Cock egg

A cock egg, in archaic English, is a yolkless egg.^[1] Since they contain no yolk and therefore cannot hatch, these eggs were traditionally believed to be laid by roosters.^[1] This gave rise to the myth that when a cock's egg was hatched, it would produce a cockatrice, a fearsome serpent which could kill with its evil stare. According to the superstition, this could be prevented by throwing the egg over the family dwelling so it smashed at the other side without touching the roof.^[1]

Reality

Eggs without yolk are called "dwarf" or "wind" eggs.^[2] Such an egg is most often a pullet's first effort, produced before her laying mechanism is fully ready. In a mature hen, a wind egg is unlikely, but can occur if a bit of reproductive tissue breaks away, stimulating the egg-producing glands to treat it as a yolk and wrap it in albumen, membranes and a shell as it travels through the egg tube. This has occurred if, instead of a yolk, the egg contains a small particle of grayish tissue. This type of egg occurs in many varieties of fowl, including chickens (both standard and bantams), guineafowl and Japanese (Coturnix) quail.

References