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 G/yptodon, 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— é 
(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. 
(6) 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 
its origin from the mesoblast 
(mesenchyme) or vascular layer, : 
assists in nourishing these Fic. 255.—Section through Elasmo- 
epidermic structures. In the branch embryo.—Ziegler. 
case of feathers and the scales C.,nervecord; WV., notochord; AU., aorta}. 
of Reptiles, the dermic papilla gut; V7 subintestina! vei MC es 
is of primary importance, but fin; C., celom: U., segmental duct; 
in the case of hairs it arises %.-M., myotome; MP., muscle plate; ~ 
late and is always small. SK., skeletogenous cells around noto- 
Fiowi the dermis are derived “Hott ) 2E= Setoderm : 
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 (1) 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 
