Candling blue/green Runner duck eggs. The first two days, nothing. The third day, a few blood vessels, maybe, perhaps. The fourth day - a beating heart! Wow! The duckling is nothing more than a red speck at the center of several blood vessels. But in that speck is a beating heart! It is humbling and terrifying (in a good way) to hold a beating heart in an egg in my hand. Literally, a beating heart, because that's the only recognizable part of the duckling.
I made the video below, by candling an egg from my duck Cinnamon.
By the way, this "heartbeat" phase lasts only a few hours, up to one day. After that, the duckling grows, so the heart disappears inside of it.