168 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
C. Tue DISTRIBUTION OF PIGMENT IN FEATHERS. 
When the pigment cells or chromatophores have reached the stage 
represented in Figure 35 (Plate 6), they send out processes (Plate 3, Fig. 
18, pre.) which take a sinuous course among the cells of the axial plates 
and at length approach the cells of the future barbules which are to be 
pigmented and in some way distribute pigment to them. The form of 
these processes varies in the feather germs of different species. In Ster- 
na hirundo they are especially regular and well defined. These pig- 
ment-cell processes usually branch one or more times, and they are 
frequently swollen or beaded at the points of branching (see Plate 7, 
Figure 38, cl. pzg.). 
I have studied many preparations to ascertain whether the cell 
wall of the pigment cells grows out in the form of a process the exist- 
ence of which can be shown by any other evidence than these rays 
of pigment granules. I have also endeavored to see whether there is 
a flow of pigment granules inside the process. In preparations fixed 
in Hermanun’s fluid and stained in iron haematoxylin there are fre- 
quently appearances suggesting the existence of regions in the processes 
which are not completely filled with pigment. In Figure 18 pre’. 
(Plate 3), I have shown such a condition, the process seeming to lack 
pigment granules for a short distance near its proximal end. This sup- 
position is further strengthened by the presence of a loose arrangement 
of the pigment rods at each end of the region apparently free from pig- 
ment, as though there were here a transition to the closely packed con- 
dition. Ordinarily the pigment process appears as a sinuous limb of 
the cell which contains pigment rods packed together so closely as to 
be indistinguishable from one another and gives no evidence of possess- 
ing an enclosing membrane. . 
Post (’94, p. 497) gave the following mechanical explanation for the 
production of these ramifications of feather pigment-cells. ‘ Bis diese 
Zellen [Barbule cells] zu verhornen beginnen, bleibt jenes vorrathige 
Pigment in den verzweigten Zellen aufgespeichert und wird erst all- 
mihlich dorthin iibergeftihrt, ein Vorgang, der durch mechanische Mittel 
wie den Wachstumsdruck der umgebenden Zellen, die wechselnde Blut- 
fille der Pulpa, Zugwirkung der Musculatur des Federbalges hinreichend 
erklart werden kann.” 
In the case of the dove, the pigment-cell processes are so irregular in 
form that it is easy to see how Post was led to such a conclusion. In 
Sterna and Cyanea, however, we have processes whose contour does not 
