§3] PROTOPLASMIC MOVEMENT 277 



absence of causes mechanically rupturing the machine, the 

 absence of agents of such intense activity as to change pro- 

 foundly its molecular constitution, and the presence of those 

 agents — food, heat, light, and water — which supply or dis- 

 tribute the energy of metabolism. 



(Jiven protoplasm under these conditions, and normal me- 

 tabolism must occur ; without them, there is no metabolism. 

 Vary the dynamical conditions quantitatively, and a quantita- 

 tive variation in metabolism will ensue. Approach a struct- 

 ural limiting condition, and metabolism begins to cease. 

 This conclusion, important for experimental morphology, is 

 now reached : A vital phenomenon occurring in a given proto- 

 plasmic mass can be reproduced only when the dynamical condi- 

 tions are reproduced, and the structural limiting conditions are 

 in no wise closely approached. 



§ 3. The Dependence of Protoplasmic Movement 

 UPON Metabolism and upon External Stimuli 



I do not propose to enter the debated ground of the cause 

 of protoplasmic motion; but shall merely summarize the re- 

 sults of our studies on this subject. First, protoplasmic 

 movement is closely related to metabolism and is probably 

 dependent upon it. This is indicated by the fact that ces- 

 sation of movement always occurs before the vital limit is 

 reached. Rigor always precedes death. A second series of 

 facts indicating the same thing is found in the closeness 

 with which the optimum for metabolism agrees with the 

 optimum for movement. Thus at about 35° C. both the 

 metabolic processes and the movements of protoplasm find 

 their optimum. These two results, then, that movement is 

 impossible in dead protoplasm even when its structure is 

 seemingly unaltered, and the close approximation of the 

 optimum points for metabolism and for movement, are the 

 best justification for the belief that movement is dependent 

 upon metabolism. 



But are the conditions essential to metabolism the only con- 

 ditions necessary for movement ? In other words, will move- 

 ment always accompany metabolism, or are external stimuli 



