The field within developmental biology that focuses on regeneration, the process by which an animal restores a lost part of its body. Broadly defined, the term regeneration can include many kinds of restorative activities shown by a wide variety of organisms. However, regeneration in this context connotes reproduction of a more perfect or complete replacement of a missing tissue or organ than usually results from processes of tissue repair or wound healing. Cells of the epidermis, blood, and digestive and reproductive tracts are regenerated continuously or periodically in all vertebrates. Within the field of developmental biology, however, most research in regeneration involves systems in which removing a complex structure or major part of an organism initiates a chain of events that produces a structure that duplicates the missing part both functionally and anatomically.