Chap, xxi.i THE TEETH. 177 



densatioii of its cells is soon noticeable into a more or 

 less continuous stratum of elongated or columnar cells, 

 the odontoblasts. 



236. The dentine is formed in connection with 

 the odontoblasts (Fig. 105) ; on its outside appears the 

 enamel formed by the enamel cells, i.e., the inner 

 layer of the enamel organ. The dentine and enamel 

 are deposited gradually, and in layers. At first they 

 are soft tissues, showing a vertical differentiation 

 corresponding to the individual cells of the enamel 

 cells and odontoblasts respectively. Soon lime salts 

 are deposited in it, at first imperfectly, but afterwards 

 a perfect petrifaction takes place. The layer of most 

 recently formed enamel and dentine is more or less 

 distinctly marked off from the more advanced layer, 

 the most recently formed layer of the enamel being 

 situated next to the enamel cells, that of the dentine 

 next to the odontoblasts. 



The milk tooth remains buried in the mucous 

 membrane of the gum. When it breaks through, the 

 enamel remains covered with i.e., carries with it 

 the inner stratum of the enamel organ only, i.e., the 

 enamel cells (Fig. 105, h) ; these at the same time as 

 the surface of the enamel increases become much 

 flattened, and, finally losing their nuclei, are con- 

 verted into a layer of transparent scales, the membrane 

 or cuticle of Nasmyth. 



237. Long before the milk tooth breaks through 

 the gum, there appears a solid cylindrical mass of 

 epithelial cells extending into the depth from the 

 connection between the enamel organ and the epithe- 

 lium of the surface of the gum mentioned above. 

 This epithelial outgrowth represents the germ for 

 the enamel organ of the permanent tooth ; but it 

 remains stationary in its growth till the time arrives 

 for the milk tooth to be supplanted by a permanent 

 tooth. Then that rudiment undergoes exactly thi 



M 1 



