Inductive role of the nerve cord in regeneration of isolated postpharyngeal body sections of Dugesia dorotocephala

Lateral cordless fragments isolated from the postpharyngeal section of Dugesia dorotocephala formed a large normal head at a 90° angle to the original antero‐posterior polarity; postcerebrally, only a hump of undifferentiated tissue developed. This “head‐hump” pattern, and also other types observed in previous studies of lateral fragments, were attributed to the absence of the nerve cord. In order to confirm the inductive role of the nerve cord and to eliminate the possibility that the “head‐hump syndrome” was due to the relative proportions of other tissues besides nerve, body fragments of two experimental groups were observed: (1) the five types of fragments which had no nerve cord but had varying proportions of other tissues present formed primarily “head‐hump” types of regenerates. (2) Almost all fragments which had varying amounts of nerve cord present but the same proportions of other tissues formed regenerates of normal body proportions. Therefore, the absence of the nerve cord does determine the “head‐hump syndrome”. Isolated postpharyngeal half segments containing one nerve cord were allowed to regenerate for varying periods of time before the lateral cordless fragment was isolated. The number of “head‐hump” regenerates from lateral fragments isolated after a one‐day or longer contact with the nerve cord gradually decreased, and the number of regenerates with incomplete head development or which were more elongated postcerebrally increased. These results indicate that the nerve cord acts gradually to determine the differentiation of specific tissues rather than rapidly to determine the overall body plan.