vi FORM AND FUNCTION 151 



pulp, as it is called, is the same in the nestling down 

 and the more lasting and stronger formation that 

 follows it. The change, therefore, bears no resem- 

 blance to the shedding of milk teeth and their re- 

 placement by permanent ones. The early tooth is 

 driven out by the later one ; the two are not in any 

 way connected. 



After the first moult, the feathers develop without 

 any nestling " downs " as precursors. Otherwise the 

 process is not, in essential points, different. The cells of 



,M.S 



^-SH 



AS 



Fig. 41. — (After Gadow). Showing development of feather. 



AS, cells forming after shaft ; b, cells forming barbs ; MS, cells forming main shaft ; 



sh, horny sheath surrounding whole feather. 



the papilla, or rather of the epidermis over it, arrange 

 themselves starwise. Two of the columns of cells 

 which cause this starlike formation grow broader and 

 longer than the rest, and go to make the rachis of the 

 feather. Two on the opposite side form a secondary 

 shaft, of which, as I have said, most feathers retain 

 some trace. At the same time there is a growth 

 inwards, so that in some cases the bone is reached. 

 On the ulna the marks of the great wing feathers are 

 easily discernible. 



The cap found on the top of young feathers is 

 formed from the outermost cells of the epidermis, the 



