Looking at the egg from the outside we see the shell, which is a hard, protective covering composed primarily of calcium carbonate. The shell is porous and the large end contains more pores than the small end of the egg. (There are about 7,000 pores in a chicken eggshell.) This permits the transfer of gases through the shell. Carbon dioxide and moisture are given off through the pores and are replaced by atmospheric gases, including oxygen.
Immediately beneath the shell are two membranes, the outer and inner shell membranes. These membranes protect the contents of the egg from bacterial invasion and prevent too rapid evaporation of liquid from the egg.
Because the body temperature of a hen is approximately 106°F, eggs are very warm at the time they are laid. The temperature of the air is usually much lower than 106°F, and the egg cools to the temperature of its surroundings. As cooling takes place, the contents of the egg contract more than does the shell of the egg. This creates a vacuum and air is normally drawn through the pores in the large end of the shell.
As a result, an air cell forms at the large end of the egg. The air cell serves as a tiny shock absorber during early embryonic development, and on the 20th day of incubation the chick pokes its beak through the shell membranes into the air cell (which by this time has enlarged greatly) and draws its first breaths of air from this space.
While the embryo is growing, the shell membranes surround and contain the white or albumen of the egg. The albumen provides the liquid medium in which the embryo develops, but it also contains a large amount of the protein necessary for proper development.
In a fresh egg, one can see white cords attached to the yolk sac. These two cords, called chalazae, are made of twisted strands of mucin fibers that are a special form of protein. The chalazae hold the yolk in the center of the egg.
The yolk contains large amounts of carbohydrates, fat, and protein. The egg white (albumen) is almost pure, high-quality protein. The yolk is also a reservoir of the vitamins and minerals that are essential for normal growth. These substances combine with the oxygen taken in through the pores of the shell and provide an abundant source of metabolic energy for the embryo. By-products of this process are carbon dioxide and water. The embryo to replace moisture lost through evaporation uses water. Carbon dioxide is transpired through the pores of the shell. Calcium absorbed from the yolk and shell are used by the embryo to make its bony structure, or skeleton.
