe."


An average-sized egg weighs approximately 57 grams (about 2 ounces). Of this weight, the shell constitutes 11 percent; the white, 58 percent; and the yolk, 31 percent. Normally, these proportions do not vary appreciably for small or large eggs. The percentage composition of the edible portions is:

Water Protein Fat Ash
Whole egg 74 13 11 1
White 88 11 .. ..
Yolk 48 17 33 1

Essential nutrients

Eggs are especially valuable as a source of protein. In fact, egg protein is used as the standard against which the quality of other food proteins is measured. One egg contains about 6 to 7 grams of protein. People of all ages need adequate protein for building and repairing body tissues.

The fat in the yolk is so finely emulsified that it is digested easily, even by infants. The ratio of unsaturated to saturated fats is about 2 to 1. This is considered very desirable. Oleic acid is the main unsaturated fat. It has no effect on blood cholesterol. Eggs contain vitamin A, the B vitamins (thiamin, riboflavin, and niacin), and vitamin D. All are necessary during childhood and adolescence for growth. Eggs also contain an abundant supply of minerals, such as iron and phosphorus, that are essential for building and maintaining strong, healthy bodies. But eggs are low in calcium (it is in the shell), and contain little or no vitamin C.

Individuals on weight-reducing programs find eggs beneficial. To lose weight, calorie intake must be reduced, while maintaining a well-balanced diet. An egg provides good nutrition and contains only about 80 calories.

Value of eggs

Food prices continue to climb, particularly for high-protein foods, and consumers are constantly searching for ways to reduce their food bill. One way is to include more eggs in the diet. Comparing protein foods on a pound-for-pound basis, eggs cost about 95 cents a pound when large eggs are selling for 64 cents a dozen. It is difficult to purchase any other high-protein food--meat or fish--for this low price.

Source: H.S. Johnson and S. F. Ridlen

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