CHAPTER II 



THE SKIN AND ITS DEEIVATIVES 



The skin of the vertebrate consists of the epidermis — the persistent 

 and less or more modified ectoderm — resting upon the superficial 

 layer of mesenchyme — the dermis — which in the higher forms 

 becomes strengthened by the formation of numerous tough inter- 

 lacing fibres. 



In studying the development of the skin in the various types 

 of vertebrate we find that the ectoderm undergoes characteristic 

 modifications to fit it for the carrying out of special functions. 

 In the fishes it becomes converted into a highly glandular mechanism 

 concerned with the production of slippery mucus for the diminution 

 of what the naval architect calls " skin-friction," in other words the 

 friction between the surface of the body and the water in contact 

 with it. Local or general specializations of this glandular apparatus 

 lead to the development of cement organs by the secretion of which 

 the young animal is able to attach itself to solid supports, to the 

 production of digestive ferments by which the eggshell is softened 

 or, in the case of the portion of ectoderm which lines the buccal 

 cavity, the digestion of the food initiated, or to the production of 

 poisonous defensive or offensive secretions. In the case of the 

 terrestrial amphibians the glandular apparatus serves to keep the 

 skin moist, while in the Birds it develops arrangements for oiling 

 the feathers. 



Again the ectoderm develops important protective functions. 

 It becomes hardened and toughened to give mechanical protection : 

 it becomes more or less loaded with opaque pigment to prevent the 

 penetration of light rays, while in those highest vertebrates, in 

 which, correlated with intensely active metabolism, the body is kept 

 at a higher temperature than its surroundings, the superficial horny 

 layer becomes as it were frayed out into a fluffy coating of feathers 

 or hair which with its entangled air retards loss of heat from the 

 surface of the body. 



Finally the ectoderm forms the great mechanism for the reception 

 of impressions from the external world. It develops sensory cells 

 which may become crowded together to form organs of special sense 



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