CHEMORECEPTORS 261 



infections, is usually insufficient in itself to successfully combat 

 the corresponding maladies, the attention of Ehrlich and his col- 

 laborators, and many other noted investigators, has within recent 

 years been largely centred upon these very infections. As a result 

 of the study of several thousand different products in regard to 

 their influence upon trypanosomes which are especially convenient 

 test objects in this respect, it has been found that there are after all 

 very few which can effect a cure in animals that have been infected 

 with the parasite in question, but these are well characterized chemi- 

 cally, and belong to three distinct groups. The first of these com- 

 prises certain arsenical preparations, notably arsenious acid, atoxyl 

 (arsanil), arsacetin, arsenophenyl glycin, and the dichlorhydrate of 

 dioxydiaminoarsenobenzol (popularly known as preparation No. 

 606), and in addition to these certain antimony preparations. The 

 second group is represented by certain azo dyes, such as trypan-red, 

 trypan-blue, and trypan-violet, while certain basic triphenylmethane 

 dyes, such as parafuchsin, methyl violet, pyronin, and others, belong 

 to the third order. 



Chemoreceptors. The study of these products in their behavior 

 to trypanosomes has led to a number of interesting discoveries. 

 While Ehrlich originally assumed that the so-called side chains of 

 the protoplasmic molecule only served purposes of nutrition, in 

 other words, that all receptors were essentially nutriceptors, and that 

 medicinal agents were not bound in this manner, he now holds that 

 receptors do exist by which such substances may be bound, and 

 terms these chemoreceptors. 



He suggests that the groups of the latter order in accordance with 

 their simpler functions are probably of a less complex structure, 

 that they are more firmly attached to the cell, and are hence less 

 readily cast off, and that as a consequence of their "sessile" character, 

 crystalline chemical substances are, generally speaking, incapable of 

 eliciting the liberation of corresponding antibodies. This conclusion 

 was based upon the following observation: 



If a given strain of trypanosomes is continuously treated with 

 chemotherapeutic agents belonging to one of the groups referred to 

 above, a race of organisms develops which can no longer be influenced 

 by that particular product and which is accordingly said to be "fast/' 

 in reference to that particular drug. It is interesting to note that 

 this acquired resistance or "fastness" is in a large measure specific. 



