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. 
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