296 ZOOLOGY FOR MEDICAL STUDENTS chap. 



takes place only on the outer side of this layer of cells, and (3) that the 

 branching processes of the cells radiate outwards through the calcified 

 matrix as long, extremely slender threads of protoplasm. In sections 

 through a dry scale the position of these protoplasmic threads is indicated 

 by empty spaces commonly known as the dentinal tubules. 



The outer surface of the spine is covered by a layer of especially hard 

 calcified material (e) in which the terminal portions of the odontoblast 

 processes are more or less obliterated. This probably corresponds to 

 the substance known in the higher vertebrates as enamel. 



The cavity of the spine is occupied by the pulp (p) — a prolongation 

 of the ordinary connective tissue of the dermis, with its irregular cells, 

 its uncalcified matrix, and its blood-vessels and nerves. 



The basal edge of the cone of dentine is prolonged as a rule into a 

 basal plate (b.p) lying in the dermis, by which it is fixed firmly in position. 

 In this the regularity of arrangement seen both in the cells and in the 

 calcified matrix of the true dentine tends to disappear, and the condition 

 approximates more to that of ordinary bone such as occurs in the higher 

 groups. 



When the development of the placoid scale is traced out in the embryo 

 it is found to make its first appearance as a concentration of the cells 

 of the dermis close under the cuticle-like basement-membrane upon which 

 the epidermis rests. This is succeeded by the formation of a dome- 

 shaped and, later, oblique and pointed dermal papilla projecting out- 

 wards into the thickness of the epidermis (Figs. 122, A and B, d.f). The 

 superficial cells of this papilla take on a regular arrangement along the 

 inner surface of the basement-membrane and become the odontoblasts, 

 while the layer of epidermal cells immediately contiguous with them 

 assume an elongated columnar shape and are known collectively as the 

 enamel-organ (Fig. 122, B, e.o). The portion of basement-membrane 

 intervening between odontoblasts and enamel-organ becomes replaced by 

 a cone of gradually increasing thickness — the dentine, over the surface of 

 which the enamel makes its appearance. As regards the precise origin of 

 dentine and enamel there is much divergence of opinion. The dentine is 

 undoubtedly formed by the odontoblasts, i.e. it is of dermal origin : the 

 main point of doubt regarding it is whether the calcified substance is to 

 be interpreted as composed of secreted material passed out of the odonto- 

 blast into the intercellular spaces or on the other hand as being formed 

 by the bodily conversion of the protoplasm of the odontoblast. While 

 the former view is more generally accepted the present writer rather 

 favours the latter. 



Regarding the enamel the question is whether it is to be looked 



