egg tooth

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A hard toothlike projection on the beak of an embryonic bird or the snout of an embryonic reptile that is used to break through the eggshell upon hatching and that later falls off or is absorbed.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun zoology A small, sharp, cranial protuberance used by offspring to break or tear through the egg's surface during hatching.