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Ultraviolet Fluorescence micrograph showing the microtubular network of an animal cell, made visible with fluorescent antibodies. Microtubules, together with microfilaments (not seen here), form a three-dimensional array known as the cytoskeleton of the cell. This fibrous network, only recently observed using ultraviolet light & fluorescent stains, is still poorly understood. Microtubules, more rigid than microfilaments, are thought to act as direction markers in the cell. (KEYSTONE/SCIENCE PHOTO LIBRARY/FRANCIS LEROY, BIOCOSMOS.)