A method of asexual reproduction among the lowest
   (unicellular) organisms by means of a process of self-division,
   consisting of gradual division or cleavage of the into two parts, each
   of which then becomes a separate and independent organisms; as when a
   cell in an animal or plant, or its germ, undergoes a spontaneous
   division, and the parts again subdivide. See Segmentation, and Cell
   division, under Division.