170 bulletin: museum of comparative zoology. 



particular cells which are to be permaneutly pigmented. It seems not 

 impossible that a condition of chemotaxis exists between the cells which 

 are to receive pigment and the pigment-cell processes. 



A unique theory has been advanced by Kromayer ('97) for the cliro- 

 matophores of the frog's epidermis. He considers the chromatophore to 

 be something more than a simple cell ; it has a cell at its centi-e, but it 

 includes parts of numerous other ei^ithelial cells lying near it. It may 

 be that in the case of the feather we have an actual connection between 

 the pigment-producing cell and the cells which receive pigment. These 

 united cells might, for the time being, be considered an organ in the 

 sense of Kromayer's hypothesis. However, the short duration of such a 

 condition for any particular cell makes such an explanation improbable, 

 even if connection actually occurs. 



The pigmentation of the differeut cells in a barbule is accomplished 

 by a distribution of pigment rods, accompanying the growth of tlie pig- 

 ment cell processes, such that the more peripheral barbule cells receive 

 pigment later than those nearer the pulp. In the case of Sterna the 

 pigment found in the barb is the last to be distributed. 



As we have already seen, the barb develops much later than its bar- 

 bules, and with its differentiation the undifferentiated epithelial cells 

 near the basal membrane are shoved farther and farther inwards and 

 away from the barbule fundaments, as can be seen in transverse sections 

 (Plate 4, Figs. 19, 20, and 21). This separation breaks the continuity 

 of the pigment-cell process, and the main mass of the cell becomes 

 widely separated from the pigmented barbule cells. The pigment seen 

 in the dorsal cortex of the barb in Sterna (Plate 5, Fig. 24, ctx.) seems 

 to come from the more proximal portion of the pigment-cell process, 

 which is now some distance away from its original position. 



I have tried to determine whether all of the ' pigment borne in the 

 processes is taken up by cells of the feather germ, but though this is 

 probable, I am unable to state it positively. Neither can I deny that 

 there is a free formation of pigment in barbule cells independently of 

 that supplied by the pigment cells, as was supposed by Klee ('86). 

 However, I have not been able to discover any evidence of such a con- 

 dition, and the fact that there is a copious supply of pigment by the 

 pigment cells makes Klee's supposition improbable. 



It is interesting to note that the amount of melanin produced is not 

 always correlated with the darkness of the feather, even in the case of 

 simple pigment colors. If a preparation such as is shown in Figure 4 

 be examined under low magnification, we see, in the case of Sterna, a 



