A soft, organic material rich in blood vessels, found in the cavities and the spongy bone layer of most bones; also known as myeloid tissue. There are two types of bone marrow. Red marrow is the primary production site for blood cells, particularly red blood cells (erythrocytes) and certain kinds of white blood cells (granular leukocytes). Red marrow contains little fat. Yellow marrow gets its color from the large amount of fat it contains; it is usually found in the body’s long bones. It produces some types of white blood cells.