146 TEXT-BOOK OF BACTERIOLOGY. 



planation of the matter, and the fact that the acquired immunity 

 extends over years, and even over decades, drives many to the 

 conviction that there must be a permanent alteration in the condi- 

 tion of the whole body which could only be accomplished by the 

 influence of the active tissue cells. 



These tissue cells form the basis and centre of a third hypothe- 

 sis, which, unlike those hitherto discussed, is founded on direct ob- 

 servations and incontestable experiments. We already know that 

 Metschnikoff ascribes the chief role in the relations between the 

 bod} 7 and the bacteria to the white elements of the blood, as phago- 

 cytes. He found that virulent anthrax bacilli were received into 

 the white blood-corpuscles of insusceptible animals, attenuated 

 ones into those of susceptible animals, and, as he believed, there de- 

 voured and digested, while a similar process was not discoverable 

 in the case of susceptible animals and virulent bacilli. From these 

 facts Metschnikoff drew the following conclusions : 



The presence or absence of immunity depends on the ability or 

 inability of the cells of the body to devour and kill off the bacteria. 

 This ability may be natural or acquired. In the latter case, the 

 cells where they have once had an opportunity of devouring atten- 

 uated micro-organisms with a milder poison which nature enables 

 them to withstand are so far accustomed to it that they can bear 

 a stronger dose without injury, and at last are able to devour the 

 most virulent material with impunity. This can be effected both 

 by gradual functional adaptation and also by a kind of selection in 

 which only the strongest and most vigorous cells remain and 

 transmit the acquired faculty to their descendants. The leucocytes 

 are but short-lived formations. A permanent resistance of the 

 organism to a disease which it has once passed through or against 

 which it has been protected by inoculation is, therefore, only con- 

 ceivable if we grant to the cells the power of transmitting an 

 acquired property unaltered to their children and their children's 

 children. 



This hypothesis, as must have been seen, presupposes an extra- 

 ordinary docility in the protoplasm of the white blood -corpuscles, 

 to which it attributes something like feeling, thinking, and acting 

 a sort of mental perception. 



But even if we raise no objection to this, there remain plenty 

 of reasons for combating the phagocytic theory. In our opinion, 

 the fact that it is essentially the excretions of the bacteria which 

 produce or are able to produce immunity is difficult to harmonize 

 with Metschnikoff s hypothesis; for if no living micro-organisms 

 are present none can be devoured, to accustom the cells to the 



