4. (Biol.) 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 Cell division, under Division.
The nucleus is sometimes termed the endoplast or endoblast, and in the protozoa is supposed to be
concerned in the female part of the reproductive process. See Karyokinesis.
5. (Zoöl.) (a) The tip, or earliest part, of a univalve or bivalve shell. (b) The central part around which
additional growths are added, as of an operculum. (c) A visceral mass, containing the stomach and
other organs, in Tunicata and some mollusks.