The blood-vessels that form inside the shell would be what absorb the oxygen and transfer it to the chick. It's the same sort of thing with reptile eggs, save they tend to have a slightly larger air pocket and, unlike bird eggs, can't be moved from the position they were laid in as it runs a high risk of smothering the air pocket and suffocating the embryo.

There's an old cemetary near town here, where the original settlement was. The dates that are still discernible on the gravestones are between 1880-1899 and there's one in particular that has I think three or four children from the same family engraved on it. Back then it was probably pneumonia or TB that did them in as I don't imagine the winters around here in those days were very forgiving.