Electron micrographs. (a) Two types of tumor cells were observed: round or polygonal cells with round nuclei and moderately clear cytoplasm (arrow) and spindle cells with irregular nuclei and dark cytoplasm (arrowhead). (b) These cells were observed in contiguity with each other. (c) Abundant rough-surfaced endoplasmic reticulum (large arrow) and free ribosomes (small arrow) were observed in the cytoplasm of the former cells. (d) Some bundles of intermediate filaments (arrow) were noted in the cytoplasms of the latter cells. (a: ×800; b: ×4000; c: ×5.0000; d: ×5000).