Does an air sac mean the egg is fertile and developing?

[Hens_And_Chicks]

We're only on day 3 but I looked at a few of the eggs today and can see an air sac on the end. Does that mean we have a chick starting to develop or do all eggs develop that sac while being incubated?

 

[speckledhen]

No. Aircells will get larger as an egg gets older anyway. You should not candle this early--the embryo is too delicate. Wait till at least a week or ten days, then you'll be able to see something.