ON THE CUTANEOUS PIGMENTARY SYSTEM OF THE FROG 53 
camera lucida,! so that accuracy of proportion is ensured. The circular black 
mass into which the colouring matter is now all collected, measures less across 
than either the length or breadth of the body of the cell in the diffused state 
of the pigment. Further, the mass is not spherical, but of flattened form, and 
its thickness is only about that of the central part of the cell in diffusion. This 
we know from the appearances presented by the spots of concentrated pigment 
in other cells seen edgewise, as is the case with some in Fig. 7, which represents 
the outline of the wall of a large blood-vessel, and the pigment contained in 
its external coat in nearly complete concentration. Hence it appears that 
all the pigment-granules contained in the body of the cell and the minutely 
ramifying processes in the diffused state, have been brought together into a space 
considerably less than was then occupied by the pale contents of the body of 
the cell alone. The coloured particles have been concentrated into a dense 
disciform mass, but the fluid in which they were suspended has been left 
behind. 
Fig. 4 shows the pigment in the same cells as Fig. 3 in an intermediate stage, 
in which the process of concentration is about half accomplished ; the upper 
one being in the condition which would appear stellate under a low magnifying 
power. The greater part of the pigment is collected in the bodies of the cells, 
especially towards their central parts: in the middle of each dark mass, how- 
ever, 1s a pale spot, doubtless due to the circumstance of the granules not having 
yet insinuated themselves between the cell-wall and the nucleus, which, as shown 
above, probably lies in contact with it. This appearance of pale central points 
was very general in the web at the time when Fig. 4 was drawn, but gradually 
disappeared as the aggregation of the pigment-molecules proceeded, and does not 
exist in Fig. 5, which represents the lower of the two cells in a more advanced 
state of concentration. The remote branches of the processes were then for the 
most part invisible, and those which did appear were generally pale, instead ~ 
of dark, as they had been during full diffusion. This difference does not depend 
on contraction of the branches, but on the granules being absent from them, 
or sparsely scattered instead of closely packed; and I have often ascertained 
from some granules remaining widely separated in a process, that it was of 
large calibre, though, without careful searching, it would have seemed invisible 
Even in Fig. 6 concentration is not represented absolutely perfect ; for a few 
molecules are to be observed near the black mass in the more circumferential 
parts of the body of the cell. The extreme delicacy of the cell-wall makes it 
very difficult to trace it among the surrounding tissues, and I have not attempted 
1 All the drawings in the plate which accompany this paper were made with the assistance of 
this very valuable instrument. 
