wall thickness of 0 002 inch 0 05 mm available from advanced polymers of salem nh an arcuate segment can be cut from the angioplasty balloon to form the clear tissue stop 96 a thin metallic foil having a thickness of about 0 005 inch or less such as a steel foil having a thickness of about 0 001 inch can then be applied to the surface of the stop 96 facing tissue receiving aperture 94 such as with an adhesive prior to attaching the foil to the stop 96 the foil can be cut to form a series of openings therethrough to provide the grid 97 shown in fig1 and 18 fig1 illustrates a cross sectional view of an overtube 86 having a noncircular cross section with a generally flattened outer surface portion in which tissue receiving aperture 94 is formed an endoscope 84 is shown positioned in the overtube 86 the generally flattened outer surface portion is located on a bottom half of the overtube 86 as viewed in fig1 providing the tissue receiving aperture 94 in such a generally flattened surface portion can be useful in positioning the aperture 94 relative to tissue to be resected fig1 also shows first and second configurations of tissue stop 96 with the second configuration shown in phantom in one embodiment the overtube 86 can be formed in two shell like halves such as a.