A medical device for the repair and reinforcement of a muscle wall defect, and that comprises, in combination: (a) a biologically compatible implantable prosthesis having a first layer of material, with a peripheral outer edge, to cover a muscle wall defect, and a second layer of material juxtaposed with said first layer and, connected to the first layer at said peripheral outer edge to provide a pocket between said layers; said second layer containing an opening to allow access to said pocket between the first and second layers; and (b) a delivery device for delivering the prosthesis to a repair site, said delivery device having a support piece made from a material having sufficient rigidity to provide an inherent bias towards a stable, self-supporting planar orientation in which the support piece is continuous over at least a substantial portion of its surface in its free body state, said material also being sufficiently flexible to facilitate deformation of said support piece against said bias so that it adopts a collapsed configuration allowing the support piece to pass through said opening before being released so that it restores into its stable, self-supporting planar orientation and nests between the first and second layers of the prosthesis, the delivery device further comprising a handle extending from said continuous surface and connected to the support piece to transmit a force thereto to deform said support piece into said collapsed configuration to facilitate removal of the support piece from the implantable prosthesis after the implantable prosthesis has been positioned relative to the muscle defect, wherein the support piece includes asingle zone of weakness that allows one portion of the support piece to overlap another portion of the support piece when said support piece is in said collapsed configuration. (a) a biologically compatible implantable prosthesis having a first layer of material, with a peripheral outer edge, to cover a muscle wall defect, and a second layer of material juxtaposed with said first layer and, connected to the first layer at said peripheral outer edge to provide a pocket between said layers; said second layer containing an opening to allow access to said pocket between the first and second layers; and (b) a delivery device for delivering the prosthesis to a repair site, said delivery device having a support piece made from a material having sufficient rigidity to provide an inherent bias towards a stable, self-supporting planar orientation in which the support piece is continuous over at least a substantial portion of its surface in its free body state, said material also being sufficiently flexible to facilitate deformation of said support piece against said bias so that it adopts a collapsed configuration allowing the support piece to pass through said opening before being released so that it restores into its stable, self-supporting planar orientation and nests between the first and second layers of the prosthesis, the delivery device further comprising a handle extending from said continuous surface and connected to the support piece to transmit a force thereto to deform said support piece into said collapsed configuration to facilitate removal of the support piece from the implantable prosthesis after the implantable prosthesis has been positioned relative to the muscle defect, wherein the support piece includes asingle zone of weakness that allows one portion of the support piece to overlap another portion of the support piece when said support piece is in said collapsed configuration.