Brain PHYSIOLOGY OF WorRMS 355 
are put into a vessel having a cylindrical form, they do not 
collect as do purely heliotropic animals on the window or 
room side of the vessel but on the right and left sides of the 
same. The Planarians from which the head together with 
the brain and the eyes have been removed behave in exactly 
the same way. All these experiments are successful as early 
as the day after the operation. Only perfectly fresh material 
must be used for these experiments. In warm weather the 
posterior pieces regenerate a new head with eyes and probably 
a brain after a week. 
3. It was formerly the custom to conclude that an animal 
possessed eyes when it reacted to light. And since no one 
doubted that the reaction to light was a reflex movement it 
was assumed that such animals possessed a central nervous 
system also. When I found that the orientation of animals 
toward light is determined by the same conditions as the 
orientation of plant organs toward the same stimulus I drew 
the self-evident conclusion that the orientation of the animals 
toward the light could not possibly rest upon characteristics 
which are possessed only by the eyes or only by the brain, 
as plants do not possess such organs. The sensitiveness of 
the eye to light must rather rest upon the fact that the eyes 
have a condition in common with heliotropic plants, namely, 
elements which suffer some change under the influence of 
the light. For the rest, however, these elements need not 
be either morphologically or chemically identical in the dif- 
ferent organisms. An analogous conclusion may be drawn 
for the brain. When an animal with a brain shows the same 
reaction as a plant which possesses no brain it follows that 
the reaction toward light is not the result of a “specific 
energy” of the brain, but that the brain in that case only 
performs a function which is possible in plants without 
nerves. This function need be nothing else than the con- 
duction of the light stimulus, which, as is well known, occurs 
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