Fertilization normally occurs in the upper third of the oviduct. Uterine and oviduct contractions may help transport the sperm.
Fertilization occurs when a sperm and egg interact.
The sperm must pass through the several layers of follicular cells surrounding the egg, called the corona radiata. The egg cell also has its own plasma membrane, a vitelline membrane, and the zona pellucida. Sperm gain entry through a species-specific mechanism of contacting the vitelline membrane, then fusing with the egg plasma membrane before the sperm nucleus enters the cell. Once the sperm and egg nuclei fuse, fertilization is accomplished. The zygote travels down the oviduct and eventually implants in the endometrium.