Broken egg shell edge. Coloured scanning electron micrograph (SEM) of the edge of a fragment of an egg shell. Hard-shelled eggs are produced by birds and some reptiles. The shell protects the embryo that develops inside, performing vital functions such as controlling the passage of air in and out of the egg through tiny pores. Thousands of these pores cover the egg shell surface. The thickness of the shell varies with its size, as normally a larger egg stretches the same amount of shell over a larger area, producing a thinner shell. Most of the egg shell material is calcium carbonate, and accounts for about a tenth of the egg's weight. Magnification unknown.