The "embryo" is already on the yolk when the egg is laid. It takes 24 - 36 hours of incubating temps to bring the cell formation division to what we think of as an 'embryo'. If you have a super bright light you can sometimes see veins at 4 days but it is bst to wait a full 7 days. Using a good light you will see veins, the dark eye spot and some swirling and swishing movement in the egg indidcating life.