The air sack is on the big end of the egg. The pointy bottom
end goes down into to turner. You can use a pencil to mark the air cell development and it shows you how the chick is progressing.
It doesn't work so well on the super dark Marans eggs though. I had no idea what was going on in most of them the time I hatched them.

It doesn't work so well on the super dark Marans eggs though. I had no idea what was going on in most of them the time I hatched them.