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ZOOLOGY: S. O. MAST 
or less, under others it requires several days. In general it is considerably 
longer for old specimens than for yoimg. It is much longer in individuals 
kept continuously on a given background than it is in those frequently 
changed from one kind to another. This is clearly shown by the follow- 
ing: An individual, after having been in a white granite pan continu- 
ously for two weeks and long since maximum white, was transferred to 
a black pan, August 18, 2.05 p.m. At 4.30 p.m. it was about one-half 
maximum black; August 19, 12 m. about three-fourths maximum black; 
August 22, 10 a.m. nearly maximum black; August 23, 10 a.m. maximum 
black. This same individual, after having been frequently transferred 
from white to black and vice versa, from August 23 to August 30, was 
taken from the white background where it was maximum white and put 
into the black pan at 7.27 a.m. One minute later, 7.28, it was already 
five-sixths maximum black, and after one minute more, 7.29, it was 
maximum black. The change from black to white however was never 
observed to be so rapid as this; in all of the experiments it required an 
hour or more. Thus while it required five days to produce a complete 
change in the skin from white to black after continuous sojourn of two 
weeks on white, it required only two minutes after repeated transfers 
from one to the other. 
Changes in color require, in general, much more time than changes 
in shade or changes in pattern. There is however much variation re- 
garding this among the different colors. Yellow for example, is a color 
that the fish assume much more readily and rapidly than green or blue. 
This may be due to the fact that yellow ordinarily predominates in their 
environment. 
3. Factors involved in adjustment to the background, a. Chromato- 
phores: In the skin of the flounders there are colored cells known as 
chroma tophores. Some of these are black and others are yellow of 
various shades. Associated with these there are other cells which con- 
tain numerous highly refractive crystals said to be guanin. These cells 
are called iridocytes. They appear pure white in reflected light. The 
pigment in all or in any group of the chromatophores can be concentrated 
in small globular masses, or it can be spread out so as to cover relatively 
large surfaces. Moreover, the iridocytes may assume such a position 
as to hide all or any portion of the pigment in the chromato- 
phores. Thus, changes in shade, color and pattern are produced by re- 
actions in these bodies. And these reactions are regulated by stimuli 
received through the eyes, as the following experimental observations 
demonstrate. 
b. Eyes: Specimens with the anterior end on white and the posterior 
