APPENDIX. 431 



Batrachian present any difficulty in the way of this view. It 

 is, at most, indistinctly fibrous and contains so large a quantity 

 of calcareous matter in proportion to the dentine, that the 

 differences between the two may well be supposed to arise — 

 as we believe they do — from this circumstance alone. 



In the higher Vertebrata, however, when the enamel in its 

 young state consists of definite fibres composed of organic 

 substance, which are added to the surface of the tooth only 

 after the formation of a subjacent scale of dentine, it becomes 

 more difficult to comprehend the development of the former. 

 There appears to be three possibilities : 



1. What we call the primary scale of dentine is not, on the 

 crown of the tooth, dentine at all, but young enamel, becoming 

 converted into the latter structure, and not into the former, as 

 development proceeds. This appears, as first sight, a startling 

 hypothesis enough; but there are, so far as we know, no 

 means of disproving it. Young dentine can only be known 

 to be such by its relations ; in structure it is neither like per- 

 fect dentine nor like perfect enamel; but might readily be 

 supposed to be converted into either by variation in the quan- 

 tity and mode of deposition of its calcareous element. If this 

 deposit be comparatively small, leaving much of the organic 

 basis, and not encroaching upon the existing cavities, we have 

 dentine; increase the quantity of calcareous salts, and break 

 up the organic basis at the same time into fibres, and enamel 

 would be produced. 



2. The enamel is the indirect product of the prismatic 

 cells of the enamel organ, whose inner extremities pass into 

 successive layers of membrane, which are applied upon and 

 indistinguishably unite with the membrana preformativa over 

 the whole surface of the developing enamel. The laminated 

 membrane thus formed receives a calcareous deposit, and 

 breaks up into the prisms of the enamel. 



This hypothesis likewise, at first sight, appears somewhat 

 improbable, but it may be strictly paralleled with what occurs 

 in the formation of prismatic shell substance, where a lami- 

 nated membranous substance is produced from the cellular 

 epidermis of the mantle, and subsequently breaks up into the 

 characteristic, large, transversely striated prisms. 



3. The enamel is neither the result of the modification of 



