A hen will lay an egg a day until she masses a dozen or so. Once she goes broody, she'll spend nearly 24/7 sitting on the eggs. The first egg laid might be 12 or more days old before she starts incubating them. Regardless, that 12 day old egg along with the last egg she laid will all start developing at the same time and 21 days latter, you will have chicks.
So...there's no development of a chick (i.e. the 12 day old egg) until the hen starts to brood the eggs. Fertilized eggs will not have a developing chick in them unless you've removed them from under a broody hen.
So...there's no development of a chick (i.e. the 12 day old egg) until the hen starts to brood the eggs. Fertilized eggs will not have a developing chick in them unless you've removed them from under a broody hen.