Eggshell. Coloured scanning electron micrograph (SEM) of cracked shell from a hen's egg. Fragments of shell membrane can be seen adhering to the shell in the lower part of the picture. Eggshells are mainly composed of calcium carbonate. As the foetus develops within the egg it absorbs calcium from the shell and builds it into its skeleton. This changes the eggshell allowing it to permit the passage of carbon dioxide and oxygen and the release of water vapour via the shell membranes. The function of the shell is to protect the developing chick from trauma and dessication. Magnification: x120 at 6x7cm size. x296 at 8x6,x157 at 10x7cm master size"