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 developmentdescribe later stages. Embryogenesis starts with the fertilization of the egg cell (ovum) by a sperm cell, (spermatozoon).
Thumbnail: Human embryo, 8-9 weeks, 38 mm. Image used with permission (CC BY-SA 3.0; ).