30 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



The corneum, on the other hand, takes part in the formation of 

 protective structures like hair, nails, claws, feathers, and other cuti- 

 cular 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 con- 

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

 extensively developed. The corium is largely composed of fibrous 

 connective tissue, intermingled with elastic tissue, blood-vessels, 

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

 vertebrates, but is much thicker in most mammals, and forms the 

 whole of ordinary leather. Pigment cells may occur in both epider- 

 mis and corium. These are mesenchyme cells, loaded with pigment, 

 which are frequently under control of the nervous (sympathetic) sys 



Fig. 20. — Section of developing scales of lizard, Scehporus. c, papilla of corium; 

 e, outer layer of epidermis which later becomes cornified; /, fibrous layer'_of_skin; m. 

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



tem, 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. 20). By a multiplication of the cells of both corium and epi- 

 dermis in definite 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 taking place at the surface of the stratum Malpighii. 

 In turtles and alligators there is a gradual wearing away of the surface. 



Closely allied to scales are claws, hoofs and nails (fig. 21). A 

 claw may be regarded as a cap on the tip of a digit, formed by two 



