EXTERNAL FEATURES 27 



The most important general fact is that the feather is 

 altogether the result of a cornification of some of the cells 

 of the Malpighian layer, and that the dermic pulp is purely 

 nutritive. Like a reptilian scale, or a bird's scale, a feather 

 is entirely an epidermic product. 



The pulp and Malpighian mantle that formed a nesthng 

 down feather (Neossoptile) will go on to form a feather of 

 the full-grown type. Professor Newton writes : " 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." 



A papilla that is going to become a quill feather shows 

 two specially thick columns of cells on its dorsal and ventral 

 surface. The former become the shaft or rhachis, the 

 others the aftershaft or hyporhachis. " In fact," to quote 

 again from Newton's Dictionary, " 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 (barbs), 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." 



A second generation of feathers is formed from the 

 persisting follicles. A residue of pulp and Malpighian 

 mantle is called into activity by the physiological conditions 

 of a moult. The old feather is pushed out and another 

 takes its place, and so on time after time, for the follicle 

 has unlimited regenerative capacity. This has a further 

 adaptive value since it may be awakened by the accidental 

 loss of a feather. Professor Newton notes at the conclusion 

 of his article on Feathers, that the lo-feet tail feathers of 

 some Japanese cocks are evoked in some unknown way by 

 checking the moult and yet stimulating the growth. 



