FIG. 185. 



NORMAL HISTOLOGY. 



of the enamel organ are separated at first by the narrow zone of 

 epithelial elements of the middle layer ; the cells of the latter soon 

 undergo characteristic changes, owing to an accumulation of fluid, 

 resulting in the complete transformation of the cells, which become 

 pressed together and reduced to thin plates, the tissue appearing 

 as if composed of irregularly anastomosing connective-tissue fibres 

 rather than of epithelial elements. The enamel organ retains for a 

 considerable time its connection with the epithelium of the oral 

 cavity, a thin atrophic cord of cells indicating the position of the 



former robust stalk. At the side 

 of this attachment a lateral cylin- 

 drical projection early marks the 

 beginning of the development of 

 the second enamel organ for 

 the permanent tooth. 



The columnar cells, of the in- 

 ner layer, the adamantoblasts, 

 alone produce the enamel. This 

 process consists essentially of a 

 gradual deposition on the inner 

 '/'f/A(\ side of the enamel cells that is, 

 next the new dentine of homo- 

 geneous prisms arranged verti- 

 cally to the surface of the inner 

 layer of the enamel organ. The 

 Section of developing tooth from cat embryo: layer of enamel increases by the 



m, mesodermic tissue condensed in dental pa- addition of increments deposited 

 pilla (/). at whose summit osteoblasts (d\ are c . . , , r 



forming young dentine (,); inner layer (a) of from Within OUt, the latest-formed 

 enamel organ is engaged in producing layer of enamel always lying immediately 



, i , .-, -, c . 



internal to the inner layer of the 

 enamel organ. During the later 

 stages the inner and outer layers are approximated at the expense 

 of the intervening middle layer, which finally becomes reduced to 

 an attenuated stratum. Subsequent deposition of lime-salts imparts 

 to the enamel its remarkable harclness. 



During the changes described in the enamel organ the central 

 dental papilla is actively engaged in producing the dentine. The 

 top and sides of the papilla are covered by a layer of elongated, 

 columnar or pyriform connective-tissue cells, the odontoblasts, 

 which are the immediate agents in causing the deposition of the 

 dentinal matrix, the formative process being similar to that producing 

 bone. The dentine is first formed at the apex of the papilla, and 

 appears as a thin lamina of homogeneous matrix into which the 

 delicate processes of the odontoblasts extend, becoming the dentinal 



young enamel (8) ; e, middle, h, outer layer of 



enamel organ. 



