An eggshell is the outer covering of a hard-shelled egg and of some forms of eggs with soft outer coats.
The generalized eggshell structure, which varies widely among species, is a protein matrix lined with mineral crystals, usually of a calcium compound such as calcium carbonate. It is calcium build-up and is not made of cells. Harder eggs are more mineralized than softer eggs.
Birds are known for their hard-shelled eggs. The eggshell is approximately 95% calcium carbonate crystals, which are stabilized by a protein matrix. Without the protein, the crystal structure would be too brittle to keep its form. The standard bird eggshell is a porous structure, covered on its outer surface with a cuticle (called the bloom if it is around a chicken egg), which helps the egg retain its water and keep out bacteria. As an egg develops within a bird's reproductive system, the shell is laid down as a liquid mineral layer around the already-formed extraembryonic membranes. This takes place in a part of the bird's oviduct which is called the uterus and serves a very different purpose from the uterus of a mammal. The shell is laid down by shell glands. In an average laying hen, the process of shell formation takes around 20 hours. Pigmentation is added to the shell by papillae lining the oviduct, coloring it any of a variety of colors and patterns depending on species. Since eggs are usually laid blunt end first, that end is subjected to most pressure during its passage and consequently shows the most color. The waxy cuticle is deposited on the shell in the bird's oviduct. The chick has an egg-tooth which it uses to start a hole in the hard eggshell to allow it to hatch.
An overlooked source of calcium is eggshell, which can be ground into a powder and mixed into food or a glass of water.
For more information: http://en.wikipedia.org/wiki/Eggshell
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