Dead Duckling in egg?

[ChickenPerson]

This is my first time hatching ducks.They started to lay in September. How do I know if one of the eggs are dead? Its day 12. Two had clear blood vessels and an embryo. The other one I could see the embryo and some blood vessels and it was blurry and clear towards the air cell. This egg is dirtier too. Some pics show its good others don't. If y'all could send right pics that would be great.Also their mother is a rouen or a mallard and the father is a Swedish.I do know mallards take quicker.

 

[oldhenlikesdogs]

I believe if you do a Google search you can find egg pictures in various stages of incubation.