Mother cell of megakaryocyte.

It was attempted to describe the morphology of the most immature cell of megakaryocytic series. The megakaryocytes were observed with the electron microscope in five cases, being traced back to their immature forms. In two cases the most immature cells of megakaryocytic series were considered to be the cells which were probably identified as lymphocytes under the light microscope, but they were not lymphocytes with the electron microscope. In other two cases it was presumed that neutrophilic and megakaryocytic series were derived from morphologically similar immature cells, since the most immature cells of neutrophilic and megakaryocytic series were not distinguished when they were traced back to their immature forms. These findings suggest that mother cells of megakaryocytes in the adult bone marrow may be identified as lymphoid cells with the light microscope.