'520 



ZOOLOGY 



.SECT. 



Teeth, as already explained in the general account of the 

 Craniata (p. 80), are developed in the epidermis and partly 

 ;from the underlying dermis. In the Mammals each tooth is 



lodged in a socket or al- 

 HI A veolus in the jaw. The 



part of the tooth developed 

 from the epidermis is the 

 enamel ; the remainder of 

 the tooth dentine, cement 

 and pulp being formed 

 from the subjacent meso- 

 dermal tissue. 



Along the oral surface of 

 the jaw is formed a ridge- 

 like ingrowth of the ecto- 

 derm the dented lamina 

 (Fig. 1113, lam.). The 

 position of this is indicated 

 externally by a groove- 

 the dental groove (gr.). 

 From this a bud is given off 

 in the position to be occu- 

 pied by each of the teeth. 

 This becomes constricted 

 off as a conical cap of cells 

 -the enamel organ which 

 remains in continuity with 

 the dental ridge only by a 

 narrow isthmus. This cap- 

 like form is brought about 

 by the development of a 

 papilla of condensed der- 

 mal tissue, the dental 

 papilla (pap. ), which 

 pushes upwards against 

 the enamel organ. On the 

 surface of this papilla, in 

 contact with the enamel or- 

 gan, the cells (odontoUasts) 

 become arranged into a 

 layer having the appear- 

 ance of an epithelium the 

 dentine forming layer. The 



cells of the enamel organ form two layers, of which that in contact 

 with the dental papilla assumes the character of a layer of long 

 cylindrical cells the enamel membrane (en. m.). The more super- 

 ficial layer consists of cubical cells. Between the two the remaining 



.Flo. 1112. Diagrammatic sections of various forms of 

 teeth. I, incisor or tusk of Elephant with pulp 

 cavity persistently open at base ; II, human incisor 

 during development, with root imperfectly formed, 

 and pulp-cavity widely open at base ; III, completely 

 formed human incisor, with pulp cavity opening by 

 a contracted aperture at base of root ; IV, human 

 molar with broad crown and two roots ; V, molar of 

 the Ox, with the enamel covering the crown deeply 

 folded, and the depressions filled up with cement ; 

 the surface is worn by vise, otherwise the enamel 

 coating would be continuous at the top of the ridges. 

 In all the figures the enamel is black, the pulp 

 white ; the dentine represented by horizontal lines 

 and the cement by dots. (After Flower and 

 Lydekker.) 



