32 IRRITABILITY 



conditional metiiod of observation saves us from error, for it 

 would be wrong to place the conception of stimulus and vital 

 conditions in contrast to one another, one excluding the other. 

 On the other hand, this method of observation shows that the 

 stimuli are likewise only conditions, but conditions producing 

 certain changes in the vital system. If a stimulus acts, that is, 

 if there is any change whatever in the vital conditions, the whole 

 complex of life in consequence of the dependency of the con- 

 stituent parts upon each other is also changed, and a new state 

 of living substance occurs. Stimuli are, therefore, also only 

 vital conditions, but vital conditions for new vital manifestations. 

 The relation of one given state to another, forms an indispensable 

 point in the understanding of vital conditions as well as that of 

 the stimulus. The stimulus becomes a vital condition for the 

 new state which it produces. It is only a stimulus relatively to 

 the original state, which previously existed. The essential point, 

 therefore, in the conception of the stimulus is that of alteration. 

 An example will serve to make this clearer. If Amoeba Umax are 

 bred in a hay infusion they appear in countless masses. Observed 

 in water in a watch glass they show at first the well-known form 

 of Amoeba proteus with short, broad, lobate pseudopods. (Figure 

 1, A. ) After a period of rest, however, they gradually assume the 

 characteristic elongated Umax form. (Figure 1, B.) In this shape 

 they constantly move about. But if I add to the water only a 

 faint trace of diluted solution of caustic potash, the amoebae first 

 assume the shape of a ball (Figure 1, C), and then after a time, 

 stretch out long, pointed pseudopods, which give them the charac- 

 teristic form of Amoeba radiosa. (Figure 1, D and E.) They 

 remain permanently^ in this form. I have observed them for sev- 

 eral hours at a time. They move in the same manner as Amoeba 

 radiosa. They draw in one pseudopod, stretch out another and 

 float freely in the water in contrast to their Umax state, in which 

 they are always attached to some support. The long, pointed, 

 often threadlike pseudopods, yield to every movement of the 

 water, bending in consequence like whipcords. In this example 



1 Max Verworn : "Die polare Erregting der lebendigen Substanz durch den 

 galvanischen Strom." In Pflugers Archiv f. d. ges. Physiologie Bd. 65, 1896. 



