These are from a hidden brood I didn't know a hen was incubating. Mallard eggs with possibly Peking father
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I sorry the pics are bad I know, my phone finds it too difficult to focus when there's so much contrast between the light and dark ( go figure I have an iphone6 too, you'd think it'd be able to) but the embryo (dark spot) it's sort of "diagonal" so in the first picture I show where it's higher and in the second pic the lower part.
Looking at metzer farms pics I assumed that it was day 20-21 as well
I'll see if I can take a better pic today
On another note, I when I candled the egg twice, I didn't see any obvious movement, I hope that doesn't mean it's dead.
Most cameras are going to have trouble with this unless you have manual exposure control and a tripod. No Obvious movement doesn't mean death, but keep candling, if you keep not seeing movement and no changes in the sizes of black blob or veining would point towards late death.
Air cells increase in size due to decreased humidity. The egg may have been in low humidity conditions but still warm enough to develop.
I do think this is what happened. Her best is very far away from any water and we have very dry air here in So Cal. I've been spraying the eggs down now
Hopefully that helps if they aren't all too far dried out by now