Okay Sally, I bucked up and opened one of the eggs, after looking at the last chicks to hatch. They are extremely small even for d'Uccles. Here is a picture, if you can tell, my air cells all looked like the ones you showed on day 18. I'm wondering if I didn't have some sort of bacteria. The liquid in the egg is thick like syrup and wet, if that makes sense. It poured out thick, it had no smell and the chick internally pipped.
Incubator Hovabator 1583 forced air
I used one of these and one of these
Here is the egg
edit to make pic bigger