Cytokinesis is accomplished in animal cells by a narrowing cleavage furrow that pulls inward and eventually splits the cell, while plant cells divide through the formation of a new cell wall that grows outward from the central portion of the cell. Because animal cells have a movable plasma membrane instead of the stiff outer cell wall which plant cells possess, the cleavage furrow can easily pull inward until the cell is physically split. A plant cell's stiff outer wall, however, does not permit this type of pliability, and requires the construction of a new cell wall to divide.
During cytokinesis, a plant cell sends vesicles to its equator that are filled with the materials needed to build a new cell wall. As the vesicles arrive at the cell's equatorial region, they collide with each other and fuse together into what is called a phragmoplast. The growing number of vesicles arriving at the equator causes the developing cell wall to expand outwards until it reaches the outer walls. When the new wall touches the outer wall, it fuses with it and effectively compartmentalizes the cell into two halves that can separate into two fully enclosed daughter cells.
The cleavage furrow that divides an animal cell is also called a contractile ring. It is believed that the course taken by the narrowing contractile ring during cytokinesis is determined by the microtubules of the cell's mitotic spindle or positioned by a spatial cue taken from the cell's medial nucleus. In plant cells, the trafficking of vesicles to the developing phragmoplast's position at the cell's equator is believed to be directed toward the remnants of the plant cell's mitotic spindle.