A theory of uric acid excretion is put forward according to which potassium or sodium acid urate is secreted in solution in the upper part of the Malpighian tube, and water and base reabsorbed from the lower part of the tube, leading to a precipitation of free uric acid; so that the same water and base are circulated and used repeatedly. The evidence for this theory is presented under four heads:

(i) General evidence from the histology of the Malpighian tube and the relative solubility of uric acid and urates.

(ii) Experiments with vital dyes. Thus, neutral red is taken up from the blood by the upper segment of the Malpighian tube, and from the lumen of the tube in the lower segment. It is suggested that water follows the same route.

(iii) The application of ligatures to the tubes at different levels shows that uric acid is not secreted by the lower segment.

(iv) Experiments with indicators show that the contents of the tube are faintly alkaline (pH 7.2) in the upper segment, and acid (pH 6.6) in the lower segment. Uric acid in solution has a pH about6.5.

It is suggested that the terminal ampullae of the Malpighian tubes, and possibly the rectal glands, also play a part in the reabsorption of water

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