Duck egg questions....

I ordered some fertile duck eggs which I am happy to report are doing great in the incubator. I know when they should hatch and when I candle them I see lots of blood vessels and their little hearts beating.
We have a pond behind the restaurant I work at that people have dropped off ducks at. Lots of pekins, some muscovy, and then we also have lots of mallards since people feed them all the time. It irks me....the mallards don't migrate because they're getting fat on bread.
Anyway, there was a broody Pekin sitting on a clutch of eggs. Then one day she wasn't there. No idea what happened to her or if she just quit sitting on her eggs..... So I took some home. They are alive. I can see movement and they are getting pretty big.
Now for the question:
Can anyone direct me to a really good visual resource so I can try to gauge how old these embryos are? I've looked all over the internet and I just can't find a good one. I'm trying to get an estimate for when these "wild" duck eggs are going to hatch.
I'm new to this incubating thing...lol. I just want to know at roughly however many days, the egg should look like this when candled.

Thanks!

 

Yes that's perfect. What's the source though? It's pretty small and I can't seem to make it bigger. Maybe if I go to the webpage? Or could u email it to me at [email protected]?