(i) Structure of NucleusIn 1831 Robert Brown first described the nucleus. During the inter phase stage in the nucleus we can see the nucleoprotein fibers called chromatin, nuclear matrix, and one or more spherical bodies called nucleoli. It is covered by a nuclear envelope, which consists of two membranes with a space in between called peri nuclear space. The outer membrane is usually continuous with the endoplasmic reticulum and it bears ribosome's on it. The nuclear envelope is interrupted at a number of places by number of pores, formed by the fusion of its two membranes. The nucleoplasm contains nucleolus and chromatin. The content of the nucleolus that is spherical structures is continuous with the rest of the nucleoplasm. The nucleolus is not a membrane bound structure. The chromatin during cell division gets condensed and is called chromosomes. (ii) Structure of Centro someCentrosome consists of two cylindrical structures called centrioles which are surrounded by amorphous pericentriolar materials. In a Centrosome both the centrioles lie perpendicular to each other. The centrioles are made up of nine evenly peripheral fibrils of tubulin. Each fibril is a triplet and the adjacent triplets are linked. The central portion of the centriole is proteinaceous.