420 STUDIES IN GENERAL PHYSIOLOGY 
be brought about, and it is therefore possible that the posi- 
tive heliotropism in both cases is determined by the same 
chemical conditions. It must be left for further experiment 
to decide this point. 
XII. ON CHANGES IN PIGMENT CELLS IN LACK OF OXYGEN 
It is a definitely established fact that the pigment cells in 
the skin of the frog become lighter after death. This 
lightening is brought about, as Biedermann has found,’ by 
the fact that the coloring matter collects into small clumps. 
A piece of the skin which has been deprived of its circu- 
lation shows the same changes. 
In the transparent portions of the skin which can be studied 
microscopically —such, for example, as the web of the amputated 
foot of Rana temporaria—it can easily be seen how the much- 
branched pigment cells which follow the course of the capillaries 
gradually change their form, in that the coloring matter moves 
toward the center of the cell until finally all the pigment is col- 
lected into clumps (p. 175). 
Increase in the carbon dioxide cannot be the cause of 
this change in the pigment cells, for Biedermann found that 
the skin does not become lighter when the frog is poisoned 
with CO,. Biedermann believes that the cause is probably 
to be found in the decrease in the amount of oxygen. 
The surface of the yolk-sac of the Fundulus embryo is 
studded with a large number of black and reddish-yellow 
pigment cells, which are at first distributed irregularly, but 
which later, as I have shown,” are compelled to creep upon 
the blood-vessels and surround them. With this the first 
physiological cause was furnished for the marking of an 
animal. Since then other authors have also found that the 
course of the embryonic blood-vessels determines the mark- 
ing of the embryo. 
1 Pfliigers Archiv, Vol. LI, 
2 Journal of Morphology, 1893. 
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