ON THE CUTANEOUS PIGMENTARY SYSTEM OF THE FROG 49 
than elsewhere. The esculent frog exhibited similar phenomena, but was less 
sensitive. The concentrated state of the pigment is attributed by all the 
observers above named to contraction of the cells, while the diffused condition 
is supposed due to their relaxation. The contents of the cells are described 
as dark granules suspended in a fluid; and both von Wittich and Harless 
have distinctly seen the granules rolling along in the offsets during the process 
of concentration. All the authorities agree in the opinion that the fluid 
and granules move together from one part of the cell to another, the offsets 
being supposed empty of both when the pigment is accumulated in the body 
of the cell.’ 
In some respects the above description agrees with my own experience 
of the common frog of this country (Rana temporaria). I find that this well- 
known animal exhibits changes of hue almost as great as those of the chameleon, 
every specimen being capable of varying from a very pale to a very dark colour, 
the former being generally greenish yellow, but in some varieties reddish ; 
and the latter brownish black, or sometimes coal black ; while between these 
extremes any intermediate shade may be assumed. The depth of tint is 
generally proportioned to that of surrounding objects: thus a frog caught 
in a recess in a black rock was itself almost black ; but after it had been kept 
for about an hour on white flagstones in the sun, was found to be dusky yellow, 
with dark spots here and there. It was then placed again in the hollow of 
the rock, and in a quarter of an hour had resumed its former darkness. These 
effects are independent of changes of temperature ; for similar results may 
be obtained by placing a frog alternately in a vessel from which luminous rays 
are excluded, and in a white earthen jar covered with glass, in the same situa- 
tion. Different examples, however, differ much in their sensitiveness to light. 
A violent struggle on the part of the animal is often followed by a speedy 
alteration from a dark to a pale state of the skin. It seems very doubtful 
whether psychical excitement has anything to do with this occurrence, any 
more than with the arterial contraction which invariably takes place under 
1 From the way in which von Wittich alludes to Briicke’s description, it is clear that the latter 
supposed the cells to be contractile. Von Wittich himself in his first paper speaks of the movement 
of the pigment induced by galvanism as ‘ satisfactorily’ showing ‘that the stellate pigment-cells are 
contractile’. In his second paper (vide Miiller’s Archiv, 1854, p. 263), he expresses some doubt regard- 
ing the contractility of the ce//-wadl, but clearly speaks of the contents (fluid and granules) as moving 
together. Harless, after describing ‘the rolling of the pigment-molecules towards the centre of the 
cell’, goes on to say, ‘ that this rolling may be possible, there must be a fluid in the cells and offsets, 
to which the molecules owe their movement.’ He takes it for granted that the movement of the fluid 
must be due to some contractile agency, and as he finds no apparatus of this nature around the cells, 
and as the unstriped muscular fibres of the skin have no special relation to them, he infers that the 
cell-wall is itself contractile. 
LISTER I E 
