tooth or toothlike structure used by the young of many egg-laying species to break the shell of the egg and so escape from it at hatching. Some lizards and snakes develop a true tooth that projects outside the row of other teeth, helps the young to hatch, and then is shed. Turtles, crocodilians, and birds have an analogous horny structure that performs a similar function. The only mammal to hatch from an egg, the duck-billed platypus, also develops an egg tooth before birth.