54 ON THE CUTANEOUS PIGMENTARY SYSTEM OF THE FROG 
to give it in these figures, which, it must be clearly borne in mind, represent 
only the colouring matter. The external parts of the body of the cell and the 
principal processes may, however, be sometimes discovered, though perfectly 
colourless in consequence of concentration: they are then found to be of the 
usual dimensions met with in full diffusion, showing that they are still full of 
fluid though destitute of granules. In fact the only change of form to which 
the cells appear liable is a slight bulging of the central part at the seat of the 
black mass in the concentrated state, which I have detected in some cases by 
camera-lucida sketching, and which is consistent with the separation of the 
cell-wall from the nucleus, implied by the ultimate disappearance of the central 
pale points of Fig. 4. 
The movement of the granules towards the centres of the cells may be seen 
without any great difficulty. The death of a healthy frog is always followed 
by complete concentration of the pigment for a time, however much diffused 
it may have previously been, and the process taking place gradually, its progress 
can be observed. Ifa frog with the skin dark, and the pigment therefore diffused, 
be killed and the web examined soon with a good glass of high power, the 
granules may be seen distinctly moving along the offsets of each cell to join 
the dark mass which is becoming accumulated in the central part. If the 
process is going on languidly, the individual molecules advance slowly with 
slightly dancing movements, indicating that they are free in the fluid and 
not confined in any way to the cell-wall. If concentration is taking place 
more speedily, the granules rush along so quickly that no time is allowed 
for observing their molecular movements, and often their motion is so rapid 
as to elude the eye altogether. In one instance a large-sized offset, which 
at first contained abundance of pigment, became gradually cleared in this 
way of its colouring matter without any change in its dimensions, till it was 
almost invisible on account of the very small number of molecules remaining 
in it. 
It is thus a matter of direct observation, that the pigment-granules move 
along into the bodies of the cells during concentration, and leave colourless fluid 
behind them in the processes. It is clear that their motion cannot be explained 
by currents in the fluid; for streams proceeding towards the centre of a cell 
would necessarily be accompanied by a returning flow in the opposite direction, 
which would carry the pigment with it unless the molecules had a special 
tendency towards the centre. The circular form assumed by the mass of pig- 
ment when concentration is complete is strongly suggestive of a central attractive 
force acting on the granules. The occurrence of the central pale points, which 
are represented in Fig. 4, showing that the nucleus was there in the middle of 
