The phenomenon of embryo formation from a vegetative cell derived from zygote is known as

The phenomenon of embryo formation from a vegetative cell derived from zygote is known as Adventive polyembryony.
Polyembryony is the phenomenon of two or more embryos developing from a single fertilized egg.
Adventitious embryonic from nucellar cells is the mechanism leading to apomixis in most plants or it can be referred to as formation of extra embryos by the impact of sporophytic budding from as adventitious cells.