Chullin 64 - The Eggs
If an idolater sells eggs, one can buy them and not be concerned that they come from birds that are nevelah or terefah . But let's be concerned that they come from a non-kosher bird altogether! - We are dealing with the case where he says which kosher bird they come from, and in additional they have the kosher signs. What are these signs? They are spherical and rounded, though not a perfect sphere, with one of its ends rounded and one tapered. Then why can't we rely on these signs alone? - Because there are crow's eggs that look kosher.
If an egg has a spot of blood in it, then if it is only in the place of the embryo, one can discard the blood and eat the egg, but if it is in another place, then one can assume the whole egg started developing, and it is forbidden. Some say that the place of an embryo is in the connection to the yolk, others that it is in the yolk, and still others that it is in the egg white. The custom has thus become to discard an egg with any blood spot.
Art: Willem Roelofs - Still life with eggs