The pocket of air found at the larger end of the egg. It is easily observed at the flattened end of a peeled, hard-boiled egg. When laid, the egg is quite warm. As it cools, the contents contract and the air cell is formed as the inner shell membrane separates from the outer membrane. The air cell increases in size as the egg ages and air enters, replacing the original moisture and carbon dioxide that has leaked through the pores of the shell as time passes. The size of the air cell is one means used for determining the grade of an egg.