In vertebrates, after the completion of gastrulation the rear end of the embryo, called the tail bud, is the mass of undifferentiated mesenchymal cells, which make up the lumbar and caudal sections of the body. Mesenchymal cells have the potency to differentiate into a variety of tissues, including neural tube secondary, small tail, somites and their derivatives. Describes some of the molecular mechanisms associated with the tail bud różnicowaniwm and discusses the role of the tail bud as the organizer of the rear part of the body of a vertebrate animal.