Dead chick

The Exchequer Leghorn eggs under the Australorp only have 4 days left before they are due to hatch.
Today I just happened to be in the shed when the hen left the nest to eat and I noticed there were only 8 eggs, but also a fully formed dead chick. A quick check seemed to confirm it was the egg with the ruptured air sac that was missing, as none of the others had the cross I had marked in pencil on the shell. If this is the case, it is not as devastating as I first thought. Very rarely do chicks go full term and hatch successfully with damage to the air sac. Also from the markings it was possibly a cockerel. Sad all the same and I am a little concerned how the hen will react when the others start to hatch, hopefully she won't kill them. She is an untried broody and these are my much wanted chicks for a new laying flock.