Merriam-Webster Collegiate Dictionary - bone marrow

 noun  Date: 1893 a soft highly vascular modified connective tissue that occupies the cavities of most bones and occurs in two forms:  a. one that is yellowish, consists chiefly of fat cells, and is found especially in the cavities of long bones  b. one that is reddish, is the chief site of blood cell formation, and occurs in the normal adult in cancellous tissue especially of certain flat bones — called also red marrow