As we draw ever closer to the 1st egg hatching many may wonder what's happening inside the egg itself.
The interior of the eggshell is lined with membranes that together form what is called the chorioallantois. The many blood vessels of the chorioallantois pass oxygen through the outer shell and to the developing embryo (and expel carbon dioxide). Metabolic wastes are also transported to this membrane. Since being laid the egg has been slowly but steadily losing water through the pores of the shell, as a result the egg actually weighs less now then when it was laid. The shell is somewhat thinner now as some of the calcium in the eggshell has been absorbed by the embryo and used to help build its developing bones.
After over 35 days now the embryo's development is largely complete and we'll expect to see the first egg hatch over the next few days. The eaglet will reposition itself inside the egg bringing its bill close to the air space that occupies the end of the egg. The embryo is absorbing what remains of the yolk into its abdomen and will begin to absorb and remaining fluid in the egg as well.
The embryo pierces the air space with its bill and breathes with its lungs for the first time. Now the real work of hatching begins. The hatchling will use two specialized structures in this demanding process: the egg tooth a small, sharp structure on the upper beak of the young bird(this will fall off shortly after hatching) and the complexus muscle (also called the pipping muscle). This muscle is greatly enlarged prior to hatching and is used to brace the head as the egg tooth is rasped repeatedly across the shell. Eventually the hatchling breaks through or pips and a small hole will be seen at the end of egg. The hatchling will continue to break through the shell until it has created an opening large enough to emerge through. This entire process - from the early repositioning to final emergence can take days.
The parents can hear and feel the activity inside the egg and may respond to it - especially at the later stages of the process. Once the hatchling has begun to breathe with its lungs it can also begin to make soft calls which the parents can hear. Once the hatching process begins we may notcie that the adult's behavior changes - they will spend less time incubating and will watch the egg closely.