434 STUDIES IN GENERAL PHYSIOLOGY 
and growing stems are energetically positively heliotropic. 
Only that part of the stem immediately behind the polyp 
bends heliotropically. While these curvatures are usually 
produced in less than two hours in blue light, no curvature 
takes place in dark-red light even after two days; nor do 
the heliotropic curvatures appear when the polyps are cut 
off. Ishall return later to this and some other facts bearing 
on the theory of heliotropism. A stem of Eudendrium which 
is illuminated from one side only develops more polyps on the 
lighted side than on the shaded side—a thing which explains 
itself from the foregoing. 
Ill. EXPERIMENTS ON FUNDULUS EMBRYOS 
A large number of experiments on Fundulus embryos 
show that they develop as completely and as quickly in the 
dark as in the light; only the supply of oxygen must be the 
same in both cases. In one experiment the eggs were kept 
in the dark in a small, tightly closed vessel; those exposed to 
the light were kept in a large vessel; in this case the eggs 
developed more quickly in the light than in the dark. Ccn- 
trol experiments showed very clearly that it was not the 
light, but the better supply of oxygen to the vessel exposed 
to the light, which caused this difference in the development 
of the eggs. Only one constant difference exists between 
the eggs cultivated in the light and in the dark, and this 
concerns their color. As I have stated repeatedly, a large 
number of black and red chromatophores are formed in the 
membrane of the yolk-sac, which gradually creep upon the 
blood-vessels and surround them like a sheath. Since the 
number of these chromatophores progressively increases, the 
egg, if developed in the light, finally becomes very dark. In 
contrast to this, the eggs kept in the dark are very light and 
transparent. This difference may possibly be due to a con- 
traction of the chromatophores in the dark, but I am not cer- 
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