The dark coating on the Marans shell prevents normal evaporation of liquid. I dry hatch all my eggs. If you crack open the eggs that don't hatch, you will probaly see liquid moving around. When the fluid isn't evaporated enough, the chick will pip into the air cell & drown. I dry hatch all my eggs, & get much better hatch success in all the breeds.
Suje
I'll try that this next hatch and see how it goes. I did set them up in cartons this time but they still did not do well. Do you not add any water at all? I hate to admit to it but I did help one chick who seem to fill up the whole egg.