36 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



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

 their arrangement in different groups of birds and are of systematic 

 importance (fig. 26). 



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

 section of lizard skin (fig. 20) might well represent an early stage in the develop- 

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

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

 papilla, 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, torms around the base of the papilla. Next, the corium or pulp of the 

 distal part of the papilla forms several longitudinal ridges (fig. 27) which gradu- 

 ally 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 



Fig. 27. — 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. 



at last are only held togeather by the periderm. Then the pulp retracts, carry- 

 ing with it the Malpighian layer. With the blood supply removed, the 

 epidermal parts dry rapidly, and the periderm ruptures, allowing the rods to 

 separate to from the down./ 



/ A contour feather h as 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. 28) until they meet below. Thus 

 there are formed a series of rods set at an acute angle to the undivided dorsal 

 strip, the future shaft. When set free, as before, by the rupture of the periderm, 

 these rods straighten out, forming the vane. In the region of the shaft there are 

 two longitudinal ridges on the ventral side. These gradually roll together, thick- 

 ening and strengthening the shaft, the groove between them forming the um- 

 bilicus. As will be understood, the dorsal and ventral sides of the feather were 

 the outside and inside of the stratum corneum of the papilla. 



