Sorry to not have written for the past several days---I have been birdswatching at Malheur National Wildlife Refuge and out of internet access. As hatching approaches, I thought I would provide a bit of information on how this process occurs:
The chicks now fill virtually the entire interior of the eggs that was once occupied by the yoke and the albumen. A large air chamber known as an "air cell" exists at one end of the egg. If hatching goes well, the chick will shift into a "tucking position" where it places its beak between its body and its wing and then extends its beak into the air chamber and inflate its lungs. Prior to this time, the embryo has receive all of its oxygen needs through a network of capillaries that connect to a membrane inside the egg known as the "chlorioallantois" which imports oxygen from and exports carbon dioxide to the outside world.
Once the chick has broken into the air chamber it will begin to peck at the interior of the air chamber working its way slowly around the edge. The first break through to the outside world is known as 'pipping." The process of pecking its way out of the egg can take a day or two. Chicks have a feature on their bill known as an "egg tooth" which allows then to break through the hard shell. They also have a special muscle on their neck known as a ""hatching muscle" that gives them the strength to accomplish this task. This muscle shrinks once the chick leaves the egg.
We should only be days away, perhaps even less, at this point from seeing the first chick emerge. However, predicting egg hatch dates is anything but an exact science.