THE SKIN. 



477 



nificance, and is frequently concerned in the excretion of 

 waste and the regulation of the body temperature. As one 

 or other of its many functions predominates, there are cor- 

 responding structural modifications. One function which 

 we find oftenest emphasised, at the expense of the others, 

 is that of protection, and yet the extinct Glyptodon, the 

 sluggish Chelonia, the decadent " Ganoids," seem to indicate 

 that this, in itself, or in its correlated variations, is not con- 

 ducive to the continuance of the species. 



The skin includes 



(a) The epidermis, usually 

 in several layers, the 

 outer "horny" stratum 

 corneum, the inner ac- 

 tively growing stratum 

 Malpighii, or mucosum ; 

 both derived from the 

 ectoderm or epiblast of 

 the embryo. 



(d) The dermis, cutis, 

 corium, or under- skin, 

 derived from the meso- 

 derm or mesoblast of the 

 embryo. 



From the epidermis are de- 

 rived feathers, hairs, and some 

 kinds of scales. The dermis, 

 as is natural when we consider 



M 



its origin from the mesoblast 

 (mesenchyme) or vascular layer, 



assists in nourishing these FlG. 255. Section through Elasmo- 



MC 



branch embryo. Ziegler. 

 NC., nerve cord ; N. t notochord ; A ( >. , aorta ; 

 G., gut ; V. sub-intestinal vein ; MC., a free 

 mesenchyme cell ; f., beginning of a paired 

 fin; C., coelom : /., segmental duct; 

 M.-M., myotome ; MP., muscle plate; 

 SK.) skeletogenous cells around noto- 

 chord ; EC., ectoderm. 



epidermic structures. In the 



case of feathers and the scales 



of Reptiles, the dermic papilla 



is of primary importance, but 



in the case of hairs it arises 



late and is always small. 



From the dermis are derived 



the bony shields of armadillos, and a few related mammals, the bony 



scutes of crocodiles and some other reptiles, and the scales of most 



bony fishes. This again is readily explained by the fact that the 



mesoblast is also the skeletal layer of the embryo. The ordinary teeth 



of Vertebrates, as well as the superficial or skin-teeth of gristly fishes, 



are largely formed from the dermis, but are usually covered by a thin 



coating of ectodermic enamel. 



The mesoderm is divided in the embryo into (l) a series of dorsal 

 segments or somites, with a transient cavity (the myoccel), and (2) an 

 unsegmented ventral portion or " lateral plate." The dorsal part 



