|MadSci Network: Agricultural Sciences|
Dear Ramiro, The eggshell consists of two membranes, a mineralised shell (which is made up of almost pure calcium carbonate) and a proteinaceous cuticle. The membranes are the slightly leathery layers which you find underneath the hardened shell when you break open an egg. Calcification of the shell begins in the female's reproductive tract in the area called the isthmus. The egg then moves to the shell gland where it spends about 20 hours, this is where most of the mineralised part of the shell is formed. The cuticle is also added here. When the egg is laid, it has every component of its shell however the shell is still reasonably soft. The shell hardens once it has been laid. Shell hardening is not related to fecundity. This information can be found in "Reproduction of Farm Animals" (1974) by Hafez.
Try the links in the MadSci Library for more information on Agricultural Sciences.