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You might not realize it while you’re frying them sunny side up on a Sunday morning, but chicken eggs (and those of other birds, and reptiles) are some pretty sophisticated pieces of packaging.
Hard-shelled bird eggs contain albumen, or “egg white,” and a yolk. The fertilized egg cell, or embryo, develops within the yolk and feeds off of it and the white. The baby bird has shelter, food, almost everything it needs inside the egg; except, it seems, a little fresh air.
Animals that develop inside their mothers, like mammals, get their oxygen from mom through the umbilical cord. A bird in an egg doesn’t have as obvious a way to take in oxygen and get rid of carbon dioxide, but the egg, again, takes care of things.
Directly under the egg's shell are two membranes. When the eggs are laid by the mother they’re very warm, and as they cool the material inside the egg shrinks a little bit. The two membranes pull apart a little and create a small pocket or sack of air. As the developing bird grows, it breathes in oxygen from the air sack and exhales carbon dioxide. Several thousand microscopic pores all over the surface of the egg allow the CO2 to escape and fresh air to get in.
These pores also allow moisture to get into the egg to keep developing bird and the egg parts from drying out, which is why hard-boiled eggs always feel a little heavier than raw ones.