The human body has hematopoietic function is the yolk sac, liver, spleen, bone marrow and other tissues. 1. Yolk sac: Its hematopoietic period begins in the 3rd week of human embryo and ends in the 9th week. The blood island on the wall of the yolk sac is the initial hematopoietic center. 2. Liver and spleen: hematopoiesis in the liver begins in the 6th week of the human embryo and reaches its peak in the 4th-5th month, with red and granulocyte hematopoiesis taking precedence. At the same time, spleen, kidney, thymus and lymph nodes are also involved in hematopoiesis. Lymphocytes form in the spleen from the 5th month of the human embryo and participate in hematopoiesis. The thymus appears at 6-7 weeks of the human embryo, and lymphocytes begin to form. The lymphoid stem cells in the thymus also originate from the yolk sac and bone marrow. 3. Bone marrow: its hematopoietic period begins in the fourth month of human embryo, and it becomes the center of hematopoiesis after the fifth month, from then on, the hematopoiesis of liver and spleen decreases gradually, and the hematopoietic function of bone marrow increases rapidly. At birth, the bone marrow of the whole body is generally hematopoietic, and after the age of 5 years, there is a centripetal regression from the distal side of the limbs, and only the red bone marrow is left for hematopoiesis in normal adults.