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. I shall 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. 



III. 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. Con- 

 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- 



