STRONG: DEVELOPMENT OF COLOR IN DEFINITIVE FEATHER. 169 
suggest a simple mechanical cause (Plate 3, Figs. 17, 18, and Plate 7, 
Fig. 38). They are more uniform in diameter than those of any 
dove whith I have observed, and they frequently branch in a manner 
that is very characteristic of chromatophores, whose processes are un- 
questionably the result of cell outgrowths. 
The transfer of the pigment granules contained in the processes of the 
pigment cells to the barbule cells is even more difficult to explain. Ac- 
cording to Post it does not take place until after cornification has 
begun. 
Riehl (84) thought that in the case of the pigmentation of hair, the 
cornifying cortex cells of the hair might take up the pigment granules 
brought to them by the pigment-cell processes in much the same way 
that an ameba engulfs particles of foreign substance. Against this hy- 
pothesis Mertsching (’89) objected that the hair cells are motionless 
and show no ameboid movements. I have found that the form of the 
barbule cells when they receive pigment is conspicuously uniform and 
constant (Figs. 17, 18, and 19, ser. cl.), with no suggestion of amceboid 
movements. 
Another explanation was suggested by Post (94, p. 494), — that the 
barbule cells of the feather fundament might receive pigment by a pro- 
cess of osmosis, which would sweep the pigment rods in through pores 
in the cell walls. ‘“‘ Auf diesen Befunden darf man schliessen, dass die 
grossen Pigmentzellen ihr Pigment allmdhlich in jene Nebenstrahlen- 
zellen iiberfiihren, und dass diese letzteren erst auf einer gewissen Stufe 
im Verhornungsprozesse das Pigment aufnehmen. JDieser Vorgang 
diirfte am einfachsten erklart werden durch die Annahme, dass die Ober- 
fliche der verhornenden Zellen porése werde. Die Pigmentstabchen 
werden vermédge des osmotischen Austausches in die Zellen eingesch- 
wemmt und in den Maschen des Protoplasmas festgehalten.” 
In Sterna, the pigment-cell processes come in contact with the bar- 
bule cells (Figs. 17, 18, 19, and 36) on their dorsal margins; at such 
points pigment rods are found in the cytoplasm of the barbule cells, 
mostly dorsal to the nucleus, where they remain permanently. The 
barbule cells of other birds, so far as I have observed, are supplied with 
melanin in a similar way, but they may have their cytoplasm packed 
with pigment on all sides of the nucleus. The pigment-cell processes 
may branch so as to supply a group of barbule cells, as is shown in Fig- 
ure 38 (Plate 7) for the Indigo bird, Passerina cyanea. 
A question naturally arises as to the factors which determine the 
direction taken by the pigment-cell processes and cause them to go to the 
