I am incubating 40 chicken eggs in my classroom in a styrofoam still air incubator. Today is day 13 and I noticed that 2 of the eggs' shells are really rough. They were not like that when I put them in or even last week. Does anyone know what is happening? They still had water in the wells when I checked this morning. It is almost like the shell has deteriorated on one end. Thank you for any help!!