In recently described experiments '40) on the isolated Ringer perfused frog kidney it has been shown that the selective secretory transport of sulfonic acid dyestuffs by the proximal convoluted tubules depends upon an appropriate molecular configuration, such as that displayed by the large group of azo dyestuffs. The azo linkage divides the molecule into two halves, each of them being formed by a ring system, which, according to the radicals attached, is either polar hydrophilic or non-polar hydrophobic. The hydrophilic character is chiefly due to the presence of one or more sulfonate groups. Azo dyestuffs which, owing to the distribution of their sulfonate groups in the molecular halves, have a polar-non-polar configuration, are concentrated by the kidney, whereas the polar-polar ones are not. Another group of dyestuffs, the disazo dyestuffs, the molecular configuration of which resembles that of the azo dyestuffs, shows a corresponding behavior. A third group, the triphenylmethane dyes, is different. Their fundamental structure fails to be built up by two halves, one of which could be hydrophilic, the other hydrophobic. This would account for our finding that the sulfonic acid triphenylmethane dyes, in general, are excluded from the secretory transport by the Ringer perfused kidney.
However, one may object that several triphenylmethane dyes, as well as azo dyestuffs, have been found to undergo secretory concentration, when, some time after injection into the blood or after incorporation into the dorsal lymphsac, the kidney was placed under the microscope. Under those conditions, the strikingly demonstrative picture is that the dye appears to be accumulated in the lumina of the proximal tubules, which are visible on the dorsal side of the kidney, whereas the surrouriding epithelial wall and likewise the contents of the capillaries and of the Bowman capsules either seem to contain no color or show but a faint diffuse coloration. This has been proved to be the case with