Human embryogenesis is a complex process that occurs during the first eight weeks after fertilization. The stages of embryogenesis are described in more detail below. Stage 1: The process of embryogenesis begins with an egg or ovum being fertilized by a sperm cell to form a zygote. The zygote is surrounded by a strong membrane made up of glycoproteins, which the sperm has managed to penetrate. The zygote is formed 24 hours after the egg and sperm nuclei fuse.