1.General feature: (1) contain more cells and less extracellular ground substance (2) Polarisaton: ---free outer surface: face air or other things ---basal surface: face underlying CT, havebasement membrane (3) Avascularity,but innervation: ---no blood vessels ---rich in nerve terminals
(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)