STUDIES ON THE SERUM OF VACCINATED ANIMALS. 63 



reaction has gone too far. Carbolated thionin is a good stain to use. 

 The slide is fixed by heat and stained for one minute with thionin 

 and then differentiated in water until there is no longer any notice- 

 able diffusion of the stain (about ten minutes when there are many 

 blood corpuscles present or four or five minutes when clear serum 

 has been used). The granules stain a purplish blue; the red blood 

 corpuscles greenish. If the preparations are left too long in water, 

 the granulations will be completely decolorized. The formation of 

 thionin crystals may be avoided by drying the preparation with 

 filter paper. The preparations may also be stained with a con- 

 centrated aqueous solution of methylene blue or by a very dilute 

 solution of Ziehl's fuchsin. When the methylene-blue stain is used, 

 fixation by a saturated solution of picric acid is recommended, with 

 a counterstain of alcoholic eosin. 



This method is evidently only a modification or rather a simplifica- 

 tion of the method that Pfeiffer has described. It depends on the 

 same principle and its value is equally open to criticism. Are we 

 indeed sure of the diagnostic value of these results? Owing to the 

 specificity of bactericidal sera, it seems to us that if an organism 

 agrees culturally and gives this phenomenon with a true anticholera 

 serum we may identify it as a true cholera vibrio. But if, on 

 the other hand, we are dealing with an organism obtained from a case 

 that clinically resembles cholera, and this organism possesses the 

 morphological and cultural peculiarities of Koch's vibrio but fails 

 to give this reaction with cholera serum, we should not be author- 

 ized in saying, with our present knowledge, that the vibrio in ques- 

 tion is not identical with the cholera organism and that the case 

 is not one of true cholera. Different cholera vibrios from various 

 sources show variations in their resistance to serum. More serum 

 is necessary, for example, to transform the Saint-Cloud vibrio than 

 the vibrio from Eastern Prussia. Although the sera that one 

 employs are active, they have nevertheless their limitations. If 

 the Saint-Cloud organism had been slightly more resistant or the 

 serum we used slightly weaker, we should not have obtained the 

 phenomenon. It is far from being proved that there may not be 

 organisms, which, although essentially true cholera organisms and 

 of the same species as the Koch organism, may not have been 

 more highly differentiated as regards resistance to destructive 



