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Animated coloured scanning electron micrograph (SEM) Chicken eggshell inner membrane (Gallus gallus domesticus). Shown here is the eggshell inner membrane that is composed of protein fibres (internal and attached to the hard eggshell). Several membranes are found around the embryo of a bird egg. The inner eggshell membrane adheres to the inner surface of the 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. Eggshells are the outer covering of hard-shelled eggs and of other eggs with soft outer coats. Eggshell structure varies widely among species. Birds and some reptiles produce hard-shelled eggs. Bird eggshells are composed of a protein matrix lined with mineral crystals, usually calcium carbonate (CaCO3). Most of the eggshell is composed of calcium carbonate crystals (approximately 95%).