A structure associated with the centrosome and found mainly in animal cells. It consists of two short cylinders, orientated at right angles to each other and composed of microtubules. When present, the centriole replicates during the nondividing phase of the cell cycle, and during the prophase of mitosis a centriole migrates with each centrosome to lie at opposite poles of the cell. Centrioles seem to act as an orientational device, or jig, in the assembly of centrosomes; they are not obligatory for the assembly of the spindle microtubules—they are not present in the cells of most higher plants, and their removal from cells does not affect spindle formation. However, centrioles are essential for the assembly of eukaryotic cilia and flagella. See also flagellum.