<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (January 2018) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (January 2018) (Learn how and when to remove this template message) </Td> </Tr> <P> 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 . </P> <P> 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 . </P>

A sequence of cell divisions that begins after fertilization is called