1882.] the Enamel of the Teeth in Vertebrates. 



163 



Thus the basement membrane may be demonstrated running up 

 from the surface of the dermis over the tooth papilla and young tooth 

 in all its stages. The whole tooth is formed under the basement 

 membrane and the entire growth is from the side of the pulp. The 

 calcification begins at the surface of the tooth, and there is no addition 

 made outside the part already calcined, no " newly formed layer of 

 enamel " appearing as a membrane on the surface, the tooth receiving 

 its additions from the side of the pulp only. 



3. The Origin of the Enamel. 



In its early stage the enamel cannot be distinguished from the 

 dentine. It is formed by a later differentiation of a dentinal basis, 

 which is the same for both dentine and enamel, and the nature of 

 which can be best made out by teasing out, in salt solution, the young 

 still soft tooth of some Plagiostome — say of a skate. It will then be 

 found that the granular basis is formed by the regular arrangement 

 side by side of the slender processes of the odontoblasts (fig. 13), and 

 that they extend right up to the basement membrane, occupying the 

 place of the future enamel as well as of that of the dentine ; and it is 

 these regularly arranged processes which cause the striae often de- 

 scribed. If the tooth be quite young, the processes are easily sepa- 

 rated and seen in connexion with the cell from which they proceed, 

 and they are frequently branched. 



If the tooth be left in ammonium bichromate for a few hours, or 

 even in salt solution, the processes, as well as the cells, can be more 

 readily isolated. Indeed, on simply breaking through the basement 

 membrane many of them escape and will be found outside. 



When the tooth becomes a little older, and the basis thicker, the 

 processes appear glued together by an intercellular matrix as it were, 

 so that the whole basis appears as one mass, the constituent parts of 

 which are separated with difficulty, and only by prolonged maceration 

 in ammonium bichromate. But, by this time, the deposition of calca- 

 reous salts begins ; and, soon after, the differentiation of the mineralised 

 portion into dentine and enamel. 



The fact that the processes of the odontoblasts extend quite up to 

 the basement membrane, occupying the place of the future enamel, 

 admits of only one view with regard to the origin of the enamel, and 

 explains the existence of tubules which have often been described as 

 extending into it, and continuous with the dentinal tubules. 



The mode of calcification and the nature of dentine will be discussed 

 in a future paper. 



The results arrived at in this paper may be summed up as follows : — 



1. The cuticula dentis is formed by the metamorphosis, either in 

 whole or in part, of the enamel cells, which have nothing whatever 

 to do directly with the formation of the enamel. In its early stages 



m 2 



