Embryonic development, also\xa0embryogenesis, is the process by which the\xa0embryo\xa0forms and develops. In mammals, the term refers chiefly to early stages of\xa0prenatal development, whereas the terms\xa0fetus\xa0and\xa0fetal development\xa0describe later stages.Embryonic development starts with the\xa0fertilization\xa0of the\xa0egg cell\xa0(ovum) by a\xa0sperm\xa0cell, (spermatozoon). Once fertilized, the ovum is referred to as a\xa0zygote, a single\xa0diploid\xa0cell. The zygote undergoes\xa0mitotic\xa0divisions\xa0with no significant growth (a process known as\xa0cleavage) and\xa0cellular differentiation, leading to the development of a multicellular embryo.Although embryogenesis occurs in both\xa0animal\xa0and\xa0plant development, this article addresses the common features among different animals, with some emphasis on the embryonic development of\xa0vertebrates\xa0and\xa0mammals.