The nucleolus is the dark circle within the larger mottled circle of the nucleus.
The nucleolus is the site of ribosomal RNA (rRNA) synthesis in eukaryotic cells. It is dense, spherical in shape, and made up principally of rRNA that is in the process of being transcribed from rRNA genes. It lies within the cell nucleus and is surrounded by the nuclear membrane. During cell division the nucleolus disappears.