The fertilisation of the egg cell and the sperm cell results in the formation of a single celled zygote. On undergoing mitotic divisions ( cleavage) the zygote further develops into a multicellular embryo.
In plants, the embryo ( along with other cells from the plant ) develops into the seed that germinates as a new plant. The zygotic embryo is formed following double fertilisation of the ovule. Embryogenesis involves cell growth and division, cell differentiation and programmed cell death.
Animal embryogenesis involves various stages such as gastrulation, formation of early nervous system and initiation of organogenesis.