I actually have a pretty amazing story which involves an embryo producing it's own heat.
It's a long story, but here's the quick version. I found an egg on the ground in the coop. It was somewhat near a broody, but not very. I took it inside and set it on the kitchen table. I forgot about it and we went camping for the weekend. It sat on the table at room temperature for 3 days and 2 nights. I found it when we got back and it crossed my mind that maybe I should candle it. I decided not to, because even if there was an embryo, it had to be dead by now, right? Nope. I took the egg outside, and with the first crack, blood started pouring. I instantly felt horrible, but it was bad enough that I knew there was nothing that I could do. I continued opening the egg and there was an 17 or 18 day old embryo. The baby looked up at me and tried to peep before passing away in my hand. It was horribly sad.
So it's a very sad story, but the amazing part is that he survived 3 days at room temperature by producing his own body heat.