I have endeavoured to render these relations obvious by the diagram (fig.304.), which may be taken for a section from centre to surface of a fcetal lirnb, or of an exogenous branch. a, outer protomorphic line between epidermis or periderma and mesophlceum in the plant ; between ecderon and enderon in the animal ; a', inner protomorphic line between liber and wood of plant, between bone and periosteum of animal ; b, b' , cork and epidermic layers of plant ; cellular epidermis and scale of animal, fish, e.g. ; c, mesophlceum, enderon (derma) ; d, liber, periosteum ; e,e', vvood and pith, bone and cartilage ; x, axis; y, surface.
The consideration of vegetable structures will aid us even further in understanding the manner in which the different varieties of in tegumentary organs, with which we shall meet, are formed. For it is well known that the outer covering of a plant may ultimately be constituted in one of three ways. 1. The original cellular ecderon may persist un changed. 2. The " epiderm" persisting, a la minated, but otherwise strnctureless "cuticula," may be developed upon its outer surface, attaining sometimes a very considerable thick ness. 3. The original epidermis is cast off, its place being taken by the development of a new layer of different, usually suberous con stitution, beneath it, which then goes on growing endogenously, and constitutes the permanent integumentary surface. Now, we find a precise parallel for all these conditions in animals. In the soft integument of most Mollusca and Vertebrata the first condition obtains, the general surface of the integument being constituted by the cellular "epidermis." In the Annulosa, on the other hand, the integument has certainly, in many casts, and I think probably in the great majority, the cha racter of a vegetable cuticle, consisting as it does of layers developed from the outer sur face of the cellular ecderon. In this way also I believe that all molluscan shells are formed.
Lastly, the fish-scale produced altogether beneath the cellular ecderon or epidermis, but growing endogenously after the manner of a true ecderonic structure, appears to be precisely analogous to the corky periderrna of the plant ; and as the latter, though it is not the original epidermis, takes its place and grows in the same way, so in the fish the scale, which is assuredly not a calcification of the cellular ecderon, yet represents it both in position and in mode of growth.
2. Morphology of the integuments.— In the embryonic state of all animals, and in the adult condition of many of the lower forms, the in tegument, constituted as above defined, forms a continuous investment over the surface of the body without any impertant processes or irregularities. Such is the case in many of the Worms, Polypes, and lower Mollusca. Prom such simple forms of integument as these the most rudimentary kinds of appendages or tegumentary organs are produced in one of two ways,—either the outer portion of the ecderon is thickened, and as a spine or as a plate projects bejond the common surface — e. g. cells of Hydroid and Polyzoic Poij pes ; or the whole integument is developed into a spine-like or plate-shaped process, as in the so-called " bracts " of the aphydw, and in all the spines, hairs, and scales of the Insecta, Crustacea, and Arachnida.
The shells and plates of Mollusca and Arti culata belong principally to the former division, being simple laminated thickenings of theouter portion of the ecderon. In the Vertebrata the integument but rarely possesses appen dages of so simple a nature. Simple plates of this kind, however, coat the surface of the beaver's tail, in which animal, according to Heusinger, " the epidermis is divided by a great number of clefts into hexagonal por tions 4 lines long, whose whole edges ad here to the cutis. They usually consist of a couple of superimposed laminm identical in structure with the rest of the epidermis " (Z. c. p. 168.). The polygonal horny plates of the Chelonia arc of the same nature. The scales on the under surface of the tail of the rat and other rodents, and on the tarsi of birds, are similarly constituted ; but here one edge is thrown up, and we have a transition to the scales of the Pangolin, — to those of Ophidia and Sauria,— and to the nails, claws, hoofs, and hollow horns of Mammals, and the horny sheaths of the beak of Birds, all of which are constructed on essentially the saine plan, being diverticula of the whole integument, the outer layer of whose ecderon has undergone horny metamorphosis.
Among these the nails, horns, and hoofs of mammalia present certain complexities of arrangement which entitle them to particular notice.