The zona pellucida is the transparent, thick outer layer of the ovum in mammals that protects the cell as it travels from the ovary to the uterus. The National Institutes of Health states the layer regulates interactions between ovulated eggs and free-floating sperm as the egg is fertilized. In order to fertilize the egg, sperm must penetrate this outer layer as it thins on the way to the uterus.
The zona pellucida is not made of cells as it is the outer layer of the specialized ovum cell. The layer forms as the egg develops in the ovary. The zona pellucida disappears when the egg reaches the uterus, so the egg can be implanted whether it is fertilized or not.
The outer layer is made of carbohydrates that determine whether or not a sperm fertilizes the egg. Once the egg is fertilized, the zona pellucida keeps other sperm out; otherwise a condition called polyspermy results. Scientists have identified the enzymes and proteins that form the zona pellucida and how the membrane hatches embryos.
The zona pellucida is a purely mammalian structure as mammals are the only creatures on Earth that use ovum fertilization to procreate. The ovum is the largest single biological cell known to scientists.