268 ROUND THE YEAR 



and nerves. Both layers contribute to the formation 

 of the feathers, but the epidermis alone furnishes the 

 formative cells, while the vessels of the dermis bring 

 nutritive substance for the supply of the rapidly 

 multiplying epidermis cells. The first stage in the 

 development of a feather shows a conical elevation of 

 the epidermis, within which the dermis forms a papilla 

 of similar form. As the papilla increases in height, its 

 base becomes sunk to a corresponding extent beneath 

 the general surface of the skin, thus obtaining pro- 

 tection against friction, which would be injurious to a 

 slender column of rapidly growing cells. The 

 epidermic sheath which encloses the dermal papilla 

 increases rapidly in thickness, and the cells arrange 

 themselves in three layers, of which the middle one is 

 much the thickest. After a time the middle layer 

 thins out along one side of the papilla, corresponding 

 to the future inner side of the feather, while it grows 

 in thickness on the opposite side, where the shaft will 

 ultimately appear. 1 At length the line of weakness is 

 broken through, and the upper part of the tube is laid 

 open, forming henceforth a flattish sheet, which is the 

 vane of the feather. Meanwhile unequal deposition 

 of material has given rise to the barbs and barbules, 

 which are due to splitting of what was in a very early 

 stage a continuous conical sheath. The tubular 

 arrangement is retained in the lower part of the 

 feather, which forms the barrel. It is obvious that if a 

 tube is split open along part of its length and laid out as 

 a more or less flattened sheet, while the lower .part re- 



1 Where an after-shaft is to be formed two lines of weakness 

 and two thick ridges form. 



