The phagocytes perform a similar function in another process which very frequently takes place in various animals that pass through metamorphoses, as in insects whose organs are transformed in changing from one stage of their existence to another, and in tadpoles which lose their tails in becoming frogs; the old parts that disappear are devoured by phagocytes. Especially in the case of infectious diseases has the protective part performed by the leucocytary phagocytes been brought into full view by M. Metchnikoff. He has shown that the white globules rush to meet the bacterides of inflammation that are introduced through any wound, absorb them, and render them powerless to do harm. In the lymphatic organs--the spleen, the lymphatic ganglions, and the marrow--the white globules are normally accumulated, and there is where the struggle is most active between the bacterides of inflammation which are swarming in the blood and the defensive agents of the organism. The same takes place with the spirilla of recurrent typhus and the microbe of erysipelas. The leucocytes are capable of adapting themselves to conditions different from those in which they usually live, provided the change is not too abrupt. It may sometimes occur that the poison secreted by a microbe will paralyze and kill the leucocyte, unless care has been taken, by inoculations of virus, at first attenuated and afterward gradually increasing in virulence, to create an immunity in the phagocyte, to make it refractory to the poison and capable of swallowing the toxic bacterium without suffering from it. Explanations have been sought in this property for the virtue of vaccination and the immunity that results from it, but they are evidently only fragmentary, and there are other theories of immunity. The leucocytes are not always victorious over the microbes, and when these excel in numbers or force it sometimes comes to pass that they are overcome and succumb. Poisoned by the substance they have incorporated, they undergo a fatty degeneration and become globules of pus. Pus is therefore formed of the cadavers of conquered leucocytes. Although that humor ought, for the good of the system, to be rejected, like every other mortified part, it is nevertheless true that the production of it is a beneficent effort, and a salutary reaction of Nature against the morbid agent.