PROGRESS OF MICROSCOPICAL SCIENCE. 
169 
It was widely believed, indeed, that the skin of certain fishes took the 
colour of the bottom on which they lived ; but exaggeration often de- 
prived statements to this effect of their value. 
In 1830 certain experiments on the subject, bj Stark, were 
described in the 'Edinburgh New Philosophical Journal.' Putting 
fishes in vessels enclosed in dark or in bright cloth, he perceived that 
the colour of the animals changed in the same direction, becoming 
darker or brighter ; but he abstained from giving an opinion as to the 
internal conditions of production of this phenomenon. 
It has been shown by physiologists, further, that the colour of the 
frog's skin may be modified from various causes, section or stimulation 
of the nerves, various conditions of habitat in water or air, &c. ; but 
it was pretty generally agreed that these changes might be explained 
by disturbances in the circulation, due to the various modes of treat- 
ment, and bringing about in their turn a change in the state of 
dilatation or of contraction of the pigmentary cells. The special 
feature of M. Pouchet's experiments is that they show the pigmentary 
cells or chromoblasts to be in direct and immediate dependence on the 
nervous system; so that they must be added to the list of anatomical 
elements, in which nervous excitation is transformed into mechanical 
work. The nerves determine contractility of the chromoblasts, as 
well as that of striated fibres of voluntary muscles and fibre- cells of 
the muscles of vegetative life. 
The author first verifies the fact that certain kinds of fish, such as 
young turbots, placed successively in water on bright and dark bottoms, 
show very rapid changes of colour, or tone, produced by dilatation or 
contraction of the chromoblasts charged with dark pigment, more 
especially those having the role of changing to brown, or abating more 
or less the proper colouration of neighbouring parts. As there are 
also, however, contractile cells charged with coloured pigments vary- 
ing from red to yellow, it may happen that, by the state of relative 
contraction of these different elements, the shade of the animal may 
be modified in a certain measure. 
If in most of the species presenting these changes it is difficult to 
make out the influences which cause them, there are other species in 
which the determining conditions may be ascertained with ease. Let 
a turbot, measuring only twelve to fifteen centimeters rest for some 
minutes over a light bottom, such as one of sand, and it becomes pale 
in unison with the sand ; let it rest, on the other hand, over a rocky 
bottom, and it grows brown like that. One has only to contrast two 
animals placed under such conditions, to ascertain that the brightness 
of their colouration corresponds exactly to that presented by the 
colour of the two bottoms. We may thus produce indefinitely, in the 
same animal, a considerable change of colour, which does not require 
more than twenty to forty minutes for its production, and is sometimes 
much more rapid. 
M. Pouchet calls this power which the animal has, its chromatic 
function. It is subject, within variable limits (according to species), 
to the influence of the nervous system. The colour of several species 
of fish was observed to change when they were irritated, or on simple 
