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.