50 ON THE CUTANEOUS PIGMENTARY SYSTEM OF THE FROG 
such circumstances. Neither oil of turpentine nor galvanism, when applied 
to the integument, produces, so far as I have seen, any effect upon its colour ; 
our species being little influenced in this respect by direct irritation. I have, 
however, frequently observed, after forcibly pinching a dark web, that a pale 
ring, about one-sixteenth of an inch in breadth, has formed around the area so 
treated ; but this was very slow in appearing, being first noticed from half an 
hour to an hour after the pinch was given. 
The webs of the hind feet, examined under a low power of the microscope, 
exhibit differences in the distribution of the dark pigment? according to the tint 
of the skin, such as will be understood by referring to Plate III (p. 68), where 
Fig. i is from a dark portion of web, and Fig. 2 from a pale part in the same 
animal. In Fig. 2 the colouring matter is seen to be collected in black spots 
of irregular angular shape. This, however, is not the state which exists when 
the colour is palest, for then the masses of pigment are in the form of round 
dots, as in the part to the right in Fig. 1, Plate V (p. 274). Neither does Fig. 1 
of Plate III give the condition met with when the skin is darkest, in which case 
all that meets the eye on superficial observation is a reticular appearance, 
such as is represented in the stripe down the middle of Fig. 1, Plate V, and 
in the lower part of Fig. 2 in the same Plate. When the colour of the integu- 
ment is about medium, the pigment is disposed in a truly stellate manner, 
as on the left side of Fig. 1, Plate V. It may be convenient for the purposes 
of description, to designate these various states as respectively the dotted, 
angular, stellate, and reticular conditions of the pigment. 
When a higher magnifying power is applied in an extremely dark state of 
the skin, the chromatophorous cells, for such they seem to be, appear as depicted 
in Plate III, Fig. 3, where two of them are given, along with an adjacent capillary 
distended with blood-corpuscles. Each cell consists of a somewhat flattened 
central part with several irregular offsets, of considerable diameter near the 
central part, but speedily breaking up into small branches. The ultimate 
ramifications, some of which are of extreme minuteness, anastomose freely 
with one another and with those of neighbouring cells, constituting a very 
delicate and close-meshed network, which pervades the whole thickness of the 
‘ Other kinds of pigment are also present in the skin of the common frog, generally of yellow colour, 
but sometimes red. My attention has not been much directed to these, but I have noticed that they 
are contained in receptacles of the same general form and structure as those which hold dark pigment ; 
and on one occasion, since the reading of the paper, I observed the colouring matter disposed in a stellate 
manner with complex ramifications in one part of a web, and in another part collected into round spots ; 
implying that these cells possess the functions of concentration and diffusion of the pigment. They 
do not, however, always act in harmony with the dark cells ; and it is probably through their agency 
that changes in tint, such as I have seen to occur in one and the same frog, independent of mere lightness 
and darkness of shade, are produced. 
