172 S. O. MAST 



largely if not entirely independent of the amount of change in 

 illumination. Numerous observations were made on the extent 

 of the bending in the tail in response to direct sunlight flashed 

 on individuals in weak light in comparison with the extent of the 

 bending in the same individuals in response to diffuse sunlight. 

 The extent of bending could be fairly accurately ascertained by 

 noting the relation between the tail in various positions and 

 stationary particles of debris found in the solution. While there 

 was considerable variation in the magnitude of the reaction, no 

 consistent difference under the two conditions of illumination 

 was observed. Like results were obtained in observation on the 

 effect of light much lower than diffuse sunlight. The response to 

 increase in illumination is consequently of the ' all-or-none ' type. 



Increase in illumination is not an activating stimulus. It 

 does not cause quiet specimens to become active and, so far as 

 the evidence at hand indicates, it does not cause active speci- 

 mens to become more active. The tail apparently vibrates as 

 rapidly in the weakest light (white or red) in which it can be 

 seen as it does in direct sunlight. 



In conclusion, then, it may be said that increase in illumination 

 probably has little or no effect on the rate of movement in 

 Amaroucium tadpoles,^ that it inhibits the effect of decrease in 

 illumination in reference to direction of movement but not 

 in reference to rate of movement, and that it acts as a stimulus 

 causing increase in bending of the tail toward the abocular side. 



FUNCTION OF THE EYE 



If a number of tadpoles are resting on a horizontal glass sur- 

 face, it is found that in some the eye is so situated that light 

 from directly below enters the opening of the pigmented cup 

 and illuminates the ends of the nerves on the inner surface of 

 the cup, and that in others it is so situated that light from below 

 strikes the outside of the opaque cup and consequently does not 

 reach the nerve endings on the inside (fig. 1). Grave maintains^ 



^ All that this statement implies is that if the period of exposure is short, 

 the intensity of the light has little or no effect on the activity of the tadpoles of 

 Amaroucium. This is probably not true if the period of exposure is long. 



^ Personal communication. 



