Thrre Iliprs, of epidermis: may lie (1 isl inguished: (l) That I if A I/11 1(1N 11.4 ; (2) that of fishes; and (3) that of the Aniniota. The Amphibia occupy a middle ground between the second and third, in that their epidermis is at first tish-like and Inter similar to that of the Anthinta. In the first type the epidermis consists of a single layer of puboidal cells—the condition of the epidermis in all invertebrates. This type may lie called 'uni stratons.' The second and third types differ from the first and agree with each other in that they possess more than one layer of cells; hence they may be called `nmItistratons.' The epidermis of fishes consists of many layers of columnar or euboid cells. the more coliminar ones lying at the base. There is a cutictile over the free surface provided with fine pore canals. From the essential similarity of the epidermal cells throughout, the epidermis cannot, Le divided. This type may therefore be called `lioniostratous.' The epiderfoi, of animal, that Ike in the air 1.1mitiota) has become mod 1111•11 toNt it desi•cation. There may now be distinguished in inn, tor instanee, two regions in the epidermis, a de( p une composed of active euboidal cells, and it more superficial one of flattened cornilied yens. The first is known as the 'stratum iiincostim,' the second as the 'stratum corneuni„' These eornitied Its have tamale such gradual flattening of the deeper-lying cells. accompanied by a change in chemical constitution by which the cells are metamorphosed into keratin. These cells are gradually worn off, and are replaced front the layers Is In seeking a knowledge of the origin of the many-lat ered epidermis we must study the eom parative ontogenetie and phylogenetie develop• meit of epidermis. At the time of gastrulation the ectoderm of mammals. from alone the epidermis is derived. is unistratons. In this condition it is like the epidermis of all inverte brates. In this condition. too, it persists in Amphioxits to an adult stage. Quite early in the embryonic. development of mammals, how ever, the nuclei arrange themselves in two layers as though forced to do so by pressure. Snell it condition prevails in the human embryo of two months. (See ENtiatvotrarr, The more superficial layer consists of flattened, transpar ent, hexagona1 elements—the deeper layer of smaller cells; so that already a separation into a 'stratum cornetim' and 'stratum Ilineosime has appeared. This partly differentiated condition of the epidermis occurs in the amphibian large. In the adult Amphibia the epidermis may become many-layered. The outer layer is cast from time to time in one piece in Amphilda and tteptilia. In birds and a:animals the pornitied cells are be ing constantly shed. In Amphioxiis and fishes the epidermis is provided with a cuticula, while in some larval forms and in certain regions of adults it is ciliated. The epidermis is separated horn the ends by a basement niendtritne.
Tvt•Es or DERMA. The derma is composed of connective tissue and is derived from inesen dlynie, This is a more compact layer than the looser subdermal tissue that lies still deeper. Three types of derma may be distinguished: I I That of .1niphioxris, consisting of a layer of flat cells. to7ether with a gelatinous substance w Web they havc secreted front their outer surfaees; (2 1 that of fishes and in which the bundles of connective tissue run in two direc tions, parallel and perpendicular to the surface; (3) that of birds and mammals, in which the above described distrilmtion of connective tissue bundles is lest. and the boundary between derma and epidermis is no longer straight, but strongly corrugated. The derma sends tip projections or papilla. into the epidermis. The derma of 'man belongs to this type. but in its ontogenetie de velopment it passes through stages much resem bling the first two.
Within the integument various structures are differentiated, such as pigment in granules or specialized cells; glands, both unicellular and multicellular—scutes or scales of reptiles, feath ers of birds, hair, nails, hoofs, and claws; and dermal bones and teeth.
Dermal bones and teeth of higher vertebrates have probably been derived from the dermal bones or bony scales and the tooth-structures of fishes. Neither Amphioxus nor cyclostomes pos sess any trace of an exoskeleton. Longitudinal and transverse rows of small dentieles are pres ent in selachians. Each denticle consists of a basal plate and a spine. The dentine of this scale is formed by the mesoderm. while the ectoderm forms the enamel which covers the denticle. The first impulse toward tooth-formation seems to reside in the derma. The teeth of fishes are fused with the tooth-cement. The teeth of some reptiles lie in sockets in the bone. The teeth of relachians lie in several rows upon the uppor and lower jaws. Only one or two of these rows are functional at a time. The outer rows are re placed from time to time by the younger and in ner rows. In mammals the process of replace ment of teeth is limited in most eases to one occurrence, and the number of teeth is also lim ited. The gap in the number of teeth in selachi ans and mammals is bridged by the gradual reduction occurring in intermediate forms. Be tween scales and the teeth of selachians al-o in termediate forms occur. The dermal hones such as occur in the vertebrate skull, or in the pectoral atch of fishes. are homologous with the basal plates of the dermal dentieles of selaehians. They are probably formed by a fusion of such plates. The dermal skeleton is phylogenetically older than the endoskeleton, but it tends gradu ally to disappear the higher we ascend in the animal scale.
Consult: Wiedershcim, Comparative Anatomy of Vertebrates (Eng. trans., 3d ed., Jena, 1893) ; Lang. Text-Book of Comparative Anatomy (Lon don, 1S91-96).