There are some microbes which will produce deadly poison if grown in the clear fluid (serum) of the blood of an animal (as, for instance, the cholera-microbe when grown in the serum of the frog's blood), yet when inoculated living into the blood of that animal never cause the slightest illness! Why? Because they are at once eaten by the vigilant phagocytes of the blood before they can produce any appreciable amount of poison. That is easily demonstrated by experiment. Our main means of defence against microbial disease, says Metchnikoff--though cleanliness and precaution against access of microbes are all very well in their way--is the activity of our phagocytes. Now it appears that just as in the other cases I have been considering, so in the production of "relish," the power to produce it resides in the blood (and perhaps the cells of its vessels), but is not set at work until the enemy is in the blood. Suppose there is an infection, an invasion of the blood and tissues by one or other disease-causing microbe. Gradually if the body is healthy the "relish" is produced and becomes attached to the invading microbes. The phagocytes swallow them greedily and make an end of the invasion. It is proved that this aroused avidity of the phagocytes is due to no change in the phagocytes themselves; since if they are transferred to the serum of a normal man they show no such predilection for the special invading microbe. The "opsonin," or "relish," is something exuded into or produced in the blood fluid when the attacking microbe arrives. It attaches itself to them: that is the essential fact. In many of us the phagocytes are not at a given moment so "avid" of this or that disease-microbe as they should be in order to protect us from its multiplication and poison production. But it is found that by injecting boiled and cooled (therefore dead) microbes of a particular kind into the blood of a man, you can start the production of the "relish" appropriate to that kind. The dead microbes answer this purpose; they excite the production of the opsonin appropriate to them and yet are not themselves dangerous, since they are dead. When subsequently (or possibly concurrently in small quantity) living microbes of the same disease enter the blood, the opsonin is ready for them. They are, to put it picturesquely, like oysters at the oyster-bar, peppered and vinegared "in no time," and then swallowed by the phagocytes by the dozen. This seems almost too comic a view of the deadly struggle of man and higher animals for health and freedom from the swarming pests which everywhere invade him. Yet it is correct, and involves a simple and fundamental truth. Our properties and appetites are but the sum of those of the protoplasmic organisms--the cells--of which we are built up. Our need for a relish with oysters is the same thing as the need of the phagocyte for a relish with its microbes, not something "poetically" compared to it. The story of "the oysters and