Hens lay eggs whether they are fertilized or not. The egg then passes through the oviduct small end first, but is laid large end first ouch! When an egg is laid, it totally fills the shell but as it cools, the inner portion of the egg contracts and forms an air cell between the two shell membranes. The pigment of the shells happens in the last hours of formation. It takes about 20 hours for the egg shell to form so while one egg is being laid there is another right behind it getting ready. The shell gets added in the uterus and is composed mainly of calcium carbonate.
