Mortality in last three days - yoke not absorbed

[Laurelm]

I have been hatching a lot of chicks lately but from time to time they die in the last 3 days. When I open up the egg I can clearly see that the fully formed chick did not absorb the yoke.

What would cause this to happen?

Thank you.

 

[arherp]

What variety? Lethal genes could be a factor..