Yeah, it really depends so much on where the crack is located and at which point in incubation the crack occurred. Eggs with cracks at the outset have never incubated for me. However, eggs I have inadvertently dropped while candling - if I can get athletic tape on them asap - have all hatched just fine. Now, that said, none of those had perforated air cells - always the cracks were lower. I think if the air cell goes, the chick won't be able to hatch.