Adam Cole of the NPR series Skunk Bear explained through very clever animation, how birds and other oviparous animals get in the necessary oxygen and expel carbon dioxide within the hard shell of an egg in order to allow the burgeoning embryo inside to thrive and eventually hatch and draw breath on its own.
A few days after an egg is laid …when you hold a fertilized egg up in front of a bright light you can see it a delicate network of blood vessels that grows out of the embryos abdomen and presses up against a membrane just inside the shell oxygen from the air comes in through the tiny holes in the shell and then diffuse into the embryos blood and the growing chick gets rid of carbon dioxide at the same time
