to partially sever the membrane 276 from base member 254 the insert assembly 246 also differs in structure from the insert assembly 46 in that the outer surface 298 of plunger body portion 293 includes a knurled flange portion 299 which extends radially outwardly therefrom adjacent the proximal end 294 and allows the plunger body portion 293 to be easily grasped and turned a second flange portion 301 extends radially outwardly from the outer surface 298 in a parallel and spaced relationship relative to the knurled flange portion 299 the flange portion 301 defines a radial shoulder 303 which as shown in fig7 is adapted to abut against the shoulder 262 defined at the proximal end 263 of the base member 254 to limit the depth to which the plunger body portion 293 can be threaded into the passageway 268 defined in the base member 254 the membrane 276 differs in structure from the membrane 176 associated with the insert assembly 46 in that the membrane 276 includes a peripheral portion 277 which is thinner than a central portion 279 thereof to further enhance the piercing of the membrane 276 by the tip or edge 208 of the piercing blade.