Sir William Bowman, who in 1842 gave the first detailed description of the microscopic structure of the kidney, concluded that, whereas “the tubes and their plexus of capillaries are probably the parts concerned in the secretion of that portion of the urine to which its characteristic properties are due (the urea, lithic acid, etc.), the Malpighian bodies [_i.e._, the glomeruli] may be an apparatus destined to separate from the blood the watery portion.” All physiologists are in accord in regarding the glomeruli as the principal seat of exudation. There is great diversity of view as to the function of the tubules. In 1844 Ludwig advanced the opinion that all the constituents of the urine pass through the glomeruli in a large excess of water, and that in the course of the tubules this excess of water is reabsorbed. This theory was based, among other considerations, upon the extreme thinness of the epithelium which covers the glomerular tufts; he judged that water would filter through it very readily. A large amount of experimental work has been directed to the solution of these two problems—viz., (1) Do urea and other similar substances pass through the glomeruli? (2) Is water returned from the tubules to the venous system? Our views as to the functions of the kidney as a whole will not be greatly influenced by the answers that may eventually be given to these questions; yet their discussion is of very great interest, owing to the nature of the evidence which may be marshalled on either side.