The exterior of an egg refers to the hard, protective covering made of calcium carbonate called the shell. The shell is porous, with about 7,000 pores in a chicken eggshell, which 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, which protect the contents of the egg from bacteria and prevent moisture from leaving the egg too quickly.