The ovum discharged by the ovary during ovulation is actually a secondary oocyte. The ovum is a rounded, haploid, non-motile female gamete. It is the largest cell of the body. It measures about 0.1 mm or 100 in diameters. It is almost free of yolk and is said to be microlecithal.
It has abundant cytoplasm called ooplasm having a large eccentric nucleus termed as germinal vesicle with a prominent nucleus as the center and surrounded by plasma membrane now called vitelline membrane. These is no centriole in the ovum. Ovum shows polarity having two poles. Its side which shows presence of polar body and nucleus is called animal pole while the opposite side is termed vegetal pole. The ovum is enclosed by two additional coats inner thin, thick cellular corona radiata. Between the vitelline membrane and zona pellucida there is a narrow perivitelline space. The zona pellucida is secreted by ovum itself while corona radiata is formed of radially elongated follicular cells which are glued together by hyaluronic acid.