608 



THE DIGESTIVE SYSTEM 



TUBERCULUM 



FORWARD GROWTH 



FROM 

 TUBERUULUM IMPAR 



Fig. 284. Development of the tongue in the chick embryo. 



replaced continuously until the time of metamorphosis when they are dispensed 

 with. The permanent teeth begin to form shortly before metamorphosis from 

 an epithelial ridge (dental lamina) which grows inward into the deeper tissues 

 around the medial portion of the upper jaw. The teeth develop from an 

 enamel organ and dental papilla in a manner similar to that of the developing 

 shark or mammalian tooth. After the young tooth is partially formed, it moves 

 upward toward the jaw, where its development is completed and attachment 

 to the jaw occurs. Teeth are replaced continuously during the life of the frog. 



d) Development of the Egg Tooth in the Chick. Modern birds do 

 not develop teeth. However, an ingrowth of epithelium does occur which 

 suggests a rudimentary condition of the dental lamina of the shark, amphibian, 

 and mammahan embryo (fig. 2871). It is possible that this represents the 

 rudiment of a basic condition for tooth development, one which is never 

 realized, for the sharp edge of the horny beak takes the place of teeth. The 

 egg tooth is a conical prominence, developed upon the upper anterior portion 

 of the upper horny jaw (fig. 287J). It is lost shortly after hatching. It appears 

 to function in breaking the shell at hatching time. 



e) Development of Teeth in Mammals. As the oral cavity in the pig or 



