Zona pellucida

A glycoprotein coating that surrounds the outer layer of the ovum (egg) and early embryo. The zona pellucida protects the developing conceptus and plays an important role in sperm binding and prevention of polyspermy (fertilization by more than one sperm).

The zona pellucida is gradually thinned and eventually disappears (is shed) just prior to implantation of the early embryo into the uterine wall.

Zygote (fertilized egg)

The single cell that results from fusion of a sperm and an ovum at fertilization. The zygote contains a full complement of chromosomes, half contributed by each parent.

After several rounds of cell division (cleavage), the zygote develops into a solid cluster of cells called a morula, which then continues to develop into a blastocyst prior to implantation in the uterine lining.