TEE DEVELOPMENT OF THE TEETH. 165 



marked oft' from the rest of the dentine organ, and the 

 dentine is wholly derived from its conversion into calcined 

 material, so that the difference between vaso-dentine and 

 hard dentine is not one of a very fundamental character. 



Indeed, as we have seen (p. 85), the same formative pulp, 

 the same odontoblast layer, is able at one time to form hard 

 tubular dentine, at another vaso-dentine. All, therefore, 

 that has been before said of the calcification of odontoblasts 

 will apply equally to those of a vaso-dentine pulp, save only 

 that in a typical tissue of this latter kind each cell calcifies 

 solidly, and does not leave the axial portion soft, to remain 

 as a dentinal fibril. 



Of the development of Plicidentine nothing more need 

 be said, as it presents no peculiarities which are not the 

 obvious result of the folding of the surface of its formative 



THE CALCIFICATION OF OSTEO-DENTINE. 



With the exception of the thin external layers (see fig. 47), 

 which are developed from a superficial layer of not very 

 highly specialised cells, osteo-dentine is built up in a 

 manner fundamentally different from that in which hard 

 dentine, plicidentine, and vaso-dentine, are constructed. 



For it is not, like these, a surface formation ; it is not 

 laid down in a regular manner upon the exterior of a pulp, 

 and it has no relation to an odontoblast layer, if we except, 

 perhaps, its thin exterior shell. 



So soon as this has been formed its inner surface becomes 

 roughened by trabeculse shooting inwards into the substance 

 of the pulp, which speedily becomes traversed completely 

 by them, as well as by the connective tissue bundles which 

 are continuous with them. Thus the pulp being pierced 

 through in every direction by these ingrowths cannot be 

 withdrawn, like the pulp of a hard or of a vaso-dentine tooth, 



