The correct answer is gases oxygen and carbon dioxide, which diffuse freely across the shell of an egg.
The four specialized membranes of the amniotic egg are amnion, chorion, yolk sac, and allantois. The chorion and the allantois exchange gases between the embryo and the air. Oxygen and carbon dioxide diffuse freely across the shell.
An eggshell refers to the outer covering of a hard-shelled egg and of some kinds of eggs with soft outer coats. The shells of the amniotic eggs are calcareous, formed of calcium carbonate.