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Day 1: The Journey Begins The Shell 
The egg shell is added last. (Why is this a good idea?) The shell is composed mostly of calcite, a crystalline form of calcium carbonate. The calcium needed to make a shell comes from the hen. During egg-laying periods, the hen stores calcium from her diet as extra bone in the marrow cavities of her long bones. This extra bone is referred to as medullary bone and occurs in female birds when they are actively laying eggs. Long bones include the femur (upper leg) and humerus (upper wing). When the shell is ready to be added to an egg, calcium is reabsorbed from the medullary bone into the hen's bloodstream and transported to a shell gland in the oviduct to be secreted as shell.
Text and graphic by Janet
Sinn-Hanlon
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