how long can a chick live in an egg??

I had a chick hatch during the night and went out to check on it this morning I notice that it looks like feathers are starting to come in on it wings! It took my other two chicks days to start getting feathers in! I had a batch of eggs due last week and this week and I was just wondering if an chick could live longer than 21 days in an egg! This chick is a pretty good size, as big as my 1 week 2 day old chick and is getting feathers but not a many as my 6 day old chick! Blew me away when I took it out of the nesting box! Anyone have a chick hatch with the start of wing feathers....know if they can survive longer than their 21 days? I'll try to get Momma Moe to let me take pics of her chicks wings!