The present paper is the first of two dealing with experiments which were planned to disclose the meaning of the colors assumed by mammalian tissues when vitally stained with some phthalein indicators. Derivatives of the tissues themselves were employed after tests had shown, in agreement with certain facts in the literature, that inferences from model experiments, so called, would be misleading.

Organs were perfused with water, or salt, or lymph of the same creature and extracted with small quantities of the fluid. The turbid extracts were colored with phthalein, the reaction determined colorimetrically with the aid of an arc light, and the findings were compared with potentiometer readings upon the material. The results of the two methods were in close agreement.

The significance of the data will be discussed in our second paper.

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