CHAPTEK XIV. 



THE EPIDERMIS AND ITS DERIVATIVES. 



In many of the Coelenterata the outermost layer of the blasto- 

 derm 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 potentialities 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 pro- 

 tective 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 differentiations those of a protective or 

 skeletal nature and those of a glandular nature may be considered 

 in this place. 



Protective epidermal structures. These structures constitute a 

 general cuticle or an exoskeleton of scales,' hairs, feathers, nails, 

 hoofs, &c. They may be entirely formed from the epidermis either 

 as (1) a cuticular deposit, or as (2) a chitinization, a cornification, 

 or calcification of its constituent cells. These two processes run 

 into each other, and are in many cases not easily distinguished. The 

 protective structures of the epidermis maybe divided into two groups 

 according as they are formed on the outer or the inner side of the 

 epidermis. Dermal skeletal structures are in many cases added to 

 them. Amongst the Invertebrata the most widely distributed type 



