The century-old controversy over the reproduction and function of the centriole is examined to elucidate the conceptual and methodological issues that have made it so resistant to closure. The study of centrioles is situated in two distinct eras punctuated by the deployment of the electron microscope. From the late 19th century to the mid-20th century, centrioles were defined largely in functional terms- as self-reproducing 'central bodies' playing a directive role in mitosis. During this period, when their structure remained unknown, their universal presence in all cells was ambiguous, and their reality was seriously debated. A conceptual switch occurred after the mid-1950s. When the centriole was made visible under the electron microscope, it was defined in terms of its characteristic cart-wheel structure, but its function and manner of reproduction have remained enigmatic. The controversy over the nature of centrioles illustrates the dynamic interplay of techniques, theories, and background assumptions in the production of scientific knowledge. It also highlights the difficulties biologists face in coming to grips with problems of cell structure and intracellular morphogenesis.