strong: development of color in definitive feather. 157 



The broad cells of the proximal barbules {brh. prx., Plate 5, Fig. 23) 

 undergo a special metamorphosis, in which their dorsal margins are 

 bent over and inwards towards the axial plate to form the well-known 

 recurved margin (Fig. 25, marg.) to which the booklets of the distal 

 barbules are ultimately to secure attachment. 



It should be noticed here that the barbule fundaments are not cut 

 exactly at right angles by cross-sections, but somewhat obliquely, 

 especially in their broad proximal portions. 



At a very early stage in the differentiation of the barbules, the barbule 

 columns lie in the plane of a radius of the feather germ (Plate 3, 

 Fig. 16, ser. cL). They also make an angle of over 60° with the 

 long axis of the feather germ. With the growth of the cells composing 

 the barbule fundaments, this angle becomes smaller and smaller, while 

 the distal, attenuated portion comes to lie nearly parallel with the axis 

 of the feather germ. 



The surface made by the barbule fundaments collectively undergoes 

 a bending, which is clearly seen to increase steadily from the stage 

 shown in Figure 16 to that of Figure 20, ser. cl. This, I think, is 

 brought about partly by the great increase in the size of the ridges near 

 their attachment to the rhachis, at the expense of tkeir distal ends, 

 wiiich lie farther away from the rhachis. It results from the fact that 

 the barbules will be largest at the proximal ends of the barbs and will 

 gradually decrease in size towards the distal ends of the latter. A 

 cross-section at a point where the ridges are first differentiated does not 

 show so great a contrast in size between sections of ridges near the shaft 

 and those on the ventral side. This increase in size must be accom- 

 panied by lateral displacement, which would account for the gradual in- 

 crease in the curvature of the rows of cells representing the barbules. 



2. The Barblcels. 



The barbicels arise as one or two processes of single barbule cells at a 

 comparatively late stage in the development of the barbule. The bar- 

 bicel appears first as a thick blunt projection of the cell (Plate 5, Fig, 

 27, brbc); its final form is not attained until the end of cornification. 



The cells of the distal halves of the distal barbules arc, except for a 

 few of the most proximal, each provided with two distinct barbicels, — 

 one ventral and one dorsal (Plate 5, Figs. 26, 27, brbc). Of these the 

 ventral is the longer. Towards the middle of the barbule the ventral 

 barbicels are of considerable size, and they are more or less recurved at 

 their distal ends to form the so-called " booklets" or "hamuli " (haml.). 



