I found an abandoned egg that I believe is from a mallard duck, and I made an incubator to hatch it at home. It was doing great- I candled it after a week and had vessels, and candled it again a week later and saw a moving embryo with vessel growth. I candled it again today (approximately day 19), and it was mostly dark with a pretty normal air sac, but I couldn't see vessels easily like the previous checks. I also noticed a slight smell today, and now I'm nervous that it's a not-so-good smell. Does any of this sound normal for this stage of development? Or is my egg a goner?