Looking at the egg from the outside we see the shell, which is a hard, protective covering made of calcium carbonate. The shell is porous. (There are about 7,000 pores in a chicken eggshell.) This allows 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 bacteria and prevent moisture from leaving the egg too quickly..
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 drawn through the pores of the egg.
As a result, an air cell forms at the large end of the egg. 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, and it also contains a large amount of the protein necessary for proper development.
In a fresh egg, we 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 is the source of food for the embryo and contains all the fat in the egg. The small white spot on the yolk is call the germinal disc. The germinal disc is where the female's genetic material is found.