DEVELOPMENT OF THE DIGESTIVE TUBE 



607 



GILL APERTURE 



RESPIRATORY 

 TUBE 



Fig. 283. Partial eversion of the oral cavity during development in the embryo of 

 Petromyzon. (Left) Longitudinal section of the head region in 19-day embryo. (Redrawn 

 and modified from Kingsley, 1912, Comparative Anatomy of Vertebrates, Blakiston, 

 Phila.) (Right) Median longitudinal section of head region of adult Petromyzon. (Redrawn 

 and modified from Neal and Rand, 1936, Comparative Anatomy, Blakiston, Phila.) 



fused to the surface of the jaw are referred to either as acrodont or pleurodont 



teeth. If the tooth is ankylosed to the upper edge of the jaw, as in many 

 teleosts and snakes, it falls within the acrodont group (fig. 287B), but if it is 

 attached to the inner surface of the jaw's edge, as in the frog and Necturus, 

 it is of the pleurodont variety (fig. 287A). 



In most vertebrates, all the teeth of the dentition are similar and thus form 

 a homodont dentition. In some teleosts, some reptiles, and in most mammals, 

 the teeth composing the dentition are specialized in various areas. Such 

 localized groups of specialized teeth within the dentition assume different 

 shapes to suit specific functions. Consequently, the conical, canine teeth are 

 for tearing; the incisor teeth are for biting or cutting; and the flat-surfaced, 

 lophodont and bunodont teeth are for grinding and crushing. A dentition 

 composed of teeth of heterogeneous morphology is a heterodont dentition. 



b) Development of Teeth in the Shark Embryo. The development 

 of teeth in the shark embryo is identical with that of the placoid scale previ- 

 ously described. However, the teeth of the shark are larger and more durably 

 constructed than the placoid scale and they are developed from a dental lamina 

 of epithelial cells which grows downward along the inner aspect of the jaw. 

 From this epithelium, a continuous series of teeth is developed as indicated 

 in figure 287E and F. Within the oral cavity and pharyngeal area, ordinary 

 placoid scales are found. Teeth are continuously replaced throughout life in 

 the shark from the dental lamina. The word polyphyodont is applied to a con- 

 dition where teeth are replaced continuously. 



c) Development of Teeth in the Frog Tadpole. The mouth of the 

 frog tadpole possesses prominent upper and lower lips (fig. 287H). Inside 

 these lips are rows of horny epidermal teeth. Three or four rows are inside 

 the upper lip, and four rows are found inside the lower lip. These horny 

 teeth represent cornifications of epidermal cells. They are sloughed off and 



