II HABITS AND NATURAL HISTORY OF THE FROG 61 



often be induced by rubbing the back and sides. If the 

 frogs are in a normal resting position, they frequently raise 

 themselves up on their legs and remain motionless and rigid 

 for some time. If when in this state they are laid on their 

 backs, the legs still retain the same attitude as before. 



Frogs may be awakened from their state of hypnosis by 

 any sudden stimulus, and their recovery is often immediate. 

 The duration of this state may be much prolonged if all 

 sensory impressions are so far as possible removed. 



REFERENCES 



See especially the works of Abbott, Allen, Boulenger, Brehm, Dume- 

 ril et Bibron, Diirigen, Fischer-Sigwart, Hay, Gadow, Leydig, Rosel 

 von Rosenhof, and Spallanzani cited in the references to the preceding 

 chapter. 



