118 APPLIED BACTERIOLOGY 



forward, till at last they were so numerous that the 

 organisms could not make any headway, and were thus 

 destroyed. The attraction possessed by living or dead' 

 bacteria for the leucocytes is remarkable, while they are 

 equally repelled by the presence of certain bodies, such as 

 quinine, chloroform, etc. 



The term. chemiotaxis has been applied to this phenomenon ; 

 when the leucocyte is attracted towards a body, the chemio- 

 taxis 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 

 experimental 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 immunity 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 products, 

 or to neutralise the toxic action of such products. 



Putrefaction and Oxidation. — The bacteria play the prin- 

 cipal part in causing the disintegration and dissolution of 

 dead animal and vegetable matter, of which the molecules 

 are, so to speak, in a condition of unstable equilibrium, 

 and by abstracting the small portion of nutriment which 

 they require for their own development destroy the 

 balance and bring about the resolution of the animal 

 or vegetable tissues into simpler inorganic bodies, the chief 

 products being water, carbonic acid, and ammonia, together 

 with smaller quantities of other products, some of them of 

 particularly evil odour and poisonous properties. 



The bacteria of putrefaction are for the most part 



