Imported post
<font color='#0000FF'>The shell is added in the uterus or shell gland portion of the oviduct. The shell is composed mainly of calcium carbonate. It takes about 20 hours for the egg shell to form. If the hen lays brown eggs, the brown pigments are added to the shell in the last hours of shell formation.
In the last portion of the oviduct, the vagina, a thin, protein coating called "bloom" is applied to the shell to keep harmful bacteria or dust from entering the egg shell pores. The egg passes through the oviduct small end first, but is laid large end first. In the vagina, the egg is turned horizontally just before laying. If the hen is disturbed on the nest, the egg may be prematurely layed small end first. "Oviposition" is the act of pushing the egg from the oviduct.
When an egg is laid, it fills the shell. As it cools, the inner portion of the egg contracts and forms an air cell between the two shell membranes. A high quality egg has a tiny air cell, indicating the egg was collected soon after being layed and was stored properly. The air cell is usually located in the large end of the egg where the shell is most porous and air can enter easily.
There, my three years of biological study came to use.