The theory of cellularization, also known as Ciliate-acoel theory, is one of the theories explaining the origin of the metazoans. It was first based on Ernst Haeckel's assumption that the earliest animals derived from ciliateprotozoans. Haeckel later abandoned this idea which is revived by Hadzi in 1953. According to the theory, a multinucleated unicellular ciliate ancestor would give rise to organisms similar to modern turbellarianflatworms by cellularization of the external layer. Recent molecular and morphologic data add increasing evidence against this view, and the alternative colonial theory, also proposed by Haeckel in the 1870s is gaining widespread acceptance.