1. It has been found that during a 12-15 hour period after the subcutaneous injection of mice with a predetermined ‘optimum’ dose of neutral red, there occurs a definite series of changes in the pancreas acinar cell in which neutral red granules are formed as separate bodies, which subsequently come together as aggregates and then disappear from the cell.

2. The cell is not damaged by the cycle of events just described. The response to the presence of neutral red appears to be physiological, not pathological.

3. Evidence is brought forward showing that neutral red granules and aggregates are not vacuoles that pre-existed in the normal cell and were subsequently stained: they are new formations produced by the presence of neutral red in the cell.

4. Little more than one-quarter of the neutral red injected is excreted in unchanged form.

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