54 



CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



Structure. Enamel is composed of fine hexagonal fibers, figures 64, 65. 

 These are set on end vertical to the surface of the dentine, and fit into cor- 

 responding depressions in the same. 



Like the dentine tubules, they are disposed in wavy and parallel curves. 

 The fibers are thus marked by transverse lines. They are mostly solid, 

 but some of them may contain a very minute canal. 



FIG. 64. 



FIG. 64. Thin Section of the Enamel and a Part of the Dentine, a, Cuticular pellicle of the 

 enamel (Nasmyth's membrane); b, enamel fibers, or columns with fissures between them and cross 

 striae; c, larger cavities in the enamel, communicating with the extremities of some of the dentinal 

 tubuli (d). X 350. (Kolliker.) 



FIG. 65. Section of the Upper Jaw of a Fetal Sheep. A. i, Common enamel germ dipping 

 down into the mucous membrane; 2, palatine process of jaw; 3, rete Malpighi. (Waldeyer.) B. 

 Section similar to A, but passing through one of the special enamel germs here becoming flask- 

 shaped; c, c', epithelium of mouth; /, neck; /', body of special enamel germ. (Rose.) C. A later 

 stage; c, outline of epithelium of gum; /, neck of enamel germ; /', enamel organ; p, papilla; 5, 

 dental sac forming; fp, the enamel germ of permanent tooth; m, bone of jaw; v, vessels cut. across. 

 (Kolliker.) Copied from Quain's "Anatomy." 



The enamel prisms are connected together by a trace of hyaline cement- 

 substance. 



Development. The first step in the development of the teeth consists 

 in a downward growth, figure 65, A, i, from the deeper layer of stratified 



