TOOTH-DEVELOPMENT. 



139 



impaired vitality. Their dentinal processes likewise grow thinner and less 

 flexible and gradually assume the characteristics of the dentinal fibres. 

 The 'portions of the dental papilla remaining after the dentine has been 

 completed persist as the definite pulp-tissue, receiving 

 a generous vascular and nervous supply. 



The Enamel-Organ. The end of the ecto- 

 dermic epithelial outgrowth, which early marks the 

 position of the future tooth, soon broadens and 

 becomes invaginated to form the young enamel-organ 

 that overlies the top of the mesodermic dental papilla 

 (Fig. 177). In contrast to the latter, which as the 

 pulp-tissue partially persists as a permanent structure, 

 the enamel-organ is only transient and ultimately 

 disappears. When fully developed, the enamel-organ 

 consists of three principal parts, the outer, middle 

 and inner layers. Since the organ is converted into 

 a cap by the pushing in or invagination of its broader 

 and deeper surface, it follows that the outer and inner 

 layers are directly continuous at the margin of the inverted area. The outer 

 layer consists of flattened epithelial cells, which send processes into the 

 surrounding vascular connective tissue that invests the tooth-germ as the 



FIG. 178. Isolated odonto- 

 blasts from tooth of new- 

 born child. X 300. (Ebner.) 



Intermediate layer of 

 enamel-organ 



Ameloblasts 



Young enamel with 

 Tomes's processes 



Dentine 



Last-formed dentine 



Odontoblasts 



Embryonal pulp-tissue 

 FIG. 179. Section of developing: tooth through junction of enamel and dentine. X 4- 



dental sac. The middle layer, conspicuous by reason of its clear loose 

 texture, consists of epithelial elements that have become highly modified 

 in consequence of an enormous distention of the intercellular clefts by fluid, 

 the epithelial plates in this manner being reduced to stellate cells connected 

 by delicate processes. The inner border of this peculiar area constitutes 



