Morphogenesis of regenerating fragments of Dugesia schubarti (Turbellaria tricladia).

An extensive study of regeneration processes from the point of view of the axial gradient theory and its corollary, the possibility of forming two heads from one fragment (the Janus head problem) was undertaken on the South American species Dugesia schubarti. Altogether 330 specimens were used, cut into 2,640 pieces and the regeneration of each of them was followed to the end result. Concerning the role of an axial gradient in the regeneration processes the results were not supportive. Anterior fragments often regenerated less completely (eyes and auricles without complete heads) than more posterior ones, although the differences were not sharp, due to individual variations of physiological nature. The Janus head regeneration, one of the postulates of the axial gradient theory, was analyzed both on preauricular (parts A) and postauricular fragments (part B). The regeneration of parts A was less perfect than that of parts B. ALthough a considerable number of the part A fragments regenerated normally, the majority of the cases remained incomplete for a long time and eventually disintegrated without completing the regeneration process. The only irregularity parts B proceeded more evenly and the death rate was also much lower. The only irregularity observed was a great number of cyclopic regenerates, i.e. heads with one single eye in the midline. Some of these cyclopic heads remained permanently in this condition but most of them later separated in two eyes but with a pigment between them, and in others the cyclopic eye separated in two normal eyes at the end of the regeneration. The Janus head phenomenon was never observed.