Coloured scanning electron micrograph (SEM) Chicken eggshell cross section (Gallus gallus domesticus). Shown from left to right in the image. hard mineralized eggshell and inner fibrous membrane. The hard mineralized eggshell contains calcium carbonate (CaCO3) mineral crystals that form many tiny pores in the eggshell. The eggshell inner fibrous membrane is composed of protein fibres (internal and attached to the hard eggshell). Several membranes are found around the embryo of a bird egg. The internal eggshell membrane adheres to the inner surface of the hard eggshell. As the chick develops changes in the eggshell allow the passage of carbon dioxide and oxygen and the release of water vapour by way of the eggshell membranes. Magnification: x48 when shortest axis printed at 25 millimetres.