REVERSION IN ORIENTATION TO LIGHT 387 



permeability, and that strong illumination, in which negative ori- 

 entation is usually found, causes decrease in permeability; and 

 according to McClendon ('17, p. 141), a considerable number of 

 investigators hold that anesthetics in low concentration stimu- 

 late. The concentrations of anesthetics that produce reversion, 

 therefore, probably cause increase in permeability, although Os- 

 terhout ('13) in his ingenious experiments did not discover any 

 such effect. We thus have considerable evidence in favor of the 

 idea that positive orientation is dependent upon increase in per- 

 meability. However, if this is true, then permeability ought, 

 under certain conditions, depend upon the time-rate of change 

 in illumination; it ought to be greater in old than in young colo- 

 nies and it ought, under certain conditions, be greater in intense 

 than in moderate illumination. Moreover, all conditions which 

 cause increase in permeability, e.g., NaCl, ought to produce posi- 

 tive orientation, while all those which cause decrease in permea- 

 bility ought to produce negative orientation. With these ques- 

 tions, concerning which there is at present no trustworthy 

 evidence, I hope to deal in the following paper in this series. 



Some investigators maintain that in many of the unicellular and 

 colonial forms orientation is due to a series of shock- reactions; 

 others maintain that there is no definite relation between shock- 

 reactions and orientation. However this may be, it is certain 

 that Euglena and Gonium and probably all other similar organ- 

 isms usually, if not always, respond with the shock-reaction to a 

 sudden increase in illumination if they are negative and to a 

 sudden decrease if they are positive. That is, the same reaction 

 may be induced either by a sudden increase or by a sudden de- 

 crease in illumination, depending upon whether the organisms 

 •are negative or positive. 



If shock-reactions are due to increase in permeability then 

 increase in permeability must be due, in negative specimens, to 

 increase and, in positive specimens, to decrease in illumination. 

 And if orientation is due to shock-reactions, then reversion in 

 orientation must be due to the phenomena which produce this 

 change in the cause of increase in permeability. In relatively 

 high temperature or moderate illumination and in solutions rela- 



