The urinary tubules are the separate pieces of apparatus of which the kidney consists. The problems connected with a single tubule are therefore the problems of the kidney as a whole. These structures are all exactly alike. The description of any one of them applies to all. Each begins as a capsule containing a glomerulus. The wall of the bulb—which is merely a thin basement membrane covered by epithelial scales—is involuted by the tuft of bloodvessels. The vessels do not penetrate its capsule. Between the tessellated epithelium which covers the tuft and the similar epithelium which lines the capsule there is a space communicating by a narrow aperture with the next portion of the tubule—termed its “contorted” part, because it is twisted about like a tangled thread in the cortex of the kidney. The contorted tubule is of relatively large calibre. The cells which line it are irregular in form and indistinct in outline. The basal half of each cell, between its nucleus and the basement membrane, is vertically striated, or “rodded,” as it is usually termed. Such an arrangement of the protoplasm of a cell is commonly associated with a habit of absorbing fluid. It would seem to indicate in this case that the cells take water and various substances dissolved in water from the direction of the basement membrane. After a time the contorted portion of the tubule, although still sinuous, becomes more nearly straight—the “spiral portion”—and assumes a radial direction. In the zone between the cortex and the medulla, the spiral portion tapers into an exceedingly slender tubule which, after running some distance in the direction of the hilus, turns back again towards the cortex, making a loop, known as the “loop of Henle.” The ascending limb of this loop is of larger calibre than the descending limb. The descending limb is lined by flattened epithelium, each cell so thin that (in microscopic sections as ordinarily prepared) its nucleus bulges into the lumen of the tube. The cells of the ascending limb are more nearly cubical in form. On reaching the cortex, the tubule again becomes contorted. The second contorted portion narrows into a “collecting portion,” which joins a ductule. The ductules unite together, until at last a single duct is formed which opens at the apex of a pyramid. The cells of the ductules are cubical or columnar. Their cell-substance is clear, whereas that of the cells lining other parts of the tubule is cloudy in appearance.