THREE FIGURES
I n an earlier investigation it was shown that exogenous inulin and creatinine were eliminated in the frog kidney solely by the process of glomerular filtration, without the intervention of either tubular escretiori or reabsorption (Forster, '38). The inulin clearance is used in this study as a measure of the rate of glomerular filtration, and is compared with simultaneous phenol red clearances to analyze the tubular process involved in the excretion of this dye.
The frog has long been a favorite subject for investigations concerned with analyzing the processes involved in urine formation, because of the dual blood supply to its kidney, and because of that organ's translucency and loose anatomical structure. These features enabled Nussbaum (1878) to isolate glomerular and tubular function by ligating the renal arteries supplying the glomeruli, and to study tubular activity with merely the renal portal blood supply intact, and prompted Hill and McQueen ( '21) and Richards and Schmidt ( '24) to lead in devising ingenious techniques concerned with direct microscopic observations of the functioning renal unit. Despite the voluminous literature which has accumulated concerning renal processes involved in phenol red elimination in the frog, through use of one or the other of these techniques, the conclusions are contradictory.
Rowntree and Geraghty ( '12) concluded that phenol red was actively excreted by tubular activity in the frog when this dye was observed in uretral urine after the renal arterial circulation was eliminated and the renal portal vein perfused with a solution containing phenol red. However, this conclusion was invalidated by the findings of Bainbridge, Collins and Menzies ('12--'13), who noted that as a result of the high renal portal perfusion pressure used, some of the perfusate was driven back up the efferent arterioles, thus effectively reestablishing the glornerular circulation. Wearn and Richards ( '24), in applying their method of direct observation, noted that phenol red was separated from the