MODIFICATIONS OF BIOLOGICAL CHARACTERS. 129 



1 'The most important source of error, however, and one which 

 must be kept in view in future experiments, is the fact that a pro- 

 tective influence has been shown to result from the injection of virus 

 the virulence of which has been modified, without being entirely de- 

 stroyed, by the agent used as a disinfectant." 



' ' Sodium hyposulphite and alcohol were the chemical reagents 

 which produced the result noted in these experiments ; but it seems 

 probable that a variety of antiseptic substances will be found to be 

 equally effective when used in proper proportion. Subsequent ex- 

 periments have shown that neither of these agents is capable of de- 

 stroying the vitality of the septic micrococcus in the proportion used 

 (one per cent of sodium hyposulphite or one part of ninety -five-per- 

 cent alcohol to three parts of virus), and that both have a restraining 

 influence upon the development of this organism in culture fluids." ' 



Cultivation in the Blood of an Immune Animal. It has 

 been shown by the experiments of Ogata and Jasuhara that when 

 the anthrax bacillus is cultivated in the blood of an immune animal, 

 such as the dog or the white rat, its pathogenic power is modified 

 so that it no longer kills susceptible animals and may be used as a 

 vaccine. 



Pasteur had previously shown (1882) that the virus of rouget can 

 be attenuated by passing it through rabbits. 



Recovery of Virulence. Pasteur has shown that when the viru- 

 lence of a pathogenic organism has been modified it may be re- 

 stored by successive inoculations into susceptible animals. Thus in 

 the case of the anthrax bacillus a culture which would not kill an 

 adult guinea-pig may be inoculated into a very young animal of the 

 same species with a fatal result ; and by inoculating the blood of 

 this animal into another, and so on, the original virulence may be 

 restored, so that a culture is obtained which will kill a sheep. In 

 the same way the attenuated virus of fowl cholera may be restored 

 to full vigor by inoculating a small bird sparrow or canary to 

 which it is fatal. After several successive inoculations the virus 

 resumes its original activity. 



In general, pathogenic virulence is increased by successive inocu- 

 lations into susceptible animals, and diminished by cultivation in arti- 

 ficial media under unfavorable conditions. Thus various pathogenic 

 bacteria which have been cultivated in laboratories for a length of 

 time are likely to disappoint the student if he makes inoculation ex- 

 periments for the purpose of demonstrating their specific action as 

 described in text books. 



1 Quoted from " Bacteria," pages 207, 208, written in 1883. 



