The tumour cells and the reaction cells in Sarcoma 37 ascitic fluids have been studied in thin sections with the electron microscope. The reaction cells were either leucocytes or much larger acidophilic peritoneal cells of the same dimensions as the tumour cells; the peritoneal cells formed as much as 20 per cent of the large cell population.

The fine structure of the cells is described and some new observations recorded. It has been found that the cell membrane of eosinophil granulocytes has a laminated composition and the characteristic granules of these cells a double limiting membrane. The pores in the double nuclear membrane of the peritoneal cells have been observed to have a fine line running across them. In the tumour cells, a rounded granular body with a central dense area has been found in the region of the centrosome; these cells were also seen to contain rows of parallel smooth surfaced cisternae lying 150 mµ apart similar to those hitherto only observed in spermatids. There was a feltwork of fine filaments in the cytoplasm of the centrosome region of the tumour cells.

The cytoplasmic fine structure underlying the basophilia of the tumour cells and the acidophilia of the peritoneal cells is compared and discussed.

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