Embryogenesis is the process by which the embryo forms and develops. In mammals, the term refers chiefly to early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.
Embryogenesis starts with the fertilization of the egg cell (ovum) by a sperm cell, (spermatozoon). Once fertilized, the ovum is referred to as a zygote, a single diploid cell. The zygote undergoes mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of a multicellular embryo.
Fertilization also known as syngamy is the process of fusing of the ovum by the sperm to create a new organism. In animals, the process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo. The development of the embryo can occur either internally or externally to the female's body.