On a chicken I'd say find the bad smell & remove immediately. If the entire mass of embryo is floating in the shell rather than moving or twitching, it's gone. You can also try tapping the shell to see if you get a response. I would get the bad one out so you don't jeopardize the rest of the...
They were external. They didn't make it. I'm so sad about it since they all looked so vigorous at lockdown. I have 5 more due this coming Thursday & 2 (a fertility test) due the next Thursday. Just trying to keep my chin up to cope with it.
Sometimes you just don't know. I had one that looked good on day 14 dead in shell at lockdown. Sometimes they are malpositioned, don't absorb all their yolk or just aren't healthy enough to make it. It's sad, and I'm having to toughen up my soft heart.