Embryogenesis is the process by which the embryo is forme and develops. It starts with the fertilization of the ovum, egg, which, after fertilization, is then called a zygote. The zygote undergoes rapid mitotic divisions, the formation of two exact genetic replicates of the original cell, with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of an embryo. It occurs in both animal and plant development, but this article addresses the common features among different animals.