. Outlines of zoology. whichwe find oftenest emphasised, at the expense of the others,is that of protection, and yet the extinct Glyptodon, thesluggish Chelonia, the decadent Ganoids, seem to indicatethat this, in itself, or in its correlated variations, is not con-ducive to the continuance of the species. The skin includes— (a) The epidermis, usuallyin several layers, theouter, horny stratumcorneum, the inner ac-tively growing stratumMalpighii, or mucosum;both derived from theectoderm or epiblast ofthe embryo. (b) The dermis, cutis,corium, or under-sldn,derived from the meso-derm or mesoblast
. Outlines of zoology. whichwe find oftenest emphasised, at the expense of the others,is that of protection, and yet the extinct Glyptodon, thesluggish Chelonia, the decadent Ganoids, seem to indicatethat this, in itself, or in its correlated variations, is not con-ducive to the continuance of the species. The skin includes— (a) The epidermis, usuallyin several layers, theouter, horny stratumcorneum, the inner ac-tively growing stratumMalpighii, or mucosum;both derived from theectoderm or epiblast ofthe embryo. (b) The dermis, cutis,corium, or under-sldn,derived from the meso-derm or mesoblast of theembryo. From the epidermis are de-rived feathers, hairs, and sotnekinds of scales. The dermis,as is natural when we considerits origin from the mesoblast(mesenchyme) or vascular layer,assists in nourishing theseepidermic structures. In thecase of feathers and the scalesof Reptiles, the dermic papillais of primary importance, butin the case of hairs it ariseslate and is always the dermis are derived. Fig. 255.—Section through Elasmo-branch embryo.—, nerve cord; iV., notochord; ^ (J., aorta; -G., gut; y. sub-intestinal vein; MC, a freemesencliyme cell; i^., beginning of a pairedfin; C., ccelom: £7., segmental duct;M.~M., myotome ; MP., muscle plate ;sk,, skeletogenous cells around noto-chord; MC., ectoderm. thebony^shields of armadillos, and a few related mammals, the bonyscutes of crocodiles and some other reptiles, and the scales of mostbony fishes. This again is readily explained by the fact that themesoblast is also the skeletal layer of the embryo. The ordmary teethof Vertebrates, as well as the superficial or skin-teeth of gristly fishes,are largely formed-from the dermis, but are usually covered by a thincoating of ectodermic enamel. The mesoderm is divided in the embryo into (l) a series of dorsalsegments or somites, with a transient cavity (the myoccel), and (2) anunsegmented ventral portion or lateral plate. The dorsal part 478 STRUCT
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
