artsyrobin -
Marking the size of the air cell is a way of checking for correct weight loss and so you can adjust your humidity as needed by visual inspection of air cells. If the incubation humidity is too low (very dry conditions), the air sac will be larger than normal and the humidity in the incubator should be increased to reduce the rate of water loss. If the air space is smaller than normal then the opposite applies.
ok, thanks! that is interesting - gonna have to do some reading on that!