DENTITION 



249 



dermal spines form a continuous series with those which invest 

 the jaws and eventually become teeth (Fig. 151). It is only 

 later, when lips become apparent, that the continuity of the 

 teeth and dermal spines is interrupted, and the two structures 

 assume their distinc- 

 tive characters. 



The tissues of which 

 the teeth of Fishes are 

 composed are (1) den- 

 tine, which is a non- 

 vascular, calcined 

 tissue, traversed by 

 numerous radiating, 

 branched, dentinal 

 tubuli, into which ex- 

 tend protoplasmic pro- 

 longations from the 

 cells (scleroblasts) by 

 which the dentine is 

 secreted. Dentine 

 forms the greater part 

 of the body of a tooth. 



(2) vasodentine and 



(3) osteodentine are 

 modifications of ordi- 

 nary dentine, the 

 former containing 

 blood - vessels ramify- 

 ing in its substance 



but no dentinal tubules, and the latter more closely resembling 

 bone. (4) enamel, an exceptionally dense, non -vascular, non- 

 tubular tissue, which may or may not exhibit traces of the 

 prismatic structure so characteristic of this tissue in the higher 

 Vertebrates, forms the outer investment of the teeth. 



As regards their fixation, the more primitive forms of teeth, 

 such as those of Elasrnobranchs, are simply embedded in the 

 gums, and are only connected with the jaws by fibrous tissue ; 

 but in some of the older fossil Sharks the fixation of the teeth 

 is effected by the mutual articulation of the basal plates of the 

 teeth with one another. The Chondrostean Polyodon, so shark- 



FIG. 151. Transverse section through the lower jaw of 

 an embryo Scyllium, to show the gradual transition 

 from dermal spines (d, d, d) on the outer surface of 

 the jaw to teeth (t, t, t) on the oral surface, c, Car- 

 tilage of the lower jaw. (From Gegenbaur.) 



