A small dense round body within the nucleus of a nondividing eukaryotic cell that is the site of ribosome assembly. It forms around the nucleolar organizer, which encodes most of the segments of ribosomal RNA. Ribosomal proteins migrate to the nucleolus from their assembly sites in the cytoplasm and are packaged into ribonucleoproteins, which then return to the cytoplasm where they become mature ribosome particles. Two or more nucleoli may be present, depending on the species of organism and stage of the cell cycle.