INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 1009 



active poison of the fowl-cholera does not always kill twenty times in 

 twenty cases, but in the observed cases which I have seen with my 

 own eyes, it has killed at least eighteen times out of twenty in those 

 instances where it has not killed twenty times. Similarly the poison 

 which has had its violence diminished has not always preserved life 

 twenty times out of twenty cases ; in the cases of inferior protection 

 this occurred sixteen or eighteen times out of the twenty. Further, 

 it does not absolutely and by a single inoculation prevent a recurrence 

 of the disease ; this non-recurrence is attained much more surely by 

 two inoculations than by one." 



Since M. Pasteur's first experiments, he has investigated the 

 conditions of immunity more fully, and has arrived at the following 

 hypothesis of the real natui'c of the protective operation and the 

 reasons for the immunity.* 



Numerous experiments had shown that the inoculation with the 

 weaker poison (which on the grounds of analogy and simplicity is 

 named vaccination) gives such diiferent results with different fowls, 

 that in one case one vaccination is sufficient to cause entire immunity 

 against the deadlier poison, while in others a once- and even twice- 

 repeated vaccination is necessary. To illustrate this, let eighty fowls 

 be taken, and twenty of them be inoculated with the violent poison, 

 these twenty will die ; inoculate the next twenty with the mitigated 

 poison and none of them will die ; if these twenty fowls which have 

 once undergone vaccination be inoculated with the stronger poison, 

 about six or eight will remain alive. A fresh series of twenty fowls 

 may be vaccinated on two occasions, one operation to follow the other 

 after an interval of seven to eight days, and inoculation with the deadly 

 poison is now without danger to from twelve to fifteen of the number. 

 If a batch of twenty new fowls is now vaccinated three or four times in 

 succession, the inoculation with the powerful poison will cause neither 

 the death or even the sickness of any more. In this last case the fowls 

 can never again take the disease. 



With regard to the reason of the immunity, we cannot avoid the 

 idea that tlio microscopic organism which causes the disease, finds in 

 the body of the animal a medium in which to grow, and tliat it alters 

 or destroys certain substances while carrying out the activities of 

 its own life. But wlion the perfect immunity is attained, one may 

 inoculate any muscle one pleases with the more deadly organism 

 without obtaining the slightest effect, that is, all cultivation in these 

 muscles is now impossible ; tliey no longer contain materials to nourish 

 the microbiou. 



The question now is, whether this suppression of the j)ossibility of 

 any cultivation of the parasite in the muscles is limited to these j)art8 

 which have undergone the protective inoculation. To decide tliis, a 

 new series of strongly vacciiuited fowls was once inoculated by intro- 

 duction of tlie poison into tlic jugular vein, and in a second series of 

 experiments by feeding witli tlie infectod muscles of a fowl wliieh had 

 died of tho parasite. The result was that in both cases tlic fully 



• 'Coniptcs Uciulus,' xc. (ISSit) pp. '.>;V2-8. Cf. ' NiiturroMolicr,' xiii (1880) 

 pp. -247-8. 



