The acidity of the macrophage granules in which litmus comes to be stored during life is considerable. It has proved possible to stain these granules in vivo with some of the phthalein indicators and the results, had they been obtained under controlled conditions, would indicate a pH of 3.0 or less. The amount of acid material which may accumulate within the cells of animals stained with litmus is great, sufficient in the case of the elements of a peritoneal exudate for the acid reaction to prevail when they are gathered together and crushed. The material is derived, not from the dye, but from living elements responding characteristically to a stimulus far from unique. Such responses may well play a rôle in normal physiological activities and in the cellular defense against microorganisms.

Vital staining with litmus demonstrates anew that the intracellular reaction during life is independent of that of the body fluids. By means of color changes in the stored indicator one can distinguish sick as well as dead cells of certain sorts and follow their distribution and fate within the organism. There are data to suggest that with the aid of the indicator the normal period of survival of certain elements at least can be determined.

By the indicator method, of which the foregoing observations afford a crude illustration, much should be learnt in the future about body processes. The present paper is the second of a series upon the theme.

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