74 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



view and having regard further to the fact that Eeptiles are typi- 

 cally covered with a coating of scales, we may safely also accept the 

 view that feathers are to be looked upon as highly specialized and 

 modified scales. 



While the mode of development of the feather fully substantiates 

 this hypothesis, perhaps the most interesting point that emerges 



PlQ. 43. — Illustrating the neonyehium or claw-pad in the developing Bird. 

 (Prom Agar, 1909.) 



A, median longitudinal section through the claw of a chick of 19 days' incubation. B, claw of a 

 chick taken in the act of hatching. TIip neonyehium is seen beginning to break away from the rest of 

 the claw. C, section similar to A, hut from a chick 12 hours after hatching, c.p, claw-plate ; c.s, 

 sole of claw ; n, claw-pad (neonyehium). 



from its study is that the successive sets of feathers — the down 

 feathers of the nestling, and the annual or other sets of feathers in 

 the adult — are not to be looked on, as has been customary, as suc- 

 cessive series of independent individual feathers. On the contrary 

 the down feather and the definitive feathers, which succeed it in the 

 series of moults, are all simply portions of a single greatly elongated 

 and basally growing structure — the first down feather being its tip, 

 and the succeeding feathers being successive portions of it. The 

 moult consists not in the shedding of the whole feather but merely 

 in the breaking off of its projecting portion. 



Claws, which make their first appearance in Anura (Xenopus), 



