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alive after three months. But there is another way in which they 

 are destroyed. "When bacteria are injected into the tissues there 

 follows a strusgle for existence between them and the cellular 

 elements. Leucocytes quickly accumulate in the neighbourhood 

 of the mass of bacteria, and then there follows a light for the 

 mastery between these cells and the bacteria. The cells take up 

 the bacteria into their interior, and where the bacteria are non- 

 pathogenic for the animal employed they are destroyed" (Cheyne). 

 The discovery of this is due to Metschnikoff. It is well known 

 that the colourless blood-cells possess the power of constantly 

 changing their shape, exhibiting undulatory movements and alter- 

 nate protrusion and retraction of processes ; they are also able to 

 take up and absorb solid bodies into their soft substance. "If 

 the foreign body comes into contact with the surface of the cell, 

 the latter puts out processes which embrace it and gradually close 

 over it as the waves close over a drowning animal, so that it lies 

 at last inside the soft cell substance. It may be cast out again at 

 some future time, but it may also suffer decomposition inside the 

 cell, be killed and disappear." These well-known facts led 

 Metschnikoff to investigate the behaviour of the colourless blood 

 cells of the vertebrate animals towards the bacillus of anthrax, 

 " He found that the virulent rods when introduced by inoculation 

 into an animal liable to take the fever, such as a rodent, were not 

 absorbed by the blood cells, or only in exceptional instances. 

 They were readily absorbed by the blood-cells of animals not liable 

 to the disease, as frogs and lizards, provided the temperature was 

 not artificially raised, and then disappeared inside the cells. The 

 same thing happened when susceptible animals were inoculated 

 with bacillus anthracis which had been attenuated to the harm- 

 less state." Metschnikoff therefore assumed "that the bacillus 



