The practical identity in structure of the kidney in birds and reptiles and mammals would seem to have an important bearing on this controversy. The urinary excretion of birds consists almost exclusively of uric acid. As seen under the microscope, it is a semi-solid white deposit, made up of crystals, supposing no special precautions have been taken to obtain it fresh. The water, pigment, and salts which are essential elements of the excretion of mammals are practically absent. Yet the kidney of a bird presents the same arrangement of glomeruli and tubules as the kidney of a mammal, although the glomeruli are relatively smaller. Uric acid diffuses with great difficulty. If it is, so to speak, washed through the glomeruli, and the water which dissolved it reabsorbed by the tubules, an enormous quantity of water must pass through the kidney in order that it may carry the uric acid in its stream. If uric acid be excreted by the epithelium of the tubules, it is difficult to account for the presence of glomeruli, since no water leaves the kidney. Crystals of uric acid are to be seen in a section of the kidney, not only in the cells of the tubules, but also in the glomeruli; but it may well be that in both situations crystallization has been induced during the preparation of the section. It jars an histologist’s conception of the constitution of a secreting cell to contemplate the formation within its network of protoplasm, and the extrusion from it, of sharp-angled crystals. As a matter of fact, it is not in its crystalline form that uric acid is excreted by birds, but as quadri-urates—_i.e._, salts containing only one-fourth of their “normal” complement of base; crystalline spheres or amorphous deposit, not angular crystals. These quadri-urates decompose very quickly, setting free crystals of uric acid. It must be confessed that, in whatever way one attempts to account for the excretion of uric acid by birds, the similarity of structure of their kidneys and those of mammals is difficult to reconcile with the wide difference in consistency and in chemical composition of the excrement. Reflecting upon all the evidence bearing upon the mechanism of the mammalian kidney, the majority of physiologists come to the following conclusions: The greatest outflow of water occurs in the glomeruli. The water is accompanied by salts, including a small quantity of urea. The contorted and spiral portions of the tubule and the ascending limbs of Henle’s loops add to the urine the remainder of the urea, together with various bodies still less readily diffusible.