XII PHYLUM CHORDATA 461 



small conical prolongation of the skin, the feather papilla. 

 A second extremely minute aperture, the superior umbilicus 

 {sup. limb), occurs at the junction of the quill with the vane 

 on the inner or ventral face of the feather, i.e., the face 

 adjacent to the body. 



The vane has a longitudinal axis or rachis (rch) contin- 

 uous proximally with the quill, but differing from the latter 

 in being solid. To each side of the rachis is attached a kind 

 of membrane forming the expanded part of the feather and 

 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 barbules, 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- 

 ute hooklets 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 cover- 

 ing of the bird and have the structure just described, are 

 found filoplumes (Fig. 274, 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 papilla? of 

 the skin formed of derm with an epidermal covering. The 

 papilla becomes sunk in a sac, the feather-follicle, from 



