52 ON THE CUTANEOUS PIGMENTARY SYSTEM OF THE FROG 
pended in a colourless fluid, in which I have often seen them moving freely : 
when in considerable mass they produce a jet-black appearance, but exhibit 
a brown tint when present only in small quantity. 
When the skin of the animal is very pale, the colouring matter is all 
accumulated in the central parts of the cells. With regard to the method in 
which this change is effected, I am compelled to differ altogether from the 
before-mentioned authorities, who suppose that the granules and fluid are 
together forced by contraction from the processes into the bodies of the cells. 
They seem to take it for granted that the depth of tint of any one part of a cell 
depends simply upon the bulk of the contents situated there, and the consequent 
thickness of the coloured medium through which the light passes before reaching 
the eye. This, however, is by no means the case, as may be seen by referring 
again to Plate III, Fig. 3. The pigment is there represented fully diffused 
through the ramifications of the offsets, and some of the smallest of these are 
darker than the bodies of the cells and the adjoining broad parts of the pro- 
cesses; yet the former are far from being thicker than the latter: on the 
contrary, some of the branches, though conspicuous for their blackness, appear 
but as delicate lines which can be seen only at one focus when a glass of very 
high power is employed; while the bodies of the cells, as above mentioned, 
possess considerable thickness, and the processes are not flat, but subcylin- 
drical. But the differences in tint are sufficiently accounted for by the cir- 
cumstance that in the dark branches the colouring particles are closely packed 
together, whereas in the bodies of the cells and the paler parts of the offsets, 
the individual granules are separated from one another by considerable colour- 
less intervals. Hence it is clear that the degree of darkness of any part of a cell 
does not depend so much on the bulk of its contents in the aggregate, as on the 
proportion which the pigment molecules in it bear to the fluid in which they are 
suspended. 
If the whole contents of the processes were forced into the central parts 
during concentration of the pigment, and driven back again during diffusion, 
the bodies of the cells would be subject to great variations in capacity, becoming 
turgid in concentration and collapsed in diffusion; and the bulk of the 
central coloured mass would be great in the former case, but small in the latter. 
The very reverse, however, really takes place. Fig. 6 represents the appearance 
of the pigment in a concentrated condition, in one of the same cells which in 
Fig. 3 show it in full diffusion. During the time in which this change took place, 
the adjacent capillary had shrunk to about half its former size, but it will be 
recognized by its general form, and will indicate which of the two cells is that 
under consideration. Both the figures were drawn on the same scale with the 
