xii PHYLUM CHORDATA 459 



composed of barbs; delicate thread-like structures which 

 extend obliquely outwards from the rachis. In an uninjured 

 feather the barbs are closely connected so as to form a con- 

 tinuous sheet, but a moderate amount of force separates 

 them from one another, and it can readily be made out with 

 the aid of a magnifying glass that they are bound together 

 by extremely delicate oblique filaments, the barbuks, having 

 the same general relation to the barbs as the barbs themselves 

 to the rachis. 



Adjacent barbules interlock by means of a system of min- 

 ule booklets and flanges, and in this way the parts of the 

 feather are so bound together that the entire structure offers 

 great resistance to the air. 



Among the contour feathers which form the main covering 

 of the Bird and have the structure just described, are found 

 filoplumes (Fig. 258, B), delicate hair-like feathers having a 

 long axis and a few barbs, devoid of locking apparatus, at the 

 distal end. Nestling Pigeons are covered with a temporary 

 investment of down feathers (C), in which also there is no 

 interlocking of the barbs : when these first appear each is 

 covered by a horny sheath like a glove-finger. 



Feathers, like scales, arise in the embryo from papillae 

 of the skin formed of derm with an epidermal covering. 

 The papilla becomes sunk in a sac, \^Q feather-follicle, from 

 which it subsequently protrudes as an elongated feather-germ, 

 its vascular dermal interior being the feather-pulp. The 

 horny substance of the feather is formed from the epidermis 

 of the feather germ. 



The feathers do not spring uniformly from the whole 

 surface of the body, but from certain defined areas (Fig. 259), 

 the feather tracts or pterylcz (sp.pt. hu.pt., &c.), separated 

 from one another by featherless spaces or apteria (v. apt, &c.) 

 from which only a few filoplumes grow. 



