Yesterday I took a picture of an egg (Green Singer) that was incubated for 3 days. I knew that it is possible to see if an egg is fertile or not before the recommended 5 days. Sometimes it is possible to see after 2 days of incubation, but this is my first picture at such a stage. I can't be sure that it will develop. The Air Cell is located on the pointed end of the egg. I assumed that it was supposed to be on the round end. But I could be wrong.
For comparison I put the collage with my earlier pictures. What I know for sure:

1- Freshly laid egg. (All freshly laid eggs look alike, with small Air Cells on the round sides of the eggs);
2- Old infertile egg. (It was incubated for about two weeks. Air Cell is much bigger, because of the moisture lost);
3- Egg after three days of incubation (my latest picture);
4- I removed this egg after being incubated for two weeks. It came from fertile pair. Originally I thought

5- This was fertile egg after 5 days of incubation. I got a Green Singer from it.
6- This egg with a late embryo death. The chick was almost developed when my hen had stopped sitting on the eggs.