AbstractLocalized UV irradiation and protargol staining were used to study the development of cirral primordia during regeneration of the promers ofUrostyla weissei. The aim was to analyze the role of preexisting structures in the formation of primordia.The preexisting frontal cirri were removed by UV irradiation and formation of a new ciliary streaks was observed. When one of the disaggregating old cirri was removed the rest dedifferentiated normally and in the precise place where the irradiated cirrus had been several new kinetosomes appeared. Thus, the formation of these kinetosomes does not depend on the presence of preexisting ciliature at this site. The results provide further support for the hypothesis of integrated cortical activation combined with localized competence of formation sites.