THE PARTIAL-FUNCTIONS OF CELLS. 689 



lecular p-aminophenylarsenoxid and 2. The further product, ob- 

 tained from the latter by reduction, the yellow diamidoarsenobenzol. 



In contrast to atoxyl, these substances exhibited marked try- 

 panocidal properties not only in the animal body but also in the 

 test tube. Thus a solution of the arsenoxid combination of a 

 strength of 1 to 1,000,000 killed the tryponasomes in an hour. The 

 closely related p-oxyphenylarsenoxid was still stronger killing in 

 1 to 10,000,000. 



This proved that the pentavalent arsenic residue possesses no 

 trypanocidal properties whatever; these are bound exclusively to 

 the trivalent, unsaturated form.' 



As long as sixty years ago, Bunsen, with extraordinary insight, 

 pointed out that cacodyl, the reduction product, is extremely 

 poisonous, while cacodylic acid is almost non-toxic. This gave him 

 the clue to the chemical character of the cacodyl combination. In 

 striking agreement with this is the fact that the unsaturated carbon 

 oxid, for example, and a number of other unsaturated combinations 

 arc so much more toxic that the corresponding saturated combina- 

 tions. We shall, therefore, have to assume that the arseno-receptor 

 of the cells is able to take up only the unsaturated arsenic residue, 

 i.e., the group possessing the greater combining affinity. 



With the aid of such reduced combinations it was simply a matter 

 to test the atoxyl strain in test-tube experiments. These showed 

 that the organisms could be killed with a suitable concentration of 

 the chemical substances, and that we were not dealing with a loss 

 of receptors as in the case of the relapse strain. A comparison, 

 however, of the lethal dose with the dose sufficient to kill the ordinary 

 strain, showed that the resistant strain required a much higher 

 concentration. Amounts which effected immediate destruction of 

 the ordinary strain did not in the least affect the vitality of the 

 resistant parasites, even after one hour. 



These test tube experiments seemed to indicate that the arseno- 

 receptor, while still preserved in the atoxyl-fast strain, had undergone 

 some modification so that its affinity had become lessened. This 

 manifests itself by the fact that it required much stronger solutions 

 to produce the poison concentration necessary to effect destruction 

 of the parasites; the normal arseno-receptor of the original strain, 

 by virtue of its higher affinity, takes up the same amount even from 

 more dilute solutions. 



We have succeeded in clearly demonstrating by biological methods 



