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 iu diameter than those of any 

 dove which 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 diflBcult to explain. Ac- 

 cording to Post it does not take place until after coruification 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 pigmeut-cell processes in much the same way 

 that an amoeba engulfs particles of foreign substance. Against this hy- 

 pothesis Mertsching ('89) objected that the hair cells are motionless 

 and show no amoeboid 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 amoeboid 

 movements. 



Another explanation was suggested by Post ('94, p. 494), — that the 

 barbule cells of the feather fundament might i-eceive pigment by a pro- 

 cess of osmosis, which would sweep the pigment I'ods iu through pores 

 in the cell walls. " Auf diesen Befunden darf man schliessen, dass die 

 grossen Pigmentzellen ihr Pigment allmahlich in jene Nebenstrahlen- 

 zellen tiberfiihren, und dass diese letzteren erst auf einer gewissen Stufe 

 im Verhornungsprozesse das Pigment aufuehmen. Dieser Vorgang 

 dtirfte am einfachsten erklart werden durch die Annahme, dass die Ober- 

 flilche der verhornenden Zellen porose werde. Die Pigmentstabchen 

 werden vermoge des osmotischen Austausches in die Zellen eiugesch- 

 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 i-emain 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 

 witli 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 



