THE INFLUENCE OF LIGHT ON ORGANS 435 



tain of this. The other possibility is that in the dark a 

 smaller number of pigment cells are formed. In embryos 

 which develop in the light toward the end of development 

 the pigment cells form a sheath around the blood-vessels. 

 When the eggs develop in the dark, only isolated chromato- 

 phores are found upon the blood-vessels ; the vessels are for 

 the most part free from pigment. 



I found no noticeable difference in the development of 

 pigment in the embryo itself. The pigment cells of the 

 retina, for example, developed apparently as numerously and 

 contained the same amount of coloring matter in the dark as 

 in the light. The yolk-sac alone showed the influence of 

 the light. 



