550 COMPARATIVE ANATOMY. 



The true teeth are the product of the mucous membrane of the 

 mouth, which is formed of connective tissue as well as of epithelium. 

 In the Selachii their structure and mode of development is exactly 

 the same as that of the dermal denticles, with which also they 

 have many external points of resemblance ; as, therefore, the matrix 

 of the two is continuous, and as in many Selachii these integu- 

 mentary scales are distributed over other portions of the wall of the 

 buccal cavity, we may conclude that the teeth and scales were 

 primitively identical. The teeth, which are developed on the 

 edges of the jaws are, therefore, however much they 

 are differentiated, nothing more than large structures 

 of the same kind as those which are found in the 

 integument. The change in them, as compared with 

 these integumentary scales, is clearly due to adapta- 

 tion to new functions, while their first appearance was 

 contemporaneous with the differentiation of the primi- 

 tive mandibular arch. The presence of these structures 

 in the primitive buccal cavity is explicable from tho 

 fact that it was formed by an invagination from the 

 exterior. 



All teeth are developed in essentially the same way ; this has 

 been already described above (p. 423), when we were speaking of tho 

 dermal denticles of the Selachii. The dental papilla, formed of 

 connective tissue, develops the dentine from an epithelial-like 

 superficial layer (odontoblasts) ; and on this an epithelial layer 

 deposits the enamel. When the teeth are developed on the surface, 

 these layers are continuous with those of the surrounding mucous 

 membrane. When the rudiment of the tooth is sunk into the 

 mucous membrane, an epithelial thickening (enamel ridge) is 

 developed, which grows into it ; the portion of it which covers the 

 dental papilla is separated off, and forms the enamel organ. The 

 cement, or bony layer, is added to these two substances, and forms a 

 third layer. 



We have described above how the presence of teeth in the buccal 

 cavity, and their deposition on the cartilaginous skeleton of its wall 

 leads to the formation of bone (§ 342). These bones are derived 

 from dentigerous plates, and therefore each of them may carry 

 teeth. In the Ganoidei and Teleostei, for instance, there are teeth 

 on the palatines, vomer, and parasphenoid, in addition to the maxillary 

 bones ; and, also, on the hyoid and branchial arches. It is generally 

 the last of the branchial arches reduced to a simple plate, which 

 is distinguished by the possession of teeth (pharyngeal teeth ; 

 Fig. 256, VI). Teeth are much more common on the upper pieces 

 of the branchial arches. 



In the Amphibia there are still teeth on the palatine and vomer; 

 more rarely on the parasphenoid; among the Reptilia, the Ophidii and 

 Saurii alone have teeth on the palatine and pterygoid ; while in the 

 Crocodilini, as in the Mammalia, the teeth are confined to the 

 maxillary bones. 



