. 



CHAPTER XIV. 

 THE EPIDERMIS AND ITS DERIVATIVES. 



IN many of the Ccelenterata the outermost layer of the blas- 

 toderm is converted as a whole into the skin or ectoderm. 

 The cells composing it become no doubt in part differentiated 

 into muscular elements and in part into nervous elements, &c. ; 

 but still it may remain through life as a simple external 

 membrane. This membrane contains in itself indefinite poten- 

 tialities for developing into various organs, and in all the true 

 Triploblastica these potentialities are more or less realized. 

 The embryonic epiblast ceases in fact, in the higher forms, to 

 become converted as a whole into the epidermis, but first gives 

 rise to parts of the nervous system, organs of special sense, and 

 other parts. 



After the formation of these parts the remnant of the 

 epiblast gives rise to the epidermis, and often unites more or 

 less intimately with a subjacent layer of mesoblast, known as 

 the dermis, to form with it the skin. 



Various differentiations may arise in the epidermis forming 

 protective or skeletal structures, terminal sense organs, or 

 glands. The structure of the epidermis itself varies greatly, and 

 for Vertebrates its general modifications have been already 

 sufficiently dealt with in chapter XII. Of its special differenti- 

 ations those of a protective or skeletal nature and those of a 

 glandular nature may be considered in this place. 



Protective epidermal structures. These structures con- 

 stitute a general cuticle or an exoskeleton of scales, hairs, 

 feathers, nails, hoofs, &c. They may be entirely formed from 



