Please understand that this is an experiment and I strongly suggest that you set your eggs and follow the advice that others give about candling. This is just an experiment, so I don't really care if they hatch or not, though the chicks will be kind of cute if they do hatch . Anyway, I think it's important to remember that excessive handling can be detrimental to you hatch percentages,
Not that I'm suggesting you do this, but I remember in one of my biology classes in college (embryology probably) we opened up eggs that had been incubated for a certain number of hours, 24, 48, or 72, to see the developmental stages. If I remember correctly we could see the heart already beating on the 72 hr egg. It was really fascinating but really sad at the same time, since we witnessed it's heart beat slow to a stop. I'm just glad we didn't have to do that for anything older than 72hrs...