348 BACTERIOLOGY. 



the usual test animals, of no appreciable diphtheria 

 toxin. Between bacilli which produce a great deal 

 of toxin and those which apparently produce none 

 we find all grades of virulence. We believe, there- 

 fore, that in accordance with Roiix and Yersiri these 

 bacilli should be considered as attenuated varieties 

 of the diphtheria bacillus which have lost their power 

 to produce diphtheria toxin. These observers, and 

 others following them, have shown that the virulent 

 bacilli can be artificially attenuated by cultivating them 

 at a temperature of 39.5 to 40 C. in a current of air. 

 So far as we know, bacilli which produce no specific 

 toxin have never later been found to develop it. In our 

 experience some cultures hold their virulence even when 

 grown at 41 C. for a number of months, while others 

 lose it more quickly. Bacilli are also found which 

 resemble diphtheria bacilli very closely except in toxin 

 production, but differ in one or more particulars. Both 

 these and the characteristic non virulent bacilli are found 

 occasionally upon all the mucous membranes, both when 

 inflamed and when apparently normal. From varieties 

 of this sort having been found in a number of .cases 

 of the condition known as xerosis conjunctivas by 

 Kuschbert and Neisser, these bacilli are often called 

 xerosis bacilli. Under this name different observers 

 have placed bacilli identical with the diphtheria bacilli 

 and others differing quite markedly from them. Fig. 

 43, though taken from virulent bacilli, gives an exact 

 picture of many of the xerosis variety. These bacilli 

 may be almost non-pathogenic in guinea-pigs, or they 

 may kill, as we have found in a number of instances, 

 in doses of 2 to 5 c.c. hypodermatically injected. Ani- 

 mals are not protected by diphtheria antitoxin from 



