Thanks for all the responses!! I'm glad the experiment may be helpful to others. I found it fascinating, and I'm especially looking forward to Cherlyn's experience with it.
jenjscott--I think walkswithdog explained it pretty clearly. But here's a pic:
		
		
	
	
Those concentric circles are the air cell at Day 7, 14, 21, and 25 (One egg is crushed--an accident--the duckling hatched out just fine). The windows in my incubator are in the top, so I placed the eggs in an egg carton with the largest portion of the air cell facing the window closest to the front of the incubator. 
Here's another view, where you can see them more or less all facing the same direction:
Does that help? Also in that picture, if you look very close, you can see a few pips on the middle left of the air cell. The one with a hole has actually already begun to zip--but the original pip was all the way at the left (I don't know if you can tell that from the picture).
Cherlyn--I'm going to go look for your new thread. I'm anxious to hear how your experiment turns out. 
For anyone else who wants to try, it's not necessary to track the air cell at each candling--just once, right before lockdown, would be sufficient. I tracked them at each candling because I was curious how it would develop--and it was interesting each time to see how it had grown.
Edited to replace repeat photo with the one I meant to post.