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 larvse lay diago- 

 nally between the two planes. 1 When I placed the plate with 

 the eggs in an absolutely dark room, the paths followed by 

 the larva? 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, w r hich modifies or abolishes the effect of the light. 

 In my preliminary communication on animal heliotropisrn 

 I mentioned an effect of light on fly larvae 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 iieirly hatched or very //<>i<it<{ larvce. The phenomenon 

 consists in this, that the animals 2 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 larvaB. 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 



1 This 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! 

 I 1903 J 



2 When kept in a test-tube. [1903] 



