Nest found, advice?

My step dad is a farmer. He was plowing a field to day and came upon a mallard nest. He said she flew out of the way and he stopped the tractor with only seconds to spare. He called me knowing I had an incubator. Two options I could come immediately and get the eggs or he was just going to plow them under.(he's not REALLY insensitive it's just he runs into this a lot out here and he can't stop plowing a field every time he comes upon wildlife) Well I went and got them and popped them in my bator. So now I will have staggered hatch. More importantly though how can I tell how far a long these are in incubation. One egg was quite cracked and weeping, I assume from the moms quick take off. I broke it the rest of the way open and there was a little fetus in there perhaps the size of a dime or a bit bigger. I am new to incubating and feel pretty comfy candleing my 'scovie's white eggs but these are light brown and I can't really see anything. Does anyone know of a website or where I might find pics of the inside of eggs at different stages of development. Or can somebody tell me here by the size I described?
Thank you all in advance for any advice.

 

I think we do need a permit here. I wasn't going to keep them though just raise them and let them leave when they like, also I have Rouens perhaps people wouldn't know the difference!? Thanks for the good wishes.