The capacity of protoplasm for producing a new ferment when it is needed is shown by such examples as the following: Blood-plasm contains a variety of proteid substances. If a solution of white of egg be added to it, the mixture is clear and uniform. Yet egg-albumin is treated by the blood as a foreign body, a poison. When injected into the veins of a living animal, some of it is excreted by the kidneys, some destroyed in the blood-stream. If several successive doses of egg-albumin are injected into an animal (it is most convenient to inject it into the peritoneal cavity), the power of the blood to destroy the intruder is greatly increased. If now a specimen of blood be taken, and the plasma or serum mixed with egg-albumin, the mixture is no longer clear. The egg-albumin is precipitated. The blood of the animal thus “prepared” has developed a ferment, termed a “precipitin,” which throws down egg-albumin. If instead of egg-albumin, which, although a foreign body, is comparatively innocent, a substance which is distinctly poisonous, toxic, be injected into an animal, the first dose, if a large one, will prove fatal. If, however, the first dose be small, and succeeding doses progressively larger, the animal acquires the power of tolerating a quantity of the poison much larger than would have proved fatal in the first instance. A classical example of this, because it afforded an opportunity of directly observing under the microscope the difference between “unprepared” blood and blood from an immune animal, is the acquisition by a mammal of the power of tolerating the injection of the blood of an eel. Eel’s blood contains a toxin which destroys the red blood-corpuscles of a mammal. The dissolution of the blood-corpuscles may be watched with the microscope. If successively increasing doses of serum of eel’s blood be injected into the body of a rabbit, the rabbit acquires the power of resisting the toxin. Further than this, the serum of the immune rabbit injected into a rabbit which has not been prepared confers immunity upon the latter. If the blood of the prepared animal be mixed with the blood of an unprepared rabbit and with eel’s serum, and the mixture examined under the microscope, it will be seen that red blood-corpuscles are no longer dissolved. The immune serum is able to save the blood-corpuscles of the unprepared blood from destruction. During its course of preparation the rabbit developed an antitoxin.