1. The argentaffin cells in guinea-pig intestine have been studied by phase-contrast microscopy and in ultra-violet light. Frozen-dried tissue has been used.

2. One such cell in a section 5-7µ thick was selected and studied throughout. In an unfixed section mounted in nonane, the cytoplasm is packed with fine granules which emit a greenish-yellow fluorescence in ultra-violet light of wavelength 2,750 Å. There is also photographic evidence of absorption of light of this wavelength.

3. After formaldehyde fixation the fluorescence changes to orange-yellow and becomes much weaker. The absorption of light by the granular cytoplasmic contents is also greatly reduced.

4. There is no fluorescence and no photographic evidence of absorption of light of wavelength 2570 Å by the granular cytoplasmic contents, either before or after formaldehyde fixation.

5. The nuclear chromatin pattern in these cells is unaltered by formaldehyde fixation, and is well demonstrated in photographs taken in ultra-violet light of both the wavelengths mentioned above.

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