Thanks, that's good to know. Some of my girls lay very dark shelled eggs and I can't see anything, but the white eggs are pretty easy to see through!If you look at that link it shows how to tell in 76 hrs or so, or even earlier - I remember seeing the circle that is shown in the 52 hr photos. But yes, checking fertility by breaking the eggs is one way to know if ANY of the eggs are being fertilised before you set them, and pinpoint likely hens to collect from for brooding!