Pores in chicken eggs

What shape are the pores in chicken eggs? The pores allow for exchange of air in to the sac to allow the chicken to breath while it is developing in the shell. the pores also allow moisture in to keep up fluids so that the chicken does not dry out on development stage. Why do they pores do this and how does the pore prevent any infection from gaining entry. Is it due to the shape of the pore? What size diameter is the poor on the inner part of the shell and on the outer part of the shell? Could any one answer these questions for me please?