A cylindrical structure some 500 nm long and 150 nm in diameter composed of nine longitudinal fibrils. Centrioles are not found in the cells of higher plants, but are commonly found in pairs in animal cells and have been identified in the cells of some primitive plants, e.g. brown algae. At the commencement of cell division fibrils appear, radiating from the centrioles and forming *asters
. These form the poles from which the *spindle
fibres arise. The basal bodies (see kinetosome
) of *flagella
and cilia arise from centrioles and have a similar structure.