eli5: How can a bird embryo breathe?

Question

In mammals, the embryo is supplied with oxygen via the placenta and the umbilical cord.
But how do the embryos of birds or reptiles get oxygen?

In: Biology

Answer 1

The embryo only begins to breathe a few days before hatching. There’s a tiny air bubble inside the egg that is pierced by a tiny calcium horn in the embryo to begin breathing. Within days, the embryo has grown up enough to break the outer shell and hatch.

If it prematurely breaks the inner bubble, it can literally drown and fail to hatch.