from Heaven & Earth: Unseen By the Naked Eye: "The structure of an eggshell is revealed as a tangled network of mineralized fibres in this scanning electron micrograph at a magnification of 500 times. Eggshell forms around a mat of proteins, which is coated and overgrown by calcium carbonate and other mineral salts. The result is a tough, waterproof package that still allows gas exchange between the inside and the outside, enabling the developing embryo to 'breath', while providing astonishing mechanical strength."