SPECIAL PART. 

 A. INTEGUMENT. 



THE skin consists of a superficial ectodermal and a deeper 

 mesodermal layer. The former is called the epidermis (scarf- 

 skin) and the latter the dermis (corium, cutis). The subcutaneous 

 connective tissue is usually not sharply marked off from the dermis, 

 but the one passes gradually into the other. The epidermis always 

 consists of cells only, while the dermis is made up principally of 

 connective tissue fibres, and may also enclose muscular fibres. 

 Bony structures may occur in the dermis, as well as vessels 

 and glands, which only rarely extend into the epidermis, from 

 which the glands are all derived and with which they usually 

 remain in connection by means of their ducts. Nerves, migratory 

 leucocytes (lymph- or white blood-corpuscles), connective-tissue cells, 

 including pigment-cells (chromatophores) and free pigment, are found 

 in both layers of the integument. 



Pigment is never formed in the epithelial or connective-tissue cells them- 

 selves, but always originates in the blood. 



In the epidermis two layers may in general be distinguished : 

 a superficial, composed of flattened and hardened cells (stratum 

 corncum, horny layer), and a deeper layer made up of soft proto- 

 plasmic cells (stratum Malpighii, mucous layer). The latter serves 

 as a matrix for the regeneration of the horny layer, the superficial 

 part of which is continually scaling off. From the epidermis the 

 cuticular organs and integumentary glands, and all other parts 

 spoken of as epidermic structures take their origin. Such are, 

 hairs, bristles, nails, claws, hoofs, &c. The peripheral sensory end- 

 organs of the skin as well as the crystalline lens of the eye also arise 

 by a differentiation of epidermic cells (p. 5) : the definite relation 

 which many of these organs have with the dermis must be looked 

 upon as a secondary acquirement. 



Amphioxus, Fishes, and Dipnoans. The surface of the 

 epidermis is covered with cilia in the larval Amphioxus (gastrula 

 stage), and this must undoubtedly be considered as inherited from 

 Invertebrate ancestors. The striated cuticular border of the outer 



