THE EPIDERMIS AND ITS DERIVATIVES. 



395 



The most primitive form of the first type of dermal structures 

 is that of the placoid scales of Elasmobranchii 1 . These consist, 

 when fully formed, of a plate bearing a spinous projection. 

 They are constituted of an outer enamel layer on the projecting 

 part, developed as a cuticular deposit of the epidermis (epiblast), 

 and an underlying basis of dentine (the lower part of which may 

 be osseous) with a vascular pulp in its axis. The development 

 (fig. 235) is as follows (Hertwig, No. SOU). A papilla of the 

 dermis makes its appearance, the outer layer of which gradually 

 calcifies to form the dentine and osseous tissue. This papilla is 

 covered by the columnar mucous layer of the epidermis (e), from 

 which it is separated by a basement membrane, itself a product 

 of the epidermis. This membrane gradually thickens and calci- 

 fies, and so gives rise to the enamel cap (0). The spinous point 

 gradually forces its way through the epidermis, so as to project 

 freely at the surface. 



The scales of other forms of fishes are to be derived from those of 

 Elasmobranchii. The great dermal plates of many fishes have been formed 

 by the concrescence of groups of such scales. The dentine in many cases 

 partially or completely atrophies, leaving the major part of the scale formed 

 of osseous tissue ; such plates often become parts of the internal skeleton. 



FIG. 235. VERTICAL SECTION THROUGH THE SKIN OF AN EMBRYONIC SHARK, 

 TO SHEW A DEVELOPING PLACOID SCALE. (From Gegenbaur ; after O. Hertwig.) 



E. epidermis ; C. layers of dermis ; d. uppermost layer of dermis ; /. papilla of 

 dermis ; e. mucous layer of epidermis ; o. enamel layer. 



1 For the most important contributions on this subject from which the facts and 

 views here expressed are largely derived, vide O. Hertwig, Nos. 306 308. 



