I found this page that describes the progression of a developing embryo in the shell http://msucares.com/poultry/reproductions/poultry_chicks_embryo.html I found it interesting and thought others might as well. I'm sure the time line isn't written in stone - the chicks under my broody started hatching at day 19 and I've talked to some that didn't have eggs hatching until day 23, but I found the page really interesting none the less. The time line is closer to the bottom of the page.
Penny
Penny