STRONG: DEVELOPMENT OF COLOR IN DEFINITIVE FEATHER. 157 
The broad cells of the proximal barbules (rd. prz., 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 hooklets 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 their distal ends, 
which 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 Barbicels. 
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 are, except for a 
few of the most proximal, each provided with two distinct barbicels, — 
one ventral and one dorsal (Plate 5, Figs. 26, 27, drbc.). ‘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 “ hooklets” or “ hamuli” (haml.). 
