<P> While many reptiles lay eggs with flexible, calcified, eggshells, there are some that lay hard eggs . Eggs laid by snakes generally have leathery shells which often adhere to one another . Depending on the species, turtles and tortoises lay hard or soft eggs . Several species lay eggs which are nearly indistinguishable from bird eggs . </P> <P> The bird egg is a fertilized gamete (or, in the case of some birds, such as chickens, possibly unfertilized) located on the yolk surface and surrounded by albumen, or egg white . The albumen in turn is surrounded by two shell membranes (inner and outer membranes) and then the eggshell . The chicken eggshell is 95 - 97% calcium carbonate crystals, which are stabilized by a protein matrix . Without the protein, the crystal structure would be too brittle to keep its form and the organic matrix is thought to have a role in deposition of calcium during the mineralization process . The structure and composition of the avian eggshell serves to protect the egg against damage and microbial contamination, prevention of desiccation, regulation of gas and water exchange for the growing embryo, and provides calcium for embryogenesis . Eggshell formation requires gram amounts of calcium being deposited within hours, which must be supplied via the hen's diet . </P> <P> The fibrous chicken shell membranes are added in the proximal (white) isthmus of the oviduct . In the distal (red) isthmus mammillae or mammillary knobs are deposited on the surface of the outer membrane in a regular array pattern . The mammillae are proteoglycan - rich and are thought to control calcification . In the shell gland (similar to a mammalian uterus), mineralization starts at the mammillae . The shell gland fluid contains very high levels of calcium and hydrogen carbonate . The thick calcified layer of the eggshell forms in columns from the mammillae structures, and is known as the palisade layer . Between these palisade columns are narrow pores that traverse the eggshell and allow gaseous exchange . The cuticle forms the final, outer layer of the eggshell . </P> <P> While the bulk of eggshell is made of calcium carbonate, it is now thought that the protein matrix has an important role to play in eggshell strength . These proteins affect crystallization, which in turn affects the eggshell structure . Moreover, the concentration of eggshell proteins decreases over the life of the laying hen, as does eggshell strength . </P>

When does the hard calcium shell form around the egg
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