I’m not sure what part of the egg you were talking about, sorry. I thought you meant the shell. In my 35+ years of chicken keeping, I have not been able to recognize a fertilized egg by candling until after at least 3-4 days of incubation, and more reliably after a week.I know that’s why you are to candle them to see the inside!!i have exactly 84 Chickens!I have had Chickens in my life going on 10 years!I think i know what i am doing.
To the OP, Mrs. K has good advice. Check for a bullseye and keep track. Go to the education section of the forum - there are some good, clear pictures of what a fertile egg looks like, vs. an infertile egg.