FROM THE SIDES OF THE NECK OF THE DOMESTIC PIGEON. 267 



On plucking a single feather, one finds that the distal portion only (Fig. 1, dst'.) 

 gives metallic colors and only from the dorsal surface. The remainder of the feather 

 including the whole ventral surface gives dull grays and browns. It is furthermore 

 very noticeable that the transition from the region of metallic colors to that of non- 

 metallic colors is abrupt. 



An examination of the barbs shows that they do not differ essentially in either 

 case. The barbules, however, present very great differences. It will be seen in 

 Figure 1 that a single barb may have its distal portion in the region of metallic colors 

 and the proximal part in the region where metallic colors do not occur. The bar- 

 bules of both the distal and the proximal rows at the proximal end of the barb have 

 the usual typical form for those of body coverts. They are flattened in their prox- 

 imal portions and have long distal attenuated portions (Figs. 3, 5, 8). The distal 

 barbules (Fig. 3, brbl. dst, Fig. 5) are provided with barbicels, and the proximal bar- 

 bules (Fig. 3, brbl. prx., Fig. 8) have the usual recurved margin for the attachment 

 of the hooklets of distal barbules. There is a comparatively sparse pigmentation 

 with typical rod-shaped granules of melanin. It will be noticed that this pigmen- 

 tation is in patches (Figs. 7, 9, mac. pig.) for all the barbules figured. The clear in- 

 tervals between the patches are the positions of inter-cell boundaries (Figs. 7, 10, 13, 

 cV. int.), a barbule being composed of a chain of cells placed end to end. The ventral 

 parts of the barbules are thinner than the dorsal and have little pigment. 



Both the distal and proximal rows of barbules are arranged so that their broad 

 sides are nearly perpendicular to the plane of the feather and are, therefore, barely 

 visible in a dorsal view. There are comparatively wide spaces between the 

 barbules, 



2. Modifications in Structure Associated with Metallic Colors. — I have shown in 

 Figure 2 a portion of a barb and its barbules from the region of transition. It 

 will be observed that there is a rapid modification in the form of the barbules in a 

 comparatively short distance, correlated with the transition in color. The attenu- 

 ated portion of both distal and proximal barbules are reduced and the barbicels are 

 mostly absent. The broad flattened portions of both rows of barbules are lengthened 

 at the same time. 



In the ontogeny of these barbules, the cells composing the distal parts are not 

 differentiated into a long attenuated extremity as is usual (Strong, :02 a ). They 

 simply take the course of differentiation followed by the more proximal cells and 

 contribute to a flattened structure. There is then a loss rather than an increase 

 in differentiation. (Compare Figs. 5-10.) 



Accompanying the changes just described is a great increase in pigmentation 



