62 STUDIES IN GENERAL PHYSIOLOGY 
from two windows the planes of which were at an angle of 
90° with each other, the paths taken by the larvee lay diago- 
nally between the two planes.’ When I placed the plate with 
the eggs in an absolutely dark room, the paths followed by 
the larve ran concentrically around the nest; the animals 
had scattered equally in all directions over the plate, but, 
contrary to the behavior of the animals in the light, which 
always moved as far as possible toward the room side, the 
circle in which the animals moved in the dark was very nar- 
row. They did not leave the glass plate. The constant 
intensity of the light acts, as in the case of the positively 
heliotropic animals, as a constant stimulus which causes the 
animals to move in one definite direction (either toward or 
away from the source of the light), until some other stimulus 
intervenes, which modifies or abolishes the effect of the light. 
In ny preliminary communication on animal heliotropism 
I mentioned an effect of light on fly larvee which I called a 
kind of anisotropy, and which I am at a loss how to in- 
clude under the other phenomena of heliotropism. The 
phenomenon under discussion appears only in intense light 
and in newly hatched or very young larve. The phenomenon 
consists in this, that the animals’ turn their ventral surfaces 
toward the source of light without placing their median 
plane in the direction of the rays. I have never seen this 
orientation in adult larve. When I put the animals into a 
test-tube placed with its longitudinal axis perpendicular to 
the window, and exposed them to the direct rays of sunlight 
or diffuse light close to the window, the animals left the 
lower side of the tube and moved to the upper. In this the 
animals, therefore, resembled positively heliotropic animals, 
and I might have believed that I was dealing with one of 
1This experiment was recently published by an American physiologist as a new 
discovery to prove that I had overlooked the importance of the intensity of light! 
[1903] 
2 When kept in a test-tube. [1903] 
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