INTEGUMENT 



27 



down 1 becomes covered or replaced by the more complicated 

 definitive contour-feathers, the proximal barbs of which usually 

 still retain their down-like character. A contour-feather (penna) 

 at first closely resembles a down-feather, but in the course of 

 further growth, two adjacent rays become enlarged to form, with 

 the relatively longer or shorter quill, a main axis or stem (scapus) : 

 the part distal to the quill, to which the barbs are attached in 

 a double row opposite one another is called the shaft (rachis). 

 At the base of the quill is a small aperture, into which the 

 vascular papilla extends ; and a second very small aperture is 

 present at the junction of quill and shaft on the inner surface. 



St 



St 



A 



FIG. 21. THREE STAGES IN THE DEVELOPMENT OF THE EMBRYONIC DOWN 

 DIAGRAMMATIC (after Davies). A, B, IN TRANSVERSE, and C IN LONGI- 

 TUDINAL SECTION. 



F, dried remains of pulp; F.S, follicle; F.Sp, quill; P, pulp, with its ex- 

 tensions towards the feather sheath at t in A, which separate the developing 

 barbs (St) : these have become free in C. 



The barbs together constitute the vane (vrxillum), and the 

 barbules arise obliquely, in a double row on each barb, so as to have 

 relations to the latter similar to those of the barbs to the shaft. 

 On the barbules a double series of barbicels are developed, certain 

 of which may bear minute booklets which interlock with one 

 another, and so connect the barbs together into a continuous 

 sheet: this is particularly the case in the row of large wing- 

 feathers (remiges) on the fore-arm and manus and of tail-feathers 

 (rectriccs) on the rump or uropygium. 



In many Birds each quill of the ordinary feathers of the body 

 bears two vexilla, which may be equal in size (Cassowary); but 

 usually one, the aftershaft (hyporachis), is smaller than the main 

 shaft. 



1 Various modifications of the down feathers occur (e.g. Jiloplumes, which 

 by some zoologists are supposed to represent the last remains of a primitive 

 feather-covering from which both down-feathers and contour-feathers have 

 become differentiated). 



