The question as to the normal duration of the life of red blood corpuscles has hitherto been considered more as a matter for speculative inquiry than for experimental proof. Attempts have from time to time been made by various observers to solve it experimentally, by injecting the blood of one animal into the circulation of another, whose blood corpuscles were of different size and shape, and then noting how long the foreign blood corpuscles remained discoverable in the body of their host. As might have been anticipated, the success of such experiments has not been very striking.