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

Hens_And_Chicks

Songster
10 Years
Feb 23, 2009
551
1
139
Ohio
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?
 

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