[Formation of the zone of paratomy in freshwater oligochaetes]

Oligochaetes Nais communis and Pristina longiseta are capable of paratomy, i.e., asexual reproduction of the cross division type, when division proceeds across the long body axis and daughter organisms retain the maternal axes. Paratomy is represented by two forms: slow and fast. Slow paratomy is accompanied by the formation of chains from no more than two zooids (N. communis), while fast paratomy leads to the formation of chains from many zooids (P. longiseta). Analysis of the appearance and development of the zone of paratomy (constriction) has shown that when its cephalogenic part is formed, only head segments appear and are formed simultaneously, while in the somatogenic part, body segments appears successively. Dedifferentiated cells of the covering epithelium appear to serve as a source of newly formed structures. It was shown using mathematical statistics that constriction is laid down in the zone defined by a system of coordinates formed by unknown factors. Although the constriction is laid down in the middle of a segment, its subsequent growth is not related to the transformation of adjacent areas of the segment.