RHEOTROPISM 129 



demonstrated previously that the compensatory motions 

 of insects on a turntable stop when their eyes art; black- 

 ened. 311 ' Such forced motions, due to the influence of 

 the motion of the retina image, can be demonstrated in 

 the Calif ornian lizard Phrynosoma blainvilli, which is 

 an ideal object for such experiments 298 and in this animal 

 it is possible to separate these effects from the compensa- 

 tory motions caused by centrifugal force. 



It was accidentally observed by the writer that when 

 the lizard Phrynosoma is kept at the window of a moving 

 train with its eyes toward the window, a nystagmus of the 

 head of the lizard ensues, the head moving slowly in a 

 direction opposite to that of the moving train, as if to keep 

 its eyes fixed on the objects outside — telegraph poles and 

 trees, etc. The head moves until it is bent maximally, 

 when it is brought back into its normal position with a 

 quick jerky movement, and then follows again the appa- 

 rent motion of the objects outside, and so on. These nys- 

 tactic motions last for hours, in fact as long as the animal 

 is kept w r ith its head toward the window. As soon as it 

 is turned around so that it cannot see the objects outside, 

 the nystactic motions of the head cease. When the animal 

 is put on a turntable and rotated slow T ly, vigorous com- 

 pensatory movements can also be observed during rota- 

 tion. If, however, the eyes of the lizard are closed during 

 rotation these movements are considerably diminished 

 though they do not cease entirely. They are also consider- 

 ably diminished when the animal with its eyes open is 

 rotated on a turntable surrounded by a high gray cylinder 

 of cardboard which excludes the possibility of images 

 of outside objects moving on the retina. "We can also 

 produce compensatory motions of the head if the animal 

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