68 ASA A. SCHAEFFER. 



What is of considerable interest in the behavior of ameba to- 

 ward light is that the character of the response may vary rapidly. 

 See Figs. 22 to 36 ; 37 to 48 ; and a number of other experiments. 

 A negative reaction may be followed by a positive and vice versa. 

 There is no definite relationship between ameba and light, on 

 account of which the ameba is always either positive or negative 

 or indifferent. Stimulation from light produces the same general 

 character of reaction as stimulation from glass or carbon. The 

 only observable difference is a quantitative one; light beams are 

 sensed at a greater distance than particles of glass or carbon. 

 This difference may however be due to a difference in intensity 

 of the stimuli. 



Ameba reacts to dark spots in much the same way that it does 

 to beams of light. The reactions are either positive, negative or 

 indifferent. But they are negative in much the greater number 

 of cases. But no sooner does one observe the reactions of an 

 ameba to perpendicular beams of light and of darkness than the 

 question arises as to the transfer of the stimulus to the ameba as 

 well as the nature of it. How can an ameba sense a beam of 

 light or darkness which never comes nearer to it than 100 or 150 

 microns? It is possible that small particles suspended in the 

 water reflect light from a beam of light so as to reach the ameba 

 in much the same way that man can observe a beam of light in a 

 dark room because of the dust particles in the air. But if so 

 the ameba, being eyeless, is wonderfully sensitive to light. But 

 as to beams of darkness the case is entirely different. Is it con- 

 ceivable that an ameba can sense a beam of darkness at a distance 

 because not as much light is reflected from the particles in the 

 dark beam as from those more brightly illuminated surrounding 

 the beam? If one did not know of reactions to beams of dark- 

 ness, one might adopt the hypothesis of the reflection of light 

 from particles in the beam ; but since similar behavior is observed 

 toward beams of darkness, this explanation is obviously not the 

 right one. Some disturbance is created by the beams which is 

 then radially transmitted; so much is certain. But just what is 

 the nature of the disturbance is not clear. 



In a preceding paper (Schaeffer, 'i6<~), in which the reactions of 

 ameba to particles of glass, carbon, and similar materials were 



