26 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



The Malpighian layer alone is concerned in the formation of the 

 glands connected with the skin, and the corresponding part of the 

 ectoderm contributes to the sensory structures like the nose and ear. 

 The corneum, on the other hand, is concerned in the formation of 

 protective structures like hair, nails, claws, feathers, and other cuticular 

 outgrowths. The epidermis is generally thicker in terrestrial than in 

 aquatic vertebrates, and in the latter, being constantly moist, shows 

 less of the horny consistency, than occurs in animals which live in the 

 air. 



The corium lies immediately beneath the epidermis and is less 

 sharply separated from the deeper tissues by a looser layer of connective 

 tissue (subcutis, tela subjunctiva) in which fat is frequently exten- 

 sively developed. The corium is largely composed of fibrous connec- 

 tive tissue, intermingled with elastic tissue, blood-vessels, nerves, 

 smooth muscle fibres, etc. It is usually thin in the lower vertebrates, 



FIG. 17. Section of developing scales of lizard, Sceleporus. c, papilla of corium; 

 e, outer layer of epidermis which later becomes cornified;/, fibrous layer of skin; m, Mal- 

 pighian layer; p, periderm; ts, tela subjunctiva. 



but is much thicker in most mammals, and forms the whole of ordinary 

 leather. Pigment cells may occur in both epidermis and corium. These 

 are mesenchyme cells, loaded with pigment, which are frequently 

 under control of the nervous (sympathetic) system, and can be altered 

 in shape (chromatophores) , thus producing color changes, which, as 

 in the chameleons, may be very marked. 



Horny scales, produced by a cornification of the epidermis, are found 

 in all groups of terrestrial vertebrates, but they are rare in amphibians 

 and mammals. The development is best seen in reptiles (fig. 17). 

 By a multiplication of the cells of both corium and epidermis in defi- 

 nite regions the skin becomes divided into thicker areas, separated 

 by thinner lines, each area corresponding to a future scale, which arises 

 by the conversion of the stratum corneum into horny material. In 

 snakes and lizards these scales, together with all of the stratum corneum 

 (even the covering of the eye) is periodically molted, the separation tak- 



