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Sandhill Crane (Grus canadensis) chick in the process of hatching. The egg tooth, a small, sharp, cranial protuberance used to break or tear through the egg's surface during hatching is visible.
Chicks have a pipping muscle on the back of their necks. It gives them the strength to force the egg tooth through the inner membrane of the eggshell.
When a chick becomes too large to absorb oxygen through the pores of its eggshell, it uses its egg tooth to peck a hole in the air...