In many cases, however, those penetrating became enclosed in cells which the body cavity contains and which correspond with the leucocytes of the blood; in these the spores were digested and destroyed.  The daphneae in which this took place recovered from the infection.  Here was a case in which all the stages of an infectious disease could be directly followed under the microscope, and the whole process was simple in comparison with infections in the higher animals.  The pathogenic organism was known, the manner and site of invasion was clear, it was also evident that if the multiplication of the parasite was unchecked the animal died, but if the parasite was opposed by the body cells and destroyed the animal recovered.  The studies were carried further into the diseases of the higher animals, and it was found the leucocytes in these played the same part as did the cells in the body cavity of the daphnea.  The introduction of bacteria into certain animals was followed by their destruction within cells and no disease resulted; if this did not take place, the bacteria multiplied and produced disease.  Support also was given the theory by the demonstration at about the same time that in most of the infectious diseases the leucocytes of the blood became increased in number,—­that in pneumonia, for instance, instead of the usual number of eight thousand in a cubic millimeter of blood, there were often thirty thousand or even fifty thousand.  At about the same time also chemotaxis, or the action of chemical substances in attracting or repelling organisms, excited attention, and all these facts together became woven into the theory.  It was soon seen, however, that this theory, based as it was on observation and supported by the facts observed, was not, at least in its first crude form, capable of general application.  Many animals have natural immunity to certain diseases; they do not have the disease under natural conditions, nor do they acquire the disease when the organisms causing it are artificially introduced into their tissues by inoculation.  Such natural immunity seemed to be unconnected with defence by phagocytosis, for the leucocytes of the animal might or might not have phagocytic reaction to the particular organisms to which the animal was immune.  It was also seen that recovery from infection in certain diseases was unconnected with phagocytosis.  It had also been demonstrated, by German observers chiefly, that the serum of the blood, the colorless fluid in which the corpuscles float, was itself destructive, and that in an animal rendered immune to a special bacterium the destructive action of the serum on that organism was greatly increased.  In this hostile serum the bacteria often became clumped together in masses, the bodies became swollen, broken up, and finally disintegrated.  This property of the serum was described as due to a substance in the serum called alexine, which in the immune animal became greatly increased in amount.  It was even denied