Thought you'd like to see this since so many have asked how to tell if their eggs are fertilized. The first picture is an INfertile egg. The next two are fertile eggs at slightly different stages, but both freshly laid. Usually, you look for the bullseye/donut appearance, but there are different stages of development of even freshly laid eggs and the two last pictures demonstrate that. This is caused by cells in the center of the blastoderm dying off and leaving a cleared out area, making that bullseye appearance.
From this web page: http://www.hatcheryoffice.com/embryology/determfert.htm