Marsupial and other Tubular Enamels, fyc. 



29 



The nature of the question renders it impossible to convey in brief 

 space the grounds upon which the conclusions have been arrived at, 

 but they are — m 



That the special cells of the enamel organ (Ameloblasts) do not 

 themselves calcify. 



That they each furnish from their free ends outgrowths or pro- 

 cesses which are continuous with their own plasm, and which may be 

 traced through the entire thickness of young enamel. 



That one ameloblast furnishes the whole length of an enamel prism. 



That the fibrillar outgrowths, previously more or less correctly 

 described by other observers in other enamels, but apparently not 

 appreciated at their full importance, do calcify from without inwards 

 in such a manner that an axial canal is left uncalcified. Hence the 

 canals of marsupials are in the centres of the prisms, and not, as 

 supposed by Yon Eboer, in the interspaces of the prisms. 



And that towards the completion of the full thickness of the enamel 

 the central axis is no longer left soft, but the whole calcifies into a 

 solid prism. 



It is claimed that other enamels, for instance human enamel, 

 calcify in the same way. It has long been known that short pro- 

 cesses hang out from the ends of the ameloblasts, and these, having 

 first been described by my father, are generally styled Tomes' pro- 

 cesses ; and also that the earliest formed layer of enamel is per- 

 forated, so that acids will peel up a perforated membrane from its 

 surface during its development. Longer fibrils have also been detected 

 by Andrews, Williams, and others ; but so small a thickness is 

 occupied by these structures, and the full solidification of the prism 

 follows so close upon the heels of any change in the direction of 

 calcification, that the true nature of these structures has not been 

 detected. 



But in marsupial enamel, owing to the tabular condition which is 

 so very transient in human and other mammalian enamels being per- 

 manently retained, the problem is presented under conditions more 

 favourable for elucidation. 



Hence it is my belief that all enamels alike are formed by the cen- 

 tripetal calcification of fibres furnished by the ameloblasts, and that 

 tubular enamels are nothing more than the perpetuation of a stage 

 which is passed through, though only for a brief period, by every solid 

 enamel prism . This view serves to explain the occurrence of the various 

 forms of tubular enamel which are found in fish, in some of whom — 

 e.g., Sargus — the reverse order of things is met with — that is to 

 say, the prisms first formed near to the dentine are solidly calcified, 

 but as their growth goes on the later-formed portions become 

 tubular, so that in the completed enamel there appears to be a 

 system of tubes entering it from its free surface. 



