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, Fi(^ 

 18, pre.) which take a sinuous course among the cells of the axial plates 

 and at length approach the cells of the future barbules wiiich are to be 

 pigmented and in some way distribute pigment to tliem. The form of 

 these processes varies in the feather germs of different species. In Ster- 

 na hii-undo 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. pig.). 



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 Hermann's fluid and stained in iron haematoxylin there ai'e 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 verzwcigten Zellen aufgcspeicliert und wird erst all- 

 mahlich dorthin iibergefiihrt, ein Vorgang, der durch mechanische Mittel 

 wie den Wachstumsdruck der umgebenden Zellen, die wechselnde Blut- 

 fiille der Pulpa, Zugwirkung der Musculatur des Federbalges hinreichend 

 erkliirt 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 



