When I candled mine at day 3 all I could see were the yolks and air sacks. Yesterday was day five, and when I candled again I could see VERY clear development in most of my eggs.
A few weeks ago, one of my Orpingtons went broody and I let her sit on a few eggs, and then one of my Australorps went broody and they've been taking turns on the eggs. Today I went and one of the eggs was half-hatched but looked like it had been squashed, and it was too late to do anything...