60% trichloracetic acid (TCA) will dissolve the cells and mesangial matrix of isolated renal glomeruli before it dissolves the basement membranes (BM). With the removal of the mesangial cells and matrix, the paramesangial BM stands out in sharp relief. It is thicker than that of the capillary loops and is provided with prominent ridges and spurs. These thick collars of BM now border the narrow channels left after the removal of the mesangium. The channels communicate with the capillary lumens. They widen asynchronously with the continued action of the TCA leading at first to more axial cavitation of the lobules of the glomerulus and finally to the conversion of these lobules into sacs or cysts. This is brought about by the unfolding of the capillaries within the investing sheath of BM which is common to the capillary loops and the widened stalks and intercapillary axial portions of the mesangium.