TREATMENT OF SYPHILIS WITH SALVARSAN 181 



with the parasites is brought about by certain 

 " chemoreceptors " of the parasites. The arsenic 

 compounds, for example, are anchored by arseno- 

 receptors. When a strain therefore becomes re- 

 sistant to the arsenicals one might suppose that 

 this was due to the parasite ridding itself of its 

 arseno-receptors ; similarly also with the other 

 classes of trypanocidal substances. But this appears 

 not to be the case, for, as we have already said, the 

 resistance is manifested only in the animal body, 

 and not in vitro. Ehrlich explains this by assuming 

 that in the resistant strain the affinity of that par- 

 ticular chemo-receptor has been reduced, so that 

 when the germicidal agent is introduced into an 

 animal infected with the resistant strain, the pro- 

 portion of distribution of the germicide is altered 

 in favor of the chemo-receptors of the organism. 

 In other words the organotropic affinity is greater 

 than the parasitotropic affinity. Had the parasite's 

 chemo-receptors quite disappeared there should 

 have been no difference in the resistance as mani- 

 fested in the animal and in vitro. 



In searching for germicidal substances whose 

 parasitotropic affinity should be great in comparison 

 to their organotropic affinity, Ehrlich made careful 

 pharmacological studies with each of the three 

 classes of trypanocidal substances already mentioned. 

 After testing homologues and substitution products 

 of almost every variety he finally concluded that 



