INTRODUCTORY. EPIBLASTIC STRUCTURES. 3 



Hairs are slender, elongated structures which arise by 

 the proliferation of cells from the Malpighian layer of the 

 epidermis. These cells in the case of each hair form a short 

 papilla, which sinks inwards and becomes imbedded at the 

 bottom of a follicle in the dermis. Each hair is normally 

 composed of an inner cellular pithy portion containing much 

 air, and an outer denser cortical portion of a horny nature. 

 Sometimes, as in Deer, the hair is mainly formed of the pithy 

 portion, and is then easily broken. Sometimes the horny part 

 predominates, as in the bristles of Pigs. A highly vascular 

 dermal papilla projects into the base of the hair. 



Feathers, like hairs, arise from epidermal papillae which 

 become imbedded in pits in the dermis. But the feather germ 

 differs from the hair germ, in the fact that it first grows 

 out like a cone on the surface of the epidermis, and that 

 the horny as well as the Malpighian layer takes part in its 

 formation. 



Nails, claws, hoofs, and the horns of Oxen are also 

 epidermal, as are such structures as the scales of reptiles, of 

 birds' feet, and of Manis among mammals, the rattle of the 

 rattlesnake, the nasal horns of Rhinoceros, and the baleen 

 of whales. All these structures will be described later. 



Nails arise in the interior of the epidermis by the thicken- 

 ing and cornification of the stratum lucidum. The outer 

 border of the nail soon becomes free, and growth takes place 

 by additions to the inner surface and attached end. 



When a nail tapers to a sharp point it is called a claw. 

 In many cases the nails more or less surround the ends of 

 the digits by which they are borne. 



Horny beaks of epidermal origin occur casing the jaw- 

 bones in several widely distinct groups of animals. Thus 

 among reptiles they are found in Chelonia (tortoises and 

 turtles) as well as in some extinct forms ; they occur in all 

 living birds, in Ornithorhynchus among mammals, and in the 

 larvae of many Amphibia. 



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