blood-cell formation at English => English (The Britannica Concise) Of Explained:
Continuous production of blood cells. Blood cells originate in blood-forming organs, notably bone marrow, which produces all the erythrocytes (red blood cells) and platelets and most of the leukocytes (white blood cells), the rest being produced by the lymphatic and reticuloendothelial systems. Red and white cells develop from undifferentiated stem cells. Platelets are formed from the giant cells of the bone marrow.