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The structural organisation of the nucleolar apparatus during oogenesis of the spotted rayTorpedo marmoratawas investigated. The observations showed that unlike other cartilaginous fishes, inT. marmoratathe nucleolar apparatus was always represented by one or two conspicuous nucleoli, whose organization significantly changed during oocyte development. In the smallest follicles(follicles<300 μm in diameter) the nucleolus was made up of granular and fibrillar components, and actively incorporated3H uridine; later it becomes more and more electron-dense so in follicles of 400 μm in diameter its components and3H uridine incorporation were no longer evident. These results indicate that inT. marmoratathe nucleolar apparatus significantly changes and undergoes a possible impairment in rRNA synthesis. After nucleolus inactivation, the synthesis of rRNA may be substained by granulosa. [ABSTRACT FROM AUTHOR]