DEVELOPMENT OF THE ENAMEL 165 



coloured the enamel a strong blue throughout the whole 

 section, showing that very little calcification has taken place, 

 and large calcospherites are seen near the dentine junction. 

 The calcified dentine is quite unstained, while the odonto- 

 genic zone and the odontoblasts are a deep blue. It is also 

 noticeable that the part of the enamel first laid down, 

 bordering on the dentine, shows signs of still more incom- 

 plete calcification than the rest of the tissue. This appears 

 a somewhat anomalous and curious condition. It would 

 appear as if the ameloblast processes, where they extend to 

 the dentine, leave spaces between them which very slowly 

 become obliterated by calcification. These spaces can be 

 clearly seen in figs. 96 (s) and 97, and lying in the spaces are 

 round calcospherites such as are seen in the teased pre- 

 parations. The presence of these spaces and of the large 

 calcospherites would appear to indicate that a gradual clos- 

 ing in upon the enamel prisms takes place, while the enamel 

 above has become more completely calcified, and may help 

 to explain the presence of the dentinal tubes in the enamel. 



C. S. Tomes says : ' The first -formed layer of dentine con- 

 tains only the fine terminations of the dentinal tubes ' (26 d), 

 but an examination of many sections shows that although 

 some tubes terminate in fine branching extremities before 

 reaching the enamel junction, others, and often the majority, 

 pass as large unbranched tubes to the enamel margin, and 

 fig. 96 shows these tubes actually passing into the open 

 spaces above described. 



It is therefore probable that these spaces, which form the 

 ' clumsy joint ' of Tomes, are not due to inclusions of 

 dentine matrix as suggested by Professor Paul (17), or to 

 dilatations of the tubes in this situation, but to the imperfect 

 calcification of the interprismatic substance surrounding 

 them. If such spaces were due to intrusions of dentine 

 matrix we should expect to find them well defined in early 

 stages of enamel development, but instead of these irregular 

 spaces we find an area of uncalcified tissue of an irregular 

 form. As calcification advances, these spaces around the 

 tubes are gradually encroached upon by the calcifying 

 substance, but in most marsupials not completely closed in, 

 and remain as the irregular spaces seen in finished marsupial 



