THE EXOSKELETON. 



The teeth, as will be more particularly described in the section on the 

 alimentary tract, are formed by a modification of the same process as the 

 placoid scales, in which a ridge of the epithelium grows inwards to meet 

 a connective tissue papilla, so that the development of the teeth takes place 

 entirely below the superficial layer of epidermis. 



In most Teleostei the enamel and dentine layers have disappeared, and 

 the scales are entirely formed of a peculiar calcified tissue developed in the 

 dermis. 



The cuticle covering the scales of Reptiles is the simplest 

 type of protective structure formed on the outer surface of the 

 epidermis. The scales consist of papillae of the dermis and 

 epidermis ; and are covered by a thickened portion of a two- 

 layered cuticle, formed over the whole surface of the body 

 from a cornification of the superficial part of the epidermis. 

 Dermal osseous plates may be formed in connection with these 

 scales, but are never of course united with the superficial 

 cuticle. 



Feathers are probably special modifications of such scales. They arise 

 rom an induration of the epidermis of papillae containing a vascular core. 

 The provisional down, usually present at the time of hatching, is formed by 

 the cornification of longitudinal ridges of the mucous layer of the epidermis 

 of the papilla; ; each cornified ridge giving rise to a barb of the feather. The 

 horny layer of the epidermis forms a provisional sheath for the developing 

 feather below. When the barbs are fully formed this sheath is thrown off, 

 the vascular core dries up, and the barbs become free except at their base. 



Without entering into the somewhat complicated details of the formation 

 of the permanent feathers, it may be mentioned that the calamus or quill is 

 formed by a cornification in the form of a tube of both layers of the epidermis 

 at the base of the papilla. The quill is open at both ends, and to it is 

 attached the vexillum or plume of the feather. In a typical feather this 

 is formed at the apex of the papilla from ridge-like thickenings of the mucous 

 layer of the epidermis, arranged in the form of a longitudinal axis, con- 

 tinuous with the cornified mucous layer of the quill, and from lateral ridges. 

 These subsequently become converted into the axis and barbs of the plume. 

 The external epidermic layer becomes converted into a provisional horny 

 sheath for the true feather beneath. 



On the completion of the plume of the feather the external sheath is 

 thrown off, leaving it quite free, and the vascular core belonging to it shrivels 

 up. The papilla in which the feather is formed becomes at a very early 

 period secondarily enveloped in a pit or follicle which gradually deepens as 

 the development of the feather is continued. 



Hairs (Kolliker, No. 298) are formed in solid processes of 

 the mucous layer of the epidermis, which project into the 



