152 



NORMAL HISTOLOGY. 



FIG. 185. 



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 inner 

 layer alone are concerned in the 

 production of the enamel. This 

 process consists essentially of a 

 gradual deposition on the inner 

 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 

 layer of enamel increases by the 

 addition of increments deposited 

 from within out, the latest-formed 

 enamel always lying immediately 

 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, the other coats of the enamel ^sac coming 

 almost in actual contact. 



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 



Section of developing tooth from cat embryo : 

 m, mesodermic tissue condensed in dental pa- 

 pilla (/), at whose summit osteoblasts (d) are 

 forming young dentine (f) ; inner layer (a) of 

 enamel organ is engaged in producing layer of 

 young enamel (b) ; e, middle, k, outer layer of 

 enamel organ. 



