Read this...>I learn ALOT from this...especially the "carbon dioxide" part.... just thought I'd pass it on to those that needed it... Like me.. I got it from a yahoo answers post..... The Hatching Egg As the embryo develops, the egg loses water by transpiration through the chorioallentoic membrane. This loss of water, as well as the loss of yolk fats used by the embryo, cause the egg to be lighter at hatching. The egg shell is thinner at hatch because the chick has absorbed most of the calcium from the inner shell lining. Baby birds have two unique structures that aid their hatching. A temporary 'egg tooth' develops on the dorsal tip of the upper beak that enables the chick to 'pip' and cut its way out of the shell. This structure is lost several weeks post-hatch. An enlargement in a muscle in the back of the neck - the complexus or pipping muscle - helps the chick force the egg tooth through the shell. After hatching, most of the fluid in this muscle is absorbed and the muscle continues to function as an extender of the neck. Just before hatching begins, the air cell expands to approximately 30% of the internal volume of the egg. As hatching begins, the beak of the embryo penetrates the inner shell membrane where it forms the inner wall of the air cell. The lungs begin to function, inhaling the air from the air cell. This is known as 'internal pipping'. The trigger for internal pipping is an increase in carbon dioxide in the blood. The 'external pip' occurs when the chick cuts the outer shell membrane and shell. The egg tooth is used to help the baby cut its way out of the shell. Increasing carbon dioxide in the air chamber triggers contractions of the hatching muscle which assists the baby's efforts. It usually takes 24 hours from internal pip to completed hatch in most parrot species.