They are called 'blood spots'. I can't remember exactly why/how they get there, but they won't hurt you. It's not the embryo though, I know that much. A fertilized egg will have a white bullseye on the yolk and that's the fertilized egg but if you pick them up everyday, then the chick can't form that much in that much time that you would have a chick. I don't even bother picking them out anymore, but the best way to do it is to take the tip of a butter knife with a small serrated edge- slips right off of spoons and forks tines are too small. If I'm making devilled eggs that are going to be served to other people, then when I dump the yolk out, the blood spot is between the yolk and the white. Picks out easy.