ON THE CUTANEOUS: PIGMENTARY SYSTEM OF THE. FROG 55 
the concentrating pigment, led me at first to suppose that this body was the 
attractive agent. I afterwards took pains to ascertain whether the nucleus 
always has this relation to the mass, and found that such is not the case. On 
October 22, 1857, I watched three adjacent cells during the process of post 
mortem concentration; in two of them the nucleus ultimately projected by 
about a quarter of its length at one side from the black spot, while in the other 
cell the aggregated molecules covered only one-third of the nucleus, so that 
no part of that body lay in the middle of the mass. The point to which the 
granules appear to havea special tendency is the middle of the body of the cell, 
which seems always to correspond with the centre of the disc of molecules, 
whereas the nucleus is often eccentrically placed in the cell. 
The diffusion of the molecules is not merely a passive result of the cessation 
of concentration, as has been hitherto supposed. In watching closely the occur- 
rence of the phenomenon, I have seen? the granules start off suddenly from 
the central mass, with a velocity which implied that they were under the in- 
fluence of forces very different from those which cause molecular movements 
in them when shed from their containing cells. That the process requires the 
vital forces of the cells to be in full operation is also proved by the fact that 
any agency, such as a galvanic shock, which temporarily paralyses their func- 
tions, arrests diffusion as well as concentration ; whereas, if the former were 
merely passive, it would take place as soon as the concentrating power was set 
at rest. 
I have already pointed out the sparsely scattered state of the granules 
in the central receptacles, compared with their accumulation in the branches 
of the offsets, in the fully diffused state shown in Fig. 3. This contrast is some- 
times much more striking, so that the bodies of the cells are almost colourless, 
and require some experience with the tissue in order to detect them. This 
indicates a special tendency on the part of the granules to leave the middle 
of the cell. Yet to however great a degree diffusion be carried, there always 
remain some molecules in the body of the cell uniformly distributed throughout 
its thickness and not attached to the parietes, as they would have been had 
their dispersion been caused by attraction on the part of the cell-wall. This 
disposition of the granules, which obtains even in the immediate vicinity 
of the nucleus, appears also distinct evidence against the operation of a central 
repulsive force ; for this would render the body and the adjoining parts of the 
processes as clear of pigment as the remote branches are made in concentration. 
* This was the view expressed in the paper as it was read. 
* This observation was made after the reading of the paper. 
