A body, usually spheroidal, in a cell or a protozoan,
   distinguished from the surrounding protoplasm by a difference in
   refrangibility and in behavior towards chemical reagents. It is more or
   less protoplasmic, and consists of a clear fluid (achromatin) through
   which extends a network of fibers (chromatin) in which may be suspended
   a second rounded body, the nucleolus (see Nucleoplasm). See Cell
   division, under Division.