Today is day 9 for the incubator. I just pulled a few eggs out of the incubator to see how they are doing. I took a flashlight and held it to the egg to see if a chick was growing inside. There are some eggs that I can see have started to develop into chicks. The shells on these eggs are not very easy to see into because they have a dark shell. When you do this it is called candling the eggs. A long time ago farmers would put the eggs next to candles to see if they could see anything. My flashlight works much better.
Day 9 - The feather buds should be growing around the tail. This is where the chicken will have feathers. It is also growing their inner eyelid and an egg tooth. The egg tooth is used to break the shell open when they are ready to come out of the egg.
The egg tooth photo is from: https://www.tes.com/lessons/dG4eKA64ZtGxEA/hatching-chick-eggs