Zona pellucida is formed by 

Zona pellucida is a glycoprotein structure that surrounds the secondary oocyte (also known as an ovum or egg cell) in mammals. It is formed by secretions from both the oocyte itself and the surrounding follicle cells. The zona pellucida plays several critical roles in reproduction, including facilitating sperm binding and preventing polyspermy (fertilization by more than one sperm). After fertilization, the zona pellucida undergoes changes to prevent further sperm from entering and to support the developing embryo until it reaches the blastocyst stage.