1) extra-embryonic blood vessels ---blood island at the middle of 3rd week, wall of the yolk sac mesenchyma proliferate and/ form isolated cell clusters, the peripheral cell become flattened and differentiate into endothelial cell to from endothelial tube central located cells are detached and develop into primitive blood cells(blood stem cell 血岛1) extra-embryonic blood vessels ---blood island: at the middle of 3 rd week, wall of the yolk sac mesenchyma proliferate and form isolated cell clusters, the peripheral cell become flattened and differentiate into endothelial cell to from endothelial tube; central located cells are detached and develop into primitive blood cells(blood stem cell)