In human skin, the papillary capillaries and the subpapillary "veins" have the same structure. The endothelium, 4000 to 5000 A thick, forms the internal layer of the structure. It shpws no light except for the intercellular spaces whose size is 100 to 150 A. With the electron microscope, neither intercellular cement nor an endocapillary layer could be demonstrated. As in other types of capillaries, numerous intraplasmatic vesicles are found; they are regarded as being the morphological substrate of the metabolism of gas and other products. Farther out the basement membrane of variable thickness covers all the spaces between the endothelium and the pericytes like an impermeable band. It is stretched uninteruptedly beyond the confines of the cells. It is composed of several layers and has a finely striated structure. At the outermost level are the pericytes which are loosely bound together. The thickness of this part of the structure grows when the diameter of the vessel is increased. These cells are covered externally by a fringe of the basement membrane which thus forms part of the structure of the capillary. They show the phenomenon of membrane vesiculation. It has not been possible to show contractile elements in their cytoplasm.