Attached to the shell are two membranes, the inner and outer shell membranes. They protect the egg from bacterial invasion and help prevent rapid evaporation of moisture from the egg. An air cell forms between the inner and outer membranes. This happens because the egg is at the hen's body temperature when it is laid. As the egg cools to room temperature, its contents shrink slightly.
Air is drawn through the pores and forms an air cell at the larger end of the egg.
During storage or incubation, the air cell increases in size. Close to the time of hatch, the chick will push its head through the inner membrane into the air cell to draw its first breath of air. The albumen contains much of the water and protein that the developing chick will need. The yolk supplies most of the energy that is required. In addition, most of the vitamins and minerals are present in the yolk.