How birds get oxygen inside their eggs. File this great explainer under "questions previously unconsidered that have interesting answers."
As Adam Cole explains, birds have something like an umbilical cord in humans, and even before a chick develops lungs, this splits into a network of delicate veins that fan out across a membrane. An eggshell has microscopic pores that allow oxygen to come in contact with the membrane, where the chick's blood vessels can absorb it. After 21 days, the chick pecks a hole in the shell and gets a big first breath of fresh air into its lungs.