If the egg starts to dry out, you can gently add a little bit of warm Pedialyte to it with a needle-less syringe.
I had one egg pip early last season, and it started to dry out. I added a little Pedialyte to it once a day, and the baby came out in perfect condition after 12 days.
You could also use distilled water, just as long as you keep it capped to prevent anything from contaminating the water.
Gale