Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus.
Embryogenesis always starts with the fertilization of the ovum (or egg) by sperm. The fertilized ovum is referred to as a zygote. The zygote undergoes rapid mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of an embryo.
Although embryogenesis occurs in both animal and plant development, this article addresses the common features among different animals, with some emphasis on the embryonic development of vertebrates and mammals.
Read more about Embryogenesis: Fertilization and The Zygote, Cleavage and Morula, Formation of The Blastula, Formation of The Gastrula, Somitogenesis, Organogenesis