CH. V.] 



DEVELOPMENT OF THE TEETH 



73 



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

 formation 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 uncalcified 

 (odontogen), but globules of calcareous matter soon appear in it. 

 These, becoming more numerous, blend into the first cap of dentine. 

 In the meanwhile the odontoblasts have formed a second layer of 

 odontogen within this (fig. 99), and this in turn becomes 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 interglobular 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 retire 

 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 odonto- 

 blasts are formed. 



The other cells of the dental 



FIG. 09. Part of section of developing tooth of a 

 young rat, showing the mode of deposition of 

 the dentine. Highly magnified, a, Outer 

 layer of fully formed dentine ; b, uncalcified 

 matrix with one or two nodules of calcareous 

 matter near the calcined parts ; c, odonto- 

 blasts sending processes into the dentine; 

 d, pulp ; e, fusiform or wedge-shape cells 

 found between odontoblasts ; /, stellate cells 

 of pulp in fibrous connective tissue. The 

 section is stained with carmine, which colours 

 the uncalcified matrix but not the calcified 

 part. (B. A. Schafer.) 



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). 



1. A layer of columnar epithelium cells in contact with the 



dentine. These are called the enamel cells, or adamanto- 

 blasts. 



2. Two or three layers of smaller polyhedral nucleated cells, the 



stratum intermedium of Hannover. 



3. A matrix of non-vascular jelly-like tissue containing stellate 



cells. 



4. An outer membrane of several layers of flattened epithelium 



cells. 



The first three layers on an enlarged scale are seen in fig. 101. 

 The enamel prisms are formed by the agency of the ends of the 



