Structure of ovum- 

Ovum refers to secondary oocyte stage of oogenesis, where the second maturation division is yet to occur. 

In this condition, matured ovum is released from ovary and enters into uterus in mammals for fertilization. Each ovum is a rounded and non motile structures. The cytoplasm of egg is called ooplasm. The peripheral layer of ooplasm is known as cortex and it contains many microvilli and cortical granules.

The nucleus is large and centrally placed in mammals but towards the animal pole in yolky eggs.

Ovum is covered by thin vitelline membrane which further covered by another primary membrane known as zona pellucida. There is a narrow space between two membrane called privitelline space.

During dischatge of ovum from the graffian follicle , sevwral layers of epithelial cells adhere to outer surface of zona pellucida and are arranged radially forming corona radiata.