170 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
particular cells which are to be permanently 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 chro- 
matophores of the frog’s epidermis. He considers the chromatophore to 
be something more than a simple cell; it has a cell at its centre, but it 
includes parts of numerous other epithelial 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 different cells in a barbule is accomplished 
by a distribution of pigment rods, accompanying the growth of the 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 
