A demountably attachable cartridge of a surgical instrument, the cartridge comprising: a housing demountably attachable to an end effector of the surgical instrument; the housing including a proximal end, a distal end, an upper surface extending between the proximal and distal ends (224, 226), a central cavity extending between the proximal and distal ends (224, 226), and a longitudinal slot extending through the upper surface; a first member mounted in the housing and movable toward the distal end; wherein a knife is pivotally coupled with the first member, the knife having a cutting edge configured to cut when moved toward the distal end, the cutting edge extending through the longitudinal slot for at least a portion of the distal movement of the first member; a second member operable to move with the first member toward the distal end through a first distance, relative movement between the first and second members (214, 218) occurring after the first and second members (214, 218) move through the first distance, the second member blocking a rotation of the knife relative to the first member while moving with the first member through said first distance and permitting rotation of the knife after a relative movement between the first and second members (214, 218); characterized by a lead screw coupled with the housing for rotation relative to the housing, the lead screw being operatively coupled with the first member and the second member to drive the coupled member along at least a portion of the lead screw in response to rotation of the lead screw, the lead screw having a threaded portion and a non-threaded portion disposed distally of the threaded portion, both the first and second members (214, 218) being driven along the lead screw when the second member moves with the first member toward the distal end at the same rate through said first distance and one of the first and second members (214, 218) interfacing with the non-threaded portion and the other one of the first and second members (214, 218) interfacing with the threaded portion to generate said relative movement between the first and second members (214, 218) in response to a rotation of the lead screw. a housing demountably attachable to an end effector of the surgical instrument; the housing including a proximal end, a distal end, an upper surface extending between the proximal and distal ends (224, 226), a central cavity extending between the proximal and distal ends (224, 226), and a longitudinal slot extending through the upper surface; a first member mounted in the housing and movable toward the distal end; wherein a knife is pivotally coupled with the first member, the knife having a cutting edge configured to cut when moved toward the distal end, the cutting edge extending through the longitudinal slot for at least a portion of the distal movement of the first member; a second member operable to move with the first member toward the distal end through a first distance, relative movement between the first and second members (214, 218) occurring after the first and second members (214, 218) move through the first distance, the second member blocking a rotation of the knife relative to the first member while moving with the first member through said first distance and permitting rotation of the knife after a relative movement between the first and second members (214, 218); characterized by a lead screw coupled with the housing for rotation relative to the housing, the lead screw being operatively coupled with the first member and the second member to drive the coupled member along at least a portion of the lead screw in response to rotation of the lead screw, the lead screw having a threaded portion and a non-threaded portion disposed distally of the threaded portion, both the first and second members (214, 218) being driven along the lead screw when the second member moves with the first member toward the distal end at the same rate through said first distance and one of the first and second members (214, 218) interfacing with the non-threaded portion and the other one of the first and second members (214, 218) interfacing with the threaded portion to generate said relative movement between the first and second members (214, 218) in response to a rotation of the lead screw.