32 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



the primitive type. The oldest fossil bird, Archceopteryx, had well 

 developed contour feathers. 



Except in the ostriches, penguins, and toucans, feathers are not 

 distributed everywhere on the surface of the body, but are gathered in 

 feather tracts (pterylae), separated by apteria in which no contour 

 feathers and but few down or hair feathers occur. These vary in their 

 arrangement in different groups of birds and are of systematic im- 

 portance (fig. 23). 



Complicated as they are, feathers are probably derived from scales, and the 

 section of lizard skin (fig. 17) might well represent an early stage in the develop- 

 ment of a feather. A down feather begins as a thickening of the corium, pushing 

 the epidermis before it. By continued growth this forms a long, finger-like papilla, 



FIG. 24. Stereogram of developing down feather, bv, blood-vessels entering pulp; 

 c, corium; ep, epidermis; /, feather follicle; p, pulp (mesenchyme) of developing feather; 

 per, periderm; r, rods of epidermis, which later dry, separate, and form the down. 



projecting from the skin. The corium extends into the outgrowth, carrying blood- 

 vessels with it, while an annular pit, the beginning of the feather follicle, forms 

 around the base of the papilla. Next, the corium or pulp of the distal part of the 

 papilla forms several longitudinal ridges (fig. 24) which gradually increase in 

 height, growing into the epidermis and pressing the Malpighian layer above them 

 against the periderm. As a result the stratum corneum is divided distally into a 

 number of slender rods arising from the base (quill), which at last are only held 

 together by the periderm. Then the pulp retracts, carrying with it the Mal- 

 pighian layer. With the blood supply removed, the epidermal parts dry rapidly, 

 the periderm ruptures, allowing the rods to separate to form the down. 



A contour feather has much the same development, differing in details, for an 

 account of which reference must be made to special papers. The ridges of the 

 corium are no longer longitudinal, but beginning on the dorsal side of the papilla, 

 run obliquely outward and downward (fig. 25) until they meet below. Thus 



