174 MICROSCOPIC ANATOMY OF THE TEETH 



dentine junction permit the penetration of the rapidly 

 growing dentinal fibril which eventually becomes closed 

 round by advancing calcification. 



According to this view there is a distinct interprismatic 

 substance, and the process of calcification is a centrifugal one. 

 Tomes considers that the process of calcification in all mam- 

 malian enamels is made clear by the view he takes of the 

 process that tubular enamel only marks a stage in the forma- 

 tion of solid enamel prisms. The process is, however, quite 

 as clearly explained by the other view, for the solid enamels 

 would be due to the early and more complete calcification 

 of the interprismatic substance preventing the penetration 

 of the fibril from the dentine, and it appears to be more in 

 accordance with the present knowledge of the development 

 and structure of mammalian enamel. 



In a recent paper (5 a) Thornton Carter has adopted 

 a new view of the nature of the tubes in marsupial enamel. 

 He considers that marsupial enamel is not tubular, but 

 that what is thought to be the tube is a colloidal product, and 

 apparently not, strictly speaking, of an organic nature, 

 and he does not consider that the Tomes' processes of the 

 ameloblasts are processes of the cell, and thus acknowledges 

 no fibrillar organic basis to enamel. He also considers 

 Leon Williams's view of the structure of the enamel prisms 

 to be erroneous. 



Mr. Carter appears to consider the so-called enamel tube 

 to be a naked fibril within the septa of a honeycomb structure 

 arising ' from the coagulation or gel formation of an. organic 

 substance not usually present in the colloidal secretion shed 

 by the enamel cells of most other mammals '. He would thus 

 interpret the definite persistent structure seen in the enamel 

 of marsupials as part of the ' gel ' formation of a colloidal 

 material secreted by the ameloblasts, yet he speaks of an 

 organic substance present in this colloidal secretion which 

 he considers to be of a protein nature. One had always 

 considered that such colloidal product of secretory cells 

 contained the lime salts in suspension, and that calcification 

 took place within it. Analogy would lead us to expect 

 that the colloid would be effused into an organic basis 

 substance, and not that it would contain this invisible 



