IMMUNITY AGAINST MICROBES. 431 



through several rats it is possible to obtain an exceedingly 

 powerful virus, which proves fatal to very old rats in three to 

 six days after inoculation. 



It happens occasionally that the bacilli of anthrax will not 

 grow in vitro in the blood of rats which have actually died of 

 anthrax ; but, as a general rule, the spores of bacilli thrive in 

 the blood of rats, which, before being killed, proved immune 

 against anthrax. The bacilli of anthrax grew abundantly and 

 produced spores even in the aqueous humour of rats which 

 had survived an inoculation of anthrax. The same holds true 

 for the subcutaneous tissue, which often contains numerous 

 leucocytes. 



Spores of anthrax inoculated into rats always germinate and 

 produce bacilli, no matter whether the inoculation be performed 

 into the anterior chamber of the eye or under the skin. One 

 such experiment is extremely interesting, and may be related 

 here. A white rat was examined four days after having been 

 inoculated subcutaneously with the blood of a guinea-pig dead 

 of anthrax ; there was considerable oedema at the point of 

 inoculation, but the exudation fluid contained degenerated 

 bacilli only. Several little silk threads loaded with anthrax 

 spores were introduced into the oedema, and twelve hours after- 

 wards a considerable number of young and perfectly normal 

 bacilli were found in the fluid. Eighteen hours after the in- 

 troduction of the threads one was withdrawn, and found to be 

 surrounded by an immense number of bacilli, staining easily, 

 and perfectly normal in every respect. The rat survived the 

 two inoculations, and only died three months after from the 

 eff'ects of an inoculation in the eye, with moderately virulent 

 anthrax. 



The inoculation of anthrax into the most resistent rats is 

 usually followed by an abundant growth of anthrax bacilli ; 

 but after repeated inoculations on the same animal the latter 

 becomes so perfectly immune that finally the spores do not 

 seem to grow at all. M. Metschnikoff inoculated an old rat 

 (female) which had withstood three previous injections of an- 

 thrax, by introducing under the skin of the back a silk thread 



