thickness was 1 0 mm viscosity average molecular weight of the polymeric membrane according to this example was 75 000 and its molecular weight distribution was 2 2 the tubular membranes were used to cover a diphyseal defect of ten rabbits two months after surgery the canal created by the membrane according to this example was filled with osseous tissue microporous resorbable membranes were prepared from poly l lactide with a viscosity average molecular weight of 15 000 to 100 000 daltons according to the method of manufacture described in example i the membrane produced according to this example had a glass transition temperature of 50 u00b0 c and showed a porosity in the range of 10 to 50 u03bcm and a thickness of 0 9 mm numerous controlled experiments by the applicants have demonstrated that the novel membranes obtained by the described methods of manufacture promote bone growth in a reliable manner and further improve the efficacy of adjunctive bone conductive and inductive substances the membrane obtained by the method according to example ii was sterilized with ethylene oxide and implanted in 15 mature rabbits to cover mid diaphyseal defects of 10 mm created in the radius of a limb an identical but untreated defect on the other limb served as a control within one week the membrane implants were filled with tissues that progressively differentiated into osseous tissue by one month further osseous formation had bridged 40 to 95 of the defect new bone was formed within.