by some that phagocytosis of living bacteria took place, and that all those included in the cells were dead, having been destroyed in the first instance by the serum.  The strife became a national one between the French and Germans,—­on the one side in France the phagocytic theory was defended, and in Germany, on the other, the theory of serum immunity.  The mass of experimental work which poured from the laboratories of the two countries in attack and defence became so great that it could not easily be followed.  It had a good influence because, without the stimulation of this national rivalry, the knowledge which gradually arose from this work would not have been so quickly acquired.  It is interesting that the mode of action of the serum in destroying bacteria was demonstrated not by a German but by Bordet, a French observer and a pupil of Metschnikoff.  He showed that the serum contained two distinct substances, each necessary for the destructive action.  The separate action of these substances can be studied since one is thermolabile, or destroyed by heating the serum to one hundred and thirty-three degrees; the other thermostabile, or capable of withstanding a greater degree of heat.  These substances are known only by their effect, they have never been separated from the serum.  The thermostabile substance, or amboceptor, as it is generally called, has in itself no destructive action on the bacteria; but in some way so alters them that they can be acted on by the thermolabile substance called complement whose action is destructive.  The amount of amboceptor may increase in the course of infection and its formation stimulated, the amount of complement remains unchanged.  The action of the amboceptor is specific, that is, directed against a single species of bacterium only; the destructive power of the blood may be very great against a single bacterium species and have no effect on others.  There seem naturally to be many different amboceptors in the blood, and the number may be very greatly increased.  It has been shown as a result of the work of many investigators that the shield has two faces,—­there is destruction both by cells and fluids and there is interaction by both.  The amboceptors so necessary for the destructive action of the serum are produced by the body cells, particularly the leucocytes.  The serum assists in pagocytosis by the action on bacteria of substances called opsonins which are contained in it, and the formation of which can be very greatly stimulated.  Again, not all inclusion of bacteria within leucocytes is indicative of phagocytosis; in many cases the bacteria seem to find the best conditions for existence within the leucocytes, and these and not the bacteria are destroyed.