264 MICROSCOPIC ANATOMY OF THE TEETH 



of a distinct process as shown in fig. 1 15, &c. The connective 

 tissue of the pulp is seen passing between these cells, which 

 lie in its meshes. The rounded cells at the circumference 

 soon assume a pyriform or subcylindrical form, and a long 

 process extends from their upper end which develops into 

 the dentinal fibril (fig. 166). These cells are described as 

 odontoblasts. The first appearance of the dentine occurs 

 in the form of a clear layer of a semi-transparent substance 

 which borders the pulp and is in contact with the enamel 



FIG. 166. Macropus. First commencement of calcification of the dentine 

 at the coronal part of the pulp. a. Ameloblasts ; o. odontoblasts ; c. com- 

 mencing calcification of the dentine ; b. odontogenic zone. ( x 650.) 



organ cells externally and with the now distinctly differ- 

 entiated odontoblasts internally, and their dentinal processes 

 can be seen passing across this area. A thin line of calcification 

 commences with rounded contours towards the pulp, and is 

 separated from the odontoblasts by the clear layer above 

 described. 



Two different views have been held as to the mode of 

 formation and calcification of dentine. These may be called 

 the ' conversion ' and ' secretion ' theories. For a long time 

 the view most widely received and embodied in all the 

 principal text-books was the conversion theory of Waldeyer, 



