All the other materials that are in the blood are held back by the filter, so that nothing escapes from it in these places except water and salts. Further along each of the tiny tubes is a section which is surrounded by a network of fine blood vessels where two things take place; the first of these is the taking out from the blood of the various kinds of waste products; the second is the absorbing back into the blood of part of the water which filtered out through the knot at the tip of the tube. Beyond the point where these things happen the tube does nothing except to convey the finished kidney secretion, or urine, down to the outlet where opens a very large tube, the _ureter_, by which the kidney secretion is carried down to the bladder. The formation of urine is made up then of three stages; first the filtration of water and salts out of the blood, second the escape from the blood into the kidney tube of the various waste products of cell activity, and third, the reabsorption into the blood of part of the water. It is estimated that about four times as much water filters out from the thousands of tiny filters at the tips of the kidney tubes as comes out at their ends to be carried down to the bladder. We do not understand exactly why this should happen; one result of it is to make the urine contain about four times as much salt as the blood; the water that filters out at the beginning of the kidney tube carries with it just the amount of salt that is dissolved; after the water is reabsorbed the salt is left behind in the kidney tube, so that all the salt that filters out stays in the urine, but only about a quarter of the water. The discharge of waste products is easier to understand than the way in which the water is handled; all organic substances which happen to be in the blood and which do not belong there pass out from the blood during its passage through the fine network surrounding the kidney tubes. Of course we do not know just how this is carried on; there must be some special features about the protoplasm in this part of the kidney to cause these substances to pass out from the blood here and nowhere else. Not only waste products of cellular activity, but also all or nearly all the organic accessories of the diet and all drugs taken as medicine leave the blood in this region.