A breakable seal is located within a housing. The housing includes a ribbed region of restricted diameter protruding from the wall into the bore. A breakable juncture in the seal joins the tip of a cannula member to the base of the cannula member. The leading portion of the tip is free of any projection that frictionally engages either the wall or the ribbed region. Thus, neither the wall nor the ribbed region contacts or constrains movement of the leading portion in the bore when the tip is broken from the base. The trailing portion of the tip carries at least one projection constructed to frictionally engage the ribbed region. The ribbed region constrains movement of the trailing portion through the bore when the tip is broken from the base. By rocking the cannula tip, the juncture can be broken and the cannula tip advanced to a secure position in the housing.