STRONG: DEVELOPMENT OF COLOR IN DEFINITIVE FEATHER. 155 



The common condition of asymmetry in the vane, with the barbs on 

 one side of the rhacliis longer than those of the other side, causes the 

 point where the distal ends of the ridges meet to be more or less ;it 

 one side of the median plane of the feather-germ (Plate 9, Fig. 41, dst^.). 

 A conspicuous out-curving of the two sides of the feather funda- 

 ment at this point is seen in a wing-feather from the dove (Plate 9, 

 Fig. 42, dsL). 



The cylinder-cell layer, which forms a continuous sheet of cells 

 covering the ridge completely on the pulp side and between adjacent 

 ridges, takes no direct part in the formation of barb or barbule. These 

 are formed exclusively from the " intermediate cells," which constitute 

 the greater portion of the ridge. These intermediate cells become 

 differentiated into three parallel structures, an axial plate, longer in a 

 radial than in a tangential direction, and two lateral plates. A large 

 portion of the cells forming the axial plate are ultimately metamorphosed, 

 or fused together, t-o form the barb; the cells wliich compose the lateral 

 plates of the ridge, and which are separated from the furrows by the 

 cylinder-cells, are to be connected into barbules, whose attachment to 

 the barb will be near the inner or pulp margin of the axial plate. In 

 each I'idge one lateral plate will form the distal barbules and the other 

 the proximal barbules of a single barb. 



Davies ('89, Taf. 24, Fig. 19) described and figured clefts or spaces, 

 which he found occurring between the plates of barbule cells and the 

 cells forming the axial plate. He called these spaces " Langsfurchen," 

 a term which seems inappropriate for a fissure-like space, and especially 

 so in this case, because he uses the same word for the spaces that he 

 found between successive ridges. The latter could with some reason 

 be called furrows, but the spaces between the barbule rows and the 

 axial plate are nothing but artificial clefts. I have never found them 

 except in preparations that had experienced shrinkage in fixation. In 

 osmic material these clefts are altogether wanting, as are also the wide 

 V-shaped furrows whicli he described and figured as occurring between 

 ridges (Davies, '89, pp. 574-5 ; Figs. 17-19). 



The growth of the cells comprising the feather fundament and the 

 proliferation of cells at its l)asal, or proximal, end brings about a lon- 

 gitudinal growth of the feather germ, the sheath preventing lateral 

 expansion. 



Davies described this extension of the feather germ as due exclusively 

 to cell pi'oliferation at the base, ignoring the growth of the cells as a 

 factor. This is partly explained by his conception that there were 



