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 Roux and Yersin 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 conjunctive 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 5c.c. hypodermatically injected. Ani- 
mals are not protected by diphtheria antitoxin from 
