96 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



ing the pulp of the future quill, while the epidermis surrounds the 

 outgrowth. A circular depression around the base of the papilla 

 is the beginning of the formation of the future feather follicle. 

 In the distal portions of this outgrowth there next appear longi- 



M -m* 



FlG. 103. Two stages in the development of a feather, after Davies. b, basal 

 layer of epidermis; d, derma; t, epitrichium ; /, pulp; x, beginning of depression 

 for feather follicle. 



tudinal ridges of the pulp which gradually encroach upon the 

 epidermis, dividing this layer into a series of cylindrical rods 

 (Fig. 104), which at last are held in position by only the layer 

 of epitrichium. Now the derma retracts into the feather follicle, 



carrying with it the basal layer of 

 the epidermis, so that there re- 

 mains a hollow epidermal out- 

 growth, the quill, bearing at its 

 extremity a number of epidermal 

 rods. The cells of these portions 

 rapidly dry and become cornified, 

 and, the epitrichium breaking 

 away, the rods separate as the 

 down of the down-feather. 



Later the contour-feathers are 

 developed from the retracted pulp 

 which grows out again as before. 

 In general these develop like 

 their predecessors, excepting in 

 certain details. The rods of pulp 

 are not longitudinal, but oblique 

 to the axis of the outgrowth, the 

 result being that the cornified rods (which form the barbs) 

 proceed from an undivided portion (shaft) on the dorsal side of 

 the outgrowth ; and when the epitrichium breaks away, these 



FIG. 104. Transverse section of 

 developing down-feather of tern 

 (Sterna wilsoni). b, basal layer of 

 epidermis; fas, blood-vessels; e, epi- 

 trichium ; ep, epidermis ; /, pulp ; r, 

 ridges of pulp extending into epi- 

 dermis. 



