558 



ZOOLOGY 



SECT. 



Teeth, as already explained in the general account of the 

 Craniata (p. 86), are developed partly from the epidermis and 

 partly from the underlying dermis. In the Mammals each tooth 



is lodged in a socket or 

 alveolus 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 dental lamina 

 (Fig. 11 82, /am,). The posi- 

 tion 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. 

 The bud becomes con- 

 stricted off as a conical cap 

 of cells — the enmnel-organ 

 — which remains in con- 

 tinuity with the dental 

 ridge only by a narrow 

 isthmus. This cap-like 

 form is brought about by 

 the development of a 

 papilla of condensed dermal 

 tissue — the dental papilla 

 {pap), which pushes up- 

 wards against the enamel- 

 organ. On the surface of 

 this papilla, in contact with 

 the enamel-organ, the cells 

 (odontohlasts) become ar- 

 ranged into a layer having 

 the appearance 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-menibrane {en. m.) ; 

 the more superficial layer consists of cubical cells. Between the 



Fig. 1181. — Diagrammatic sections of various forms of 

 teeth. I, incisor or tusk of Elephant with pulp- 

 cavity persistently open at Ixise ; 11, human incisor 

 during development, with root imijerfectly formed, 

 and i)ulp-cavity widely open at base ; 111, 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 use, 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 

 Lydekkcr.) 



