THE PARTIAL-FUNCTIONS OF CELLS. 687 



itself give us an insight into the way in which this is brought about. 

 A large number of different explanations can be brought forward. 

 Xot until we can prove that we are dealing with a function which is 

 capable of being altered and varied in a specific manner is it possible 

 to regard the existence of preformed groups as demonstrated. 



Unfortunately it seems to be impossible to utilize the method 

 employed in demonstrating the preformed existence of nutri-recep- 

 tors, namely, by causing the liberated receptors to be thrust off 

 into the blood. The chemo-receptors appear to be much more 

 simply constituted, and remain attached to the cell, so that no 

 thrusting-off occurs. 



By indirect means, however, we succeeded in getting light on 

 this phase of the subject. With the aid of my esteemed collab- 

 orators, Franke, Browning and Rohl, I was able to show that it is 

 possible, by systematic treatment, to produce strains of trypano- 

 somes possessing immunity against the three trypanocidal poisons 

 now known to us. These poisons, it will be remembered are 

 (a) substances of the arsenic group, (b) fuchsin, and (c) the acid 

 azo dye known as trypanred belonging to the benzoburpurin series. 

 The immune strains are marked by tw r o characteristics: 



1. A stability of the acquired character. This is very great. 

 Thus our arsenic strain, after having been passed some 380 times 

 through mice in the course of two and one-half years, still possesses 

 the same drug immunity as the original strain. 



2. An essential feature of the immunity to drugs is the strict 

 specificity. This manifests itself by the fact that the immunity is 

 related not against a certain definite elementary combination, but 

 against the entire chemical group of which this combination is a 

 part. Thus the strain made immune against fuchsin is resistant 

 not only to that substance but also against a large number of related 

 triphenylmethane dyes, e. g., malachite green, ethyl green, hexae- 

 thyl violet. In contrast to this, however, the strain has remained 

 susceptible to the action of the two other types, i. e., against try- 

 panred and against an arsenical. A corresponding specific resistance 

 is exhibited by the strain made fast against trypanred and by the 

 one made fast against arsenic preparations. That we are here 

 dealing with three different functions is further shown by the fact 

 that by successive treatment of a given strain with the three sub- 

 stances mentioned above we can produce a strain which is resistant 

 against all three classes of substances, i. e., one which is triple fast. 



