THE EPIDERMIS AND ITS DERIVATIVES. 



327 



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; 2^- papilla of 

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



The teeth, as will be more particularly described in the section on the 

 alimentary tract, are formed by a modification of the same process as the 

 placoid scales, in which a ridge of the epithelium grows inwards to meet 

 a connective tissue papilla, so that the development of the teeth takes 

 place entirely below the superficial layer of ejtidermis. 



In most Teleostei the enamel and dentine layers have disappeared, and 

 the scales are entirely formed of a peculiar calcified tissue develoj)ed in 

 the dermis. 



The cuticle covering the scales of Reptiles is the simplest type of 

 protective structure formed on the outer surface of the epidermis. 

 The scales consist of papillae of the dermis and epidermis ; and are 

 covered by a thickened portion of a two-layered cuticle, formed over 

 the whole surface of the body from a cornification of the superficial 

 part of the epidermis. Dermal osseous plates may be formed in con- 

 nection with these scales, but are never of course united with the 

 superficial cuticle. 



Feathers ai*e probably special modifications of such scales. They arise 

 from an induration of the epidermis of papillae containing a vascular core. 

 The provisional down, usually present at the time of hatching, is formed by 

 the cornification of longitudinal ridges of the mucous layer of the epidermis 

 of the papillae ; each cornified ridge giving rise to a barb of the feather. 

 Tlie horny layer of the epidermis forms a provisional sheath for the de- 

 veloping feather below. When the barbs are fully formed this sheath is 

 thrown oflf, the vascular core dries up, and the barbs become free except at 

 their base. 



Without entering into the somewhat complicated details of the forma- 

 tion of the permanent feathers, it may be mentioned that the calamus or 



