I've had chicks hatch on day 25. The hatch can take 3 days to happen. If it starts on day 21, then it can easily continue until day 24 and at least once for me, day 25. So I don't remove any until day 25 is over.
The easiest way to know if an egg was fertilized - on Day 3 you candle the egg in darkness - if it isn't completely clear - it's fertile and there's something developing. If you can see through it and there's no darkness at all - it wasn't fertilized or it stopped being fertile.
The way to know if it's developing - look at the egg closely and even if it's a dark egg you should be able to see veins and movement near the air sac - if you see that - there's development. If you don't see any veins around the air sac - then it's no longer developing. You can also search for how big the chick is supposed to be on what Day in growth - and go by that if you want.
Here's my plan as I look by candling - any cracked or smelly - I toss
Day 3 - should see a dot with veins or at least not all clear
Day 7 - should see a larger dark mass with veins
Day 14 - should see mostly just a large dark mass and veins
Day 18 - should be able to not see inside egg anymore as this has a mostly developed chick - lay it down and leave it alone to hatch.
Day 19/21/22 - chicks should hatch
Day 24 - remove any eggs unhatched
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