The allantois develops early on in chick embryonic development, as an elongated, sausage-shaped (this is what “allantois” means) pouch which grows out of the bottom part of the chick’s developing gut. But instead of being contained inside the body of the embryo, the allantois protrudes out, and fuses with another membrane which envelopes the chick and its yolk, called the chorion. This fused “chorioallantoic” membrane has a network of blood vessels within it. Lying up against the inner surface of the porous eggshell, this is where gas exchange takes place. Oxygen diffuses through the egg shell, then through the walls of capillaries in the chorioallantoic membrane into the blood, while carbon dioxide passes out in the other direction. The blood in the membrane is on the move – the pumping of the chick’s heart pushes blood around its body – but also out to through the allantois, to the chorioallantoic membrane, and back.
Carbon dioxide isn’t the only waste product that the chick needs to get rid of – and this is where the inside of the hollow allantois comes in: it’s used as a bag to store nitrogen-containing waste. When the chick hatches, it leaves its chorioallantoic membrane behind, still attached to the inside of its eggshell. Don’t bother looking for traces of the allantois in your unfertilised breakfast egg though – it only forms as part of a developing embryo. If there’s no chick inside the egg, there’s no allantois.