Determination in regenerating tissues of Dugesia dorotocephala: the influence of nerve cord grafts.

Lateral fragments which contained no nerve cord were isolated from the postpharyngeal body section of Dugesia dorotocephala and fused with nerve cord grafts soon after isolation and at daily intervals through 8 days of regeneration. Fragments fused soon after isolation formed "headless" regenerates but had normal body proportions. If the lateral cordless fragment was allowed to regenerate for 1 day or longer before fusion with the nerve cord fragment, the head always developed and the body proportions were normal. Therefore, head structures become determined in the lateral fragment within the first 24 h of regeneration; during this time these tissues can also respond to the head-inhibiting influence of the nerve cord. The competence to form particular structures of the postcerebral body regions must emerge after head-forming competence is lost, that is about 24 h after isolation; however, it persists at least through the first 8 days of regeneration. Normal body proportions can be induced by nerve cord grafts throughout the first 8 days of regeneration. Lateral fragments fused at any time after isolation with another fragment containing no nerve cord developed head structures but failed to differentiate tissues of the postcerebral regions. This confirms that the nerve cord is responsible for inhibition of head structures and induction of differentiation of body regions and normal body proportions.