The electron microscopic study of a sacral chordoma, the cells of which contain variable amounts of glycogen, allowed to characterize different stages in the evolution of the tumoral cells. The young cells contain almost no glycogen ; progressively it becomes more abundant and fills up almost the whole cytoplasm. At the end of the evolution glycogen disappears while the cell becomes vacuolated. This suggests that enzymatic lysis of this polysaccharide could be the cause of the vacuolation which finally produces the physaliferous cell. Comparison between the cells of the chordoma and those of the normal notochord shows some morphological similarities in the development of these cells. These different data may be useful for a classification of the various types of chordomas.