Bone marrow is a semi-solid tissue found within the spongy (also known as cancellous) portions of bones. Bone marrow is the primary site of new blood cell production in birds and mammals. It is composed of hematopoietic cells, marrow adipose tissue, and supportive stromal cells. In adult humans, bone marrow is primarily located in the ribs, vertebrae, sternum, and bones of the pelvis.

Source: Wikipedia