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, 1 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. 1 75). 



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 2 . 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, 2 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 PflUgers Archiv, Vol. LI. 



2 Journal of Morphology, 1893. 



