Day ten is a better time to candle as the developing embryo is more visible.
backyardchickens.com
great for all kinds of information with a discussion forum for your questions to be answered by long timers.
So I have 3 RIR eggs going in a mini incubator and I decided to candle them today. When I did, I found 1, the early developer, had a darker spot (the embryo!) and very very faint veins around it. One of the other ones had a small darker spot, but I couldn't see veins because of the yolk, and the other looked like it might be developing, but nothing visible yet.
Does this sound normal for Day 5?
FYI: All of them have air sacs on the large end of the egg, no bubbles. They have been kept at a solid 100 degrees F with 50% humidity and have been turned 4 times a day. All the yolks are movable and not sticking to the sides.
