112 APPLIED BACTERIOLOGY 



chemiotaxis is said to be positive, and when it is repelled, 

 negative. 



The above are the principal hypotheses advanced ; the 

 truth probably lies not with any one alone ; all four play 

 their part, one predominating over the others in different 

 cases. They are each based on some amount of experi- 

 mental evidence, and more than one may, in fact, play a 

 part in the phenomena which they seek to explain. But it 

 cannot be said that any one of them has at the present time 

 been sufficiently verified to be regarded as an experimental 

 theory. For the time being it can only be stated that 

 acquired iTnmunity is a capacity either to prevent the 

 growth of disease-organisms, of which the pathogenic action 

 may lie in their intercellular tissue or their metabolic pro- 

 ducts, or to neutralise the toxic action of such products. 



Artificial Imnmnity. — We will now proceed to briefly con- 

 sider the various means by which artificial immunity may 

 be induced in the animal body. As has already been stated 

 immunity may be ' active ' and ' passive.' 



Active Immunity. — This may be produced by an injection 

 or a series of injections of the organisms in a vi/rulent or 

 attenuated condition, or by sub-lethal doses of their meta- 

 bolic products. 



Thus active immunity may be produced by one of the 

 following methods : 



1. By the injection of the virulent organisms in non- 

 lethal doses. 



2. By injection of the dead organisms. 



3. By the injection of the toxic products prepared from 

 filtered broth cultures of the organism. 



4. By the injection of the living, but attenuated, organism 

 prepared by one of the undermentioned methods : 



(a) By passing through one animal, whereby it becomes 

 attenuated for another animal. 



