Invertebrate: The process of regeneration is similar to the process of embryonic development. Undifferentiated embryonic cells are retained i.e. blastema. These cells are responsible for the formation of the missing part.
Vertebrates: Near the cut surface of amputation or injury, cells revert to an essentially embryonic state by losing their structural and functional specialization in a process called differentiation. These cells become part of blastema. Division and re-growing continues as long as needed to replace the lost parts.