The egg is protected by very thin shell limestone, but very hard; this allows breathing since it allows the oxygen to pass through the tiny pores of its surface.

 

In addition to the porous shell, the egg contains two membranes (outer and inner) that also influence the development of the chick . See Exercise 1 [PDF].

 

During incubation, the air chamber located on the wider end of the egg is created as a result of separation of the two membranes.