egg tooth

noun
a calcareous prominence at the tip of the beak or upper jaw of an embryonic bird or reptile, used to break through the eggshell at hatching.

Origin:
1890–95

Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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Collins
World English Dictionary
egg tooth
 
n
(in embryo birds and reptiles) a temporary tooth or (in birds) projection of the beak used for piercing the eggshell

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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American Heritage
Science Dictionary
egg tooth  
A hard, toothlike projection from the beak of embryonic birds, or from the upper jaw of embryonic reptiles, that is used to cut the egg membrane and shell upon hatching and that later falls off.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

egg tooth

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.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Chicks lack an egg tooth at hatching and kick their way out of the egg.
Chicks lose their egg tooth within a few days of hatching.
They have a nonfunctional egg tooth which suggests that they may have evolved
  from egg layers.
Newly hatched diamondback terrapin with egg sac and egg tooth.
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