1. The secretion of neutral red reproduces those variations which are observed in the mitochondrial apparatus of the renal tubule cells of animals living under native conditions. The tubular absorptive processes concerned with water, salts and sugars do not produce these effects.
2. The changes in the mitochondria consist of both structural and constituent alterations. These have been shown to be not merely phenomena concomitant with secretion, but a determining factor in one part of this process; namely, in the concentration of the dye within the cell.