<prompt> Site of RBC formation in a 20 year old healthy male is:Flat bonessho bonesLiverYolk sac  <response>: Erythropoiesis : 1st trimester - in mesoderm of yolk sac Mesoblastic stage; 2nd trimester - liver spleen Hepatic stage; 6 months onward - bone marrow Myeloid stage. At bih - all bones have red bone marrow, producing cells. Progressively, bone marrow turns yellow; shafts of long bones first turn yellow, followed by their ends. By 20 years - red marrow exists only in sternum, ribs, veebrae, pelvis and skull. 