396 



THE PIGEON. PHYLUM CHORDATA 



Each feather develops as an ectodermal papilla with a meso- 

 dermal plug (Fig. 312) ; the papilla grows at its base, and so 

 elongates to form a cyHnder ; this splits and unfolds to form 

 the rachis and vane ; a double spht at the base gives the after- 

 shaft, which in some birds such as the emu is greatly developed 



Fig. 311. — The structure of a feather.^ — From Young, The Life of Vertebrates, 

 1950. Clarendon Press, Oxford. — After Pycroft. 



/, Whole feather with a small portion as seen under a lens ; //, block section of barbs in the plane of the 

 barbules ; ///, one anterior and two posterior barbules ; B, barb ; Bs. barbule ; Bsa, anterior barbule ; 

 Bsp, posterior barbule ; c, calamus ; H, hamulus ; R, rachis ; r, ridge on posterior barbule which 

 interlocks with hamulus ; V, vane. 



so that the feather is double. Variations on the general plan of 

 development give different types of feathers ; the contour feathers 

 of the body have poorly developed barbules, so that the barbs 

 are easily separated ; scattered over the body are filoplumes, 

 in which a fine rachis has a small tuft of barbs at the apex, and 

 down feathers, which consist of a tuft of barbs with no rachis. 

 Feathers are shed or moulted and replaced at intervals ; most 

 birds have a single annual moult, in the autumn, but in some 

 there are two. 



