1. The female reproductive cell which, after fertilization,
develops into a new member of the same species (von Baer, 1827),
an egg.  2. The human ovum: a round cell about 0.1 mm. In
diameter, produced in the ovary, where there is deposited around
it a noncellular covering (oolemma, zona pellucida, zona
radiata). It consists of protoplasm which contains some yolk,
enclosed by a thin cell wall (vitelline membrane). There is a
large nucleus (germinal vesicle), within which is a nucleolus
(germinal spot). By extension, the word is also used to
designate any early stage of the conceptus, when the embryo
itself constitutes a tiny and insignificant part of the whole.
(11 Mar 2008)