Its protoplasm can isolate and place out of action any of the substances which enter it. If observations eventually prove to us that water passes from the urinary tubules into the blood, “in the face of osmotic force,” we shall be constrained to explain this antiphysical phenomenon as due to the action of living cells. The cells, we shall say, take up fluid from the urinary tubules, fix its urea and other salts in their protoplasm, discharge its water into the venous blood, return the urea and other salts to the urine. Given this property of protoplasm, such a process is strictly in accordance with physical laws. * * * * * Enough has been said regarding the theory, or want of theory, of the action of the kidney. Turning now to matters of observation, it can easily be shown that the epithelium of the tubules has the power of excreting into the urine highly complex materials which diffuse with difficulty. If a substance soluble in blood, but insoluble in urine, an alkaline salt of indigo, for example, be injected into the vascular system, it is rapidly excreted by the kidney. The indigo is precipitated even before it comes in contact with the acid urine. If the animal be killed a short time after the administration of the indigo, the contorted portions of its tubules and the ascending limbs of the loops of Henle are strongly coloured blue. An ammoniacal solution of carmine may be used for a similar experiment; but the results are not nearly so sharply limited to the large-celled portions of the tubules. Even the glomerulus is coloured red, a fact which has been interpreted as showing that, although the greater part of the carmine is excreted into the tubules, some of it accompanies the water which exudes from the blood through the glomerular tufts.