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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 alternate 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 harmless state." Metschnikoff therefore assumed "that the bacillus
