The air sac develops at the fat end of the egg. After a week or so you are able to see it. If there isn't one the egg is not fertile. I label all my eggs before they go in, A, B, C or 1,2,3 ect on one side and an x on the other. I have a note pad and write the number/letters in a line going down the papper. When I candle them I write down on a peice of of paper good vein, no, can't see yet. This way I only check the ones I can't tell yet or are no good. I throw out on day 10 unless I can't see in them because they are too dark. I Hand turn them 3x a day. I don't have a turner :-(