TEETH 



103 



the two layers together form the epithelial sheath of the root (Fig. 102). 

 Over the upper part of the dental papilla, the inner enamel cells elongate 

 and become enamel-producing cells or ameloblasts (Fig. 93). 



The ameloblasts produce enamel along their basal surfaces, which 

 are toward the dental papilla, but they become so transformed that 

 their basal surfaces appear like free surfaces, and the entire cells seem 

 inverted. In columnar epithelial cells the nuclei are generally basal, 

 and the secretion gathers near the free surface, but in the ameloblasts 

 these conditions are reversed. The nuclei are toward the enamel pulp, 



- 



If'; 



Thickened ' W!-^ff&$i& 



oral 

 ithelium. . '. 



Outer'enamel cells 



Enamel pulp 

 Inner enamel cells 



Free edge of the 

 dental lamina . 



Papilla. 



FIG. 92. FROM A CROSS SECTION OF THE UPPER JAW OF A HUMAN EMBRYO OF FIVE MONTHS. X 43. 



and the latter forms a dense layer over the ameloblasts, suggesting a 

 basement membrane (Fig. 93). According to Cohn (Verh. phys.-med. 

 Ges. Wiirzburg, 1897, vol. 31, No. 4) both ends of the ameloblasts are 

 encircled by terminal bars. These bars may be regarded as modifica- 

 tions of the thin film of cement substance found between the ameloblasts. 

 Near the center of each cell, and therefore on the basal side of the nucleus, 

 Cohn has described typical centrosomes or diplosomes. 



Toward the dental papilla the protoplasm of the ameloblasts contains 

 granules or droplets which blacken with osmic acid and presumably 

 indicate secretory activity. The basal surface of each ameloblast presents 



