FEA THERS 247 



off as scurf, it sets free the rami of the young product. While 

 the papilla grows to the length the young feather is to reach, the 

 cells of the middle stratum arrange themselves in longitudinal rows, 

 causing the pulp to assume in transverse section a somewhat star- 

 shaped appearance. These rows are transformed into the hair-like 

 rami of which most Neossoptiles consist, and their formation pro- 

 ceeds from the apex downwards, while the radii seem to be 

 produced by secondary splitting. Ultimately all the rami meet 

 at the base of the feather and there form, with the basal 

 portion of the sheath, a very short cylindrical tube, which is the 

 calamus of the Neossoptile, while the pulp having fulfilled its 

 function has withdrawn towards the base of the follicle, leaving 

 only its horny sheath in the form just above stated, its projecting 

 portion. 



The development of the feathers of adults is merely a con- 

 tinuation of the process now described, because each Teleoptile is 

 produced by the same pulp and Malpighian cells as gave rise to 

 its predecessor of the first generation. ' The short calamus of 

 a Neossoptile is not closed at its base, but is again split into a 

 number of columns of cells, which though not yet horny are the 

 tips of the rami of its successor. As a rule the whole follicle 

 sinks deeper into the skin, and thus comes to lie in a sort of 

 pocket, which, occasionally reaching the periosteum of underlying 

 bones, produces on the ulna the well-known roughnesses that 

 correspond with the number of cubital quills. 



Those papillae which give rise to the larger feathers, such as 

 the rectrices, become much thickened and greatly elongated, each 

 being surrounded by a horny sheath which peals off as the feather 

 attains maturity. A transverse section shews nearly the same 

 conditions as those of the growing Neossoptiles, but both on the 

 dorsal and ventral side are two columns longer than the rest, 

 and especially the two on the former which are transformed into 

 the rhachis, while the two on the latter give rise to the hypo- 

 rhachis. The intermediate columns and their secondarily -split 

 parts form the rami. A transverse section of the growing feathers 

 shews that both rhachis and hyporhachis pass into the calamus, 

 but that the former occupies by far the greater part of the ring, 

 or the whole of it when there is no hyporhachis. It is moreover 

 observable in an entire quill that the rami of both outer and inner 

 webs converge toward the ventral side and ultimately surround the 

 umbiliciform pit. In fact, the rhachis is only a vast elongation 

 and thickening of more than the dorsal half of the growing ^ 

 calamus which during its rapid increase carries with it most of the 

 rami, while only those nearest the ventral median line of the quill 

 remain in their original position, unless an hyporhachis be 

 developed as a ventral elongation of the calamus. The pulp 



