DEVELOPMENT OF THE ENAMEL 179 



about by the cells of the dentine pulp, but takes place under 

 a distinct layer of large ameloblasts derived from the epi- 

 blastic epithelium of the mouth. It is seen that a deposit 

 of enamel takes place within the tissue which is previously 

 laid down by the mesoblastic dentine papilla, and we have 

 the apparent anomaly of a deposit of enamel by true enamel 

 cells within a matrix derived from the dentine papilla. 



It was considered by Hertwig and Rose that these amelo- 

 blasts are concerned only in the production of a surface 

 membrane (Schmelzoberhdutchen) , but C. S. Tomes states that 

 ' no such membrane can be raised from the surface when 

 advanced in calcification'. Tomes, to whom our know- 

 ledge of the development of the enamel in Plagiostome fish 

 is chiefly due, sums up his important researches on the 

 subject as follows : 



' The outer hard layer which covers the teeth of Selachia 

 and Ganoids does not correspond exactly either with the 

 dentine or the enamel of mammalia. 



' In structure it ranges by slight gradations from the 

 simple and thoroughly enamel-like tissue met with in 

 the Rays to the complex tubular tissues ' in many sharks. 



' It is not a dentine, because it has no collagen matrix, 

 the organic tissue which it contains being easily soluble in 

 weak acids. 



' While this organic matrix is beyond question furnished 

 by the mesoblastic dentine papilla, the epiblastic amelo- 

 blasts over it are in a state of development which implies 

 that they take an active part, and that the tissue is a joint 

 production. 



' Though it is not fully demonstrable what that may be, 

 it upon the whole seems probable that they furnish it with 

 its calcifying salts. 



' Just as the entire teeth of Selachians present the problem 

 of tooth formation reduced to its simplest aspects, so this 

 layer appears to be the first introduction of enamel as 

 a separate issue, and therefore, to avoid multiplication of 

 terms, it may be appropriately called enamel.' 



The structure of the enamel organ and development of in osseous 

 the enamel in osseous fish has only been thoroughly studied 

 in a few families, and much further research is necessary, 



N 2 



