DEVELOPMENT OF THE ENAMEL 177 



Development and Calcification of the Enamel in Fish. 



The first appearance of a true enamel organ in vertebrates 

 is seen in the development of the placoid scales or dermal 

 spines of the Sharks and Rays (fig. 99). 



Hert wig's well-known researches on this subject demon- 

 strated that a distinct enamel organ and dentine germ is 

 concerned in the calcification of the placoid scale, and this 

 is clearly comprehensible in view of the fact before referred 

 to that teeth and dermal scales are similar structures, the one 

 merging into the other at the margin of the jaws. 



FIG. 99. Vertical section through the skin of an embryo shark. C. Der- 

 mis ; c, c, c, d. layers of the dermis ; E. epidermis ; e. enamel organ ; 

 o. enamel layer ; p. papilla of the dermis. From Wiedersheim in Camb. 

 Nat. History. 



These scales are developed in the mucous membrane 

 and have no developmental relation to the bones. The 

 teeth are produced in the same manner and their connexion 

 with the bones is purely secondary, so that the teeth 

 in all vertebrates may be accurately described as dermal 

 appendages. 



'These external skeletal appendages are formed both 

 from the ectodermic cells and from the mesoderm. In 

 general histology the cells which secrete the hard structures 

 of both dentine and bone are termed scleroblasts, a term 

 which would include both odontoblasts and osteoblasts. 

 The scales of the bony fish (Teleostomi) and of the Dipnoi 

 (Lepidosiren, &c.) are derived solely from the dermis and 



