PRINCIPLES OF CHEMOTHERAPY 



847 



ical substances, including salvarsan, given by intramuscular injection, 

 in spirillosis of fowls, and illustrates the particular value of "606," as 

 dependent upon the fact that it is highly parasitotropic in an amount 

 that is far below the organotropic or toxic dose : 



Chemoreceptors. Of considerable interest in this connection is the 

 question of how substances can be modified in order to render them pro- 

 gressively more parasitotropic and less organotropic in action. When 

 ordinary germicides, such as phenol, mercury bichlorid, formalin, etc., 

 are added to suspensions of bacteria, we believe that the latter are killed 

 mainly as the result of poisoning of their protoplasm, and that the 

 chemical substance enters into chemical union with the bacterial al- 

 bumins by direct toxic action, accompanied by such physical changes as 

 that of coagulation, and alters bacterial metabolism and brings about 

 death of the cells. Such germicides do not appear to exert any selective 

 action on any particular albumin. When mercury bichlorid is added 

 to a mixture of bacteria in a serum, many of the microorganisms may 

 escape destruction through the formation of protective envelops of an 

 albuminate of mercury formed with the serum albumins; a similar 

 action may be noted with phenol. 



In chemotherapeutic research, therefore, it is the aim to start with a 

 substance that primarily shows a more marked affinity for the proto- 

 plasm of the parasite than it does for the body-cells, and then, by sub- 

 tracting or adding to the molecule or by inducing an intramolecular re- 

 arrangement, an effort is made to develop its parasitotropic action. 

 The question then arises, does the increased parasitotropism of the 

 chemical substance depend upon its higher direct and simple action 

 upon the protoplasm of the microparasite, or is this effect to be explained 

 upon its increased combining power or affinity for the molecules of the 

 parasites or other cells because the latter are provided with special groups 

 for effecting the union? Ehrlich has endeavored to answer this question 

 by maintaining that both body-cells and microparasites possess special 

 receptors or side-arms by which chemical substances may be bound; 



