How do baby birds and reptiles breathe inside their eggs?

The eggs of birds and reptiles are one of the greatest inventions in packaging! The egg contains the albumen and the egg yolk. Within the egg yolk is the fertilised egg cell or embryo which slowly develops into the baby bird or reptile. As it develops, it feeds on the albumen and the yolk. The egg shell is made so well that it holds the liquids so that the inside doesn't dry up. At the same time the egg shell is porous enough to allow the gases to pass in and out. That is how the baby inside breathes till it is old enough to break out of it's shell.