612 



THE DIGESTIVE SYSTEM 



EPIDERMAL 



DENTAL LEDGE 



LABIAL PAPILLAE 



Fig. 287. Tooth development and arrangement in various vertebrates. (A-D) Tooth 

 relationships with the jaw. (Redrawn and modified from Rand, 1950, The Chordates, 

 Blakiston, Phila. After Wilder.) (E) Dental ledge and developing teeth in the dog shark, 

 Acanthias. (Redrawn and modified from Rand, 1950, The Chordates, Blakiston, Phila. 

 After Kingsley. ) (F) Section of the shark's lower jaw indicating a continuous replace- 

 ment of teeth, i.e., a polyphyodont condition. (Redrawn and modified from Rand, 1950, 

 The Chordates, Blakiston, Phila.) (G) Incisor tooth of rodent. (Redrawn and modified 

 from Rand, 1950, The Chordates, Blakiston, Phila. After Zittel.) (H) Horny teeth of 

 12 mm. frog tadpole. (I) Rudimentary dental lamina in upper jaw of chick. (Redrawn 

 from Lillie, 1930, The Development of the Chick, Holt & Co., N. Y.) (J) Anterior 

 portion of upper jaw of 18-day chick showing egg tooth. 



the mesenchyme surrounding the developing tooth. Around the base, sides, 

 and crown of the tooth, this mesenchyme condenses and forms the outer and 

 inner layers of the dental sac (fig. 286C). The latter is a connective-tissue 

 sac which surrounds the entire tooth, continuing around the outside of the 

 outer enamel cells of the enamel organ. As the dentine and enamel are de- 



