94 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



portions which are to be cast. The claws of the reptiles and 

 the horny beaks of the turtles and birds are also cornifications 

 of the epidermis. 



Scales (upon the legs and feet) and claws of the same char- 

 acter (i.e., epidermal cornifications) reappear in the birds, but 

 dermal bones are never found. Birds have besides a peculiar 

 epidermal covering, the feathers, which must have more atten- 

 tion. In the following account it must be 

 borne in mind that in the development of 

 the feathers, as in the scale of a snake, a 

 dermal papilla takes the initiative, and that 

 the ultimate structure is entirely formed of 

 cornified epidermis. 



Feathers occur only in the group of 

 birds, and here three principal types are 

 found, down-feathers, pin-feathers, and 

 contour-feathers, differing much in ap- 

 pearance, but of essentially the same struc- 

 ture. Contour-feathers are those which 

 cover the body in the adult bird, giving it 

 its outlines, and forming the broad expanse 

 of wings and tail. In a typical contour- 

 feather are to be distinguished an axial 

 portion, composed of a proximal hollow 

 part, the quill, and a distal and more solid 

 shaft, the latter bearing on either side 

 lateral outgrowths, the barbs ; shaft and 

 barbs making up the vane. Inside the 

 quill occur thin structureless partitions, 

 the pith, while the shaft bears on its so- 

 called inferior surface a longitudinal 

 FIG. ioo. Contour- groove, the umbilicus. The barbs bear 

 feather. , quill; 4, shaft ; on their sides sma n er projections, the bar- 



c, barbs. . 



bules, which are usually provided with 



minute hooks ; these, interlocking with similar hooks on the 

 adjacent barbules, convert the whole vane into a continuous 

 sheet. In many cases a second or aftershaft joins the axis of 

 the feather on the lower surface near the Junction of the main 



