Light micrograph of a segregated nucleolus (dark) in the nucleus of an onion (Allium cepa) meristematic cell during interphase of mitosis (nuclear division). The nucleolus is responsible for producing components of ribosomes, the cell's protein-manufacturing organelles. It can become segregated when this production is halted as the cell begins mitosis, the formation of two daughter nuclei from one parent nucleus. Silver staining has been used to highlight the nucleolus.