Why do some eggs have no air sac?

Being a natural product, eggs are subject to natural variations. Generally speaking the air sac starts small and grows larger as the egg ages and looses moisture. Sometimes an egg may appear not to have an air sac at all. This may be because it's a particularly fresh egg, or simply had not developed an air sac yet. Either way, just enjoy your eggs, there's absolutely nothing to be concerned about.

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