That is a very interesting question. I would assume like with us, we are born with all the "eggs" we will have in our lifetime. As they mature, hormones and growth of this egg forms the 1/2 genetic path to creating a chick. Due to the genetics of the female, the body utilizes the calcium that it consumes and forms a leathery "container" that when it comes to the air will harden.
Now that isn't very biologically sounding, but as kids when mom and dad would clean out the dispatched birds, you can remove eggs in sizes from the hardening stage, watch it firm up, then find eggs down to pea sized.
Hope this helps and I will keep an eye on this question, I'd love to read someones educated explanation!