CH. V.] 



DEVELOPMENT OF TEETH. 



79 



The method of the formation of the dentine by the odontoblasts 

 is given in Quain's Anatomy as follows : 



These cells either by secretion, or as some think by direct 

 transformation of the outer part of each, form a layer of dentinal 

 matrix on the apex of the papilla, or if the tooth has more 

 than one cusp, then at the apex of each cusp. This layer is 

 first uucalcified (odontogen), but globules of calcareous matter 

 soon appear in it. These, 

 becoming more numerous, 

 blend into the first cap of 

 dentine. In the mean- 

 while the odontoblasts have 

 formed a second layer of 

 odontogen within this (fig. 

 99), and this in turn be- 

 comes calcified ; thus layer 

 after layer is formed, each 

 extending laterally further 

 than its predecessor ; the 

 layers blend except in some 

 places ; here portions of 

 odontogen remain, which 

 in a tooth macerated for 

 histological purposes get 

 destroyed, and appear as 

 the interglobidar spaces 

 (fig. 95), so called because 

 bounded by the deposit of 

 calcareous salts, which 

 occurs, as we have already 

 seen, in the form of globules. 



As the odontoblasts re- 

 tire towards the centre, 

 depositing layer after layer 

 of dentine, they leave 



behind them long filaments of their protoplasm around which the 

 calcareous deposit is moulded ; thus the dentinal tubules occupied 

 by the processes of the odontoblasts are formed. 



The other cells of the dental papilla form the cells of the 

 pulp. 



Formation of the enamel. The portion of the enamel or dental 

 germ that covers the dental papilla is at this stage called the 

 enamel organ. This consists of four parts (see figs. 100 and 

 101). 



Fig. 100. Vertical transverse section of the dental 

 sac, pulp, &c., of a kitten, a, dental papilla 

 or pulp ; '', the cap of dentine formed upon 

 the summit ; c, its covering of enamel ; d, inner 

 layer of epithelium of the enamel organ ; 

 , gelatinous tissue ; /, outer epithelial layer 

 of the enamel organ ; .</. inner layer, and A, 

 outer layer of dental sac. x 14. (Thiersch.) 



