The author gives the complete history of the mesoderm in Ostrea. He shows that it is derived from cell 4d (as designated in cell lineage studies and that extensive mesoderm bands are formed which lie on either side of the intestine or rectum. Several cells separate from the anterior ends of the mesoderm bands and migrate forward to form a loose mesenchyme tissue vvhich later gives rise to the muscles and connective tissue of the adult. The remainder of the mesoderm bands give rise to the kidneys and pericardium. At first they form 2 distinct masses of cells but finally push laterally to join beneath the intestine, thus assuming the position of the pericardium typical of lamellibranchs. At first there is no connection between the pericardial space and the kidney tubes, both of which seem to arise by separation of the cells of the mass (Schizocoel). The conclusion is that the pericardium and kidneys of mol-lusca are derived from mesoderm only.