Structure of Ovum:
The ovum is nearly spherical in shape. It has an outer covering called the vitelline membrane. External to that there is another covering called zona pellucida. The space between the vitelline membrane and zona pellucida is called perivitelline space. The ovum discharged from the Graafian follicle of the ovary has a layer of granular cells. This is called corona radiata.
The cytoplasm of the ovum is called ooplasm. Cytoplasm is coved by a plasma membrane. Very small vesicles termed cortical granules are present under the plasma membrane. The cytoplasm contains a large nucleus called a germinal vesicle. There is no centriole in the ovum. The human ovum loses its ability to be fertilized about 24 hours after ovulation.