ANATOMY

Detailed explanation-1: -Directly under the shell are two membranes. Between the membranes is a small air cell, also called an air sack, filled with oxygen. As the animal develops it uses the oxygen, which must be replenished, and it also has to release carbon dioxide.

Detailed explanation-2: -At the large end of the egg there is a pocket of air. The air cell is formed because when an egg is first laid it is very warm, and as the egg cools the contents inside contract leaving this empty space or air pocket inside the shell. The air pocket gets larger as the egg ages.

Detailed explanation-3: -All eggs contain an air pocket at the base (the wide, rather than the pointed, end). When this air pocket is heated, it expands. If it is heated too quickly, it more like explodes, cracking the shell.

Detailed explanation-4: -Over the course of incubation the air cell will grow larger as water evaporates from the egg and escapes through thousands of microscopic pores in the shell. These pores also facilitate the diffusion of oxygen and CO2 in and out of the egg.

Detailed explanation-5: -This is called pipping. On the 21st day, the chick begins to break out of the shell. Using its egg tooth, it first pecks a hole through the shell. Then it pecks a circle around the end of the egg.