178 MICROSCOPIC ANATOMY OF THE TEETH 



possess no enamel cap, but the dermoid scales of the Elasmo- 

 branchii (Sharks and Rays) are composed of enamel 

 formed from the ectodermic epithelial cells, and of dentine 

 produced from the mesodermic scleroblasts. 1 The papilla 

 of the dermis forms a pulp cavity around which the dentine 

 is secreted, and the dentine is covered by a layer of true 

 enamel formed by ameloblasts. Beneath the dentine is 

 a basal plate of bone which is perforated for the passage of 

 blood-vessels to the pulp cavity. It is thus seen that placoid 

 scales and teeth are both structurally and developmentally 

 identical, but the true teeth of the sharks, which are developed 

 under the thecal fold of mucous membrane which covers 

 them, attain to a larger size and more complete differentia- 

 tion of the tissues composing them. 



In the Plagiostome fish the nature of the outer layer of the 

 tooth has been the subject of much controversy, Rose and 

 several other authors considering it to be a form of dentine, 

 but Tomes, after a consideration of all the evidence, looks 

 upon it as enamel. 



In the developing tooth of the sharks the cells of the 

 dentine papilla do not appear to be specialized as an odonto- 

 blast layer, but the surface of the mesoblastic dentine 

 papilla consists of a delicate fibrillar tissue, the fibres lying 

 parallel to the surface, and long processes of the underlying 

 cells, which rapidly increase in number, are continued 

 into this layer. As Tomes says, ' The great and most 

 striking peculiarity of these tooth germs lies in the fact 

 that the first apparent calcification of the true dentine, 

 whether it is a fine -tubed dentine as in Car char ias, or an 

 osteodentine as in Lamna, takes place, not at the outside of 

 the whole papilla as invariably happens in mammals, but 

 at the inner side of the specialised layer, thus cutting it off 

 from the rest of the pulp' (26 a). Beneath this specialized 

 layer the calcification of the dentine proceeds as in ordinary 

 tooth germs, and it is within this previously formed layer 

 that the calcification of the tissue considered to be enamel 

 takes place. This calcification is not, however, brought 



1 It has been affirmed by KJaatsch that the scleroblasts are also of 

 epidermic origin, but this statement has not been generally received. 

 Klaatsch, M&rph. Jahrb., xvi, 1890, pp. 97 and 209. 



