Coiiipfiisatorv Motions 



459 



a given rotation, unaftected by the position of the animal on the 

 turntable; when the rotation is clockwise, the spin is clockwise, 

 and vice versa; when the spin is to the right, the head of the frog 

 turns to the left, and vice versa. It is the "spin," therefore, that 

 determines the compensatory movement. 



To test the validity of this interpretation, it is necessary to 

 eliminate this factor from the rotation of the animal. For this 

 purpose an eccentric was arranged on the turntable in such a 



F,c2 



manner that the animal could be moved in a circle with its long 

 axis always parallel to its first position (Fig. 2). 



When the frog is rotated on this eccentric turntable, the re- 

 sponse takes the form of a pendulous movement of the head; 

 during one portion of the revolution the head is turned to the right, 

 and then the head is turned in the opposite direction. This back 

 and forth swinging of the head continues as long as the rotation 



