soft tissue filling the cavities of bones; exists in two types, yellow and red; yellow marrow is found in the large cavities of large bones and consists mostly of fat cells and a few primitive blood cells; red marrow is a hematopoietic tissue and is the site of production of erythrocytes and granular leukocytes; bone marrow is made up of a framework of connective tissue containing branching fibers with the frame being filled with marrow cells.