THE STRUCTURE OF A FEATHER 269 



mains tubular, there must be an orifice where the tube 

 and the sheet join. This orifice exists in every feather, 

 and is called the umbilicus ; it is usually choked up 

 by a tissue which will be noticed a little later. The 

 apex of the feather is formed first, and may be quite 

 complete while the base is still pulpy ; it becomes 

 gradually pushed upwards by the new growth at its 

 base. The outermost epidermic layer forms a cylin- 

 drical sheet enclosing the future feather ; it adheres 

 strongly to the barrel, but is free from the vane. When 

 the feather first appears above the surface it is 

 enclosed within this outer sheath, from the summit of 

 which a pencil-like bunch of barbs projects. The 

 sheath afterwards dries, and gradually crumbles away 

 from the top downwards, exposing the feathers. 



While the growth of the feather is in progress the 

 papilla is relatively large and highly vascular, but 

 shortly before the completion of the barrel, which is 

 the last part to be formed, the papilla begins to shrink. 

 During its retreat from the barrel the papilla leaves 

 behind it successive layers of dry and horny substance, 

 once charged with living protoplasm, and abounding 

 in vessels, but now shrunk to hollow capsules, super- 

 posed upon one another. These capsules form a chain, 

 which extends from the base of the barrel to the 

 umbilicus, and in young, unworn feathers may often 

 be seen to project through the umbilical orifice at the 

 base of the vane. At length the feather is completed, 

 and the formative papilla comes to rest. It will 

 however renew its activity periodically during the 

 whole life of the Bird, forming fresh feathers which 

 push out the old ones at the seasons of moult. The 



