56 ON THE CUTANEOUS PIGMENTARY SYSTEM OF THE FROG 
The hypothesis which would seem most consistent with the appearances 
described, is that of a mutual repulsion on the part of the pigment-granules, 
induced by some agency strongest at the centre of the cell and feeble in the 
remotest branches of the offsets. 
On October 27, 1857, I was observing a cell in which post mortem concen- 
tration had occurred, the pigment being in the angular condition. At one of 
the angles movements of the granules were going on, of which I will content 
myself with giving two examples. At one time a number of molecules started 
off together with great rapidity from the black mass, but stopped after having 
proceeded a certain distance, some of them remaining in their new position, 
while others returned at various rates towards the centre. At another time 
an individual granule moved slowly away for a little space, and then came 
back by a circuitous route to a different part of the mass from that which it 
had left. What I then saw has led me to believe that the movements of the 
pigment-molecules are of a complex character that will perhaps never be fully 
explained. In the meantime it is clear that concentration and diffusion are 
both active vital functions, and that both imply peculiar relations of the centre 
of each cell to the pigment-molecules, as distinguished from the fluid in which 
they are suspended. 
These conclusions invest the pigmentary changes with deep physiological 
interest. In the movements of the granules towards and from the centres of 
their containing cells, we now have ocular demonstration that a particular kind 
of material may have impressed upon it by vital action, independently of 
muscular contraction or ciliary motion, tendencies to rush energetically to or 
from certain fixed points in the tissues, through distances equal to nearly twice 
the thickness of a villus of the human intestine, and several times greater than 
the average breadth of a human capillary interspace. Whether we be able to 
explain the means by which such results are accomplished or not, it is obvious 
that forces of similar powers and range of operation, if suitably modified 
according to the circumstances of each case, would be more than adequate 
to cause the passage of particles of fat from the cavity of the intestine into 
the central lacteals of the vill, or the transit of the material required for 
a particular secretion or act of nutrition out of a capillary into a neighbouring 
gland cell or other portion of tissue; and, again, for the discharge of an elaborated 
product of secretion into a duct, or the return of waste matter into the blood- 
vessels or lymphatics. We thus obtain a basis of fact for what has hitherto 
been merely conjectural, in the explanation of the processes of absorption, 
secretion, and nutrition generally. 
The functions of the pigment-cells are under the control of the nervous 
