Problem of Polarity in Regeneration in Planarians
As in Hydra, flatworm regeneration as well appears to occur in a polar fashion. There seems to be ananterior-posterior gradient system as the cut anterior surface will make a head and the cut posterior surface will form a head. As in Hydra, middle pieces are able of regenerating both a head and a tail. Identical to the Hydra there is an exception to this rule, for if the middle piece is too thin as compared to the planarian segment will grow two heads.
It is believed that the anterior-posterior gradient is not efficiently delineated in such a small segment and that the piece cannot determine "Which end is up", as neither end of the segment has an advantage over the other. Evidence exists that the polarity gradient is due to metabolic activity and that the much more metabolically active end makes the head. Two-headed flatworms are so produced by removing anterior posterior metabolic gradient. It has also been observed that flatworms in addition to retaining anterior-posterior polarity can also regenerate the missing half if split longitudinally. It is believed that the pattern of planaria regeneration is similar to hydra as it also controlled by diffusible gradients.