ii THE SKIN AND ITS DEKIVATIVES 73 



and tail, originate from the basal portions of down feathers which 

 undergo a great increase in length. The basal part of the rudiment 

 in this case increases much in diameter. The epidermis here again 

 becomes incised on its inner surface to form barb rudiments. These 

 however are much more numerous (Fig. 42, F) than in the typical 

 down feather and, further, instead of being arranged strictly longi- 

 tudinally they are arranged somewhat spirally, starting from a con- 

 tinuous epidermal thickening (r) which runs along the outer side of 

 the feather rudiment. This thickening is the rudimentary rachis or 

 shaft and the barb rudiments run from it spirally round the feather 

 rudiment until their tips meet along its inner side. 



The feather is thus in early stages curled into a cylindrical form 

 round the central dermis or pulp — the whole being enclosed in a 

 continuous sheath which disintegrates sooner or later setting free 

 the elastic barbs and allowing them to flatten out to form the 

 vexillum or vane. 



As is well known the barbs are united together in the fully- 

 developed feather into a functionally continuous web, through the 

 agency of the barbules which project from the two sides of the barbs 

 much as the barbs do from the rachis. The mode of origin of the 

 barbules is seen in a longitudinal radial section through a barb such 

 as that shown in Fig. 42, G, where the outer portion of the barb 

 rudiment is seen to be splitting up into barbules (lb) while its inner 

 portion remains continuous to form the definitive barb (6). 



Traced downwards, towards the base of the feather, the rachis 

 increases in width so as to extend round the whole periphery of the 

 feather rudiment. Its outer layer assumes a translucent character 

 and forms the cylindrical quill (calamus), the basal end of which 

 becomes somewhat narrowed, bounding the umbilicus, the opening 

 through which the dermal pulp extends up into the interior of the 

 quill. The pulp of the feather undergoes a gradual shrinkage 

 leaving behind it the series of cornified caps (H, c) formed on its 

 apical surface as already mentioned and which eventually lie loose 

 within the quill. 



The lips of the umbilicus are continued (Fig. 42, H) into a deep 

 rim of uncornified epidermis (g). This with the dermal papilla pro- 

 jecting into the feather base remains inactive until the period of 

 moulting when it springs into activity, grows rapidly, and becomes 

 converted into a new feather which pushes the old one out and takes 

 its place. 



The scales which frequently occur upon the legs and feet of birds 

 are probably not, as might at first sight be supposed, to be looked 

 upon as having persisted from the Eeptilian condition. They fre- 

 quently bear feathers in the young condition and are probably 

 secondary developments replacing an earlier feathery covering. 



In view of the convincing evidence offered by comparative 

 anatomy and palaeontology we are compelled to believe that Birds 

 have been evolved out of Eeptile-like ancestors. Accepting this 



