The ventral nerve cord signals positional information during segment formation in an annelid (Ophryotrocha puerilis,...

Growth and regeneration of segments were recorded in the polychaeteOphryotrocha puerilis. In one experiment the ventral nerve cords (VNCs) of the animals were cut; in the other, VNCs were left intact. VNC lesion in some specimens resulted in the outgrowth of supernumerary posterior parts from the site of operation. The characteristics of outgrowth of these supernumeraries were essentially the same as in normal specimens without double tails. After removing different numbers of caudal setigers, each of the two tails of the same double-tail monster independently regenerated different segment numbers within a given time. A simple model is proposed, allowing for these results, which states that the larval body of a polychaete consists of two regions with completely different positional values (episphere - prostomium; hyposphere - pygidium). During growth, segments with intervening positional values are intercalated. The rate of segment formation is high when there is a wide gap in positional values between pygidium and adjoining budding zone and the posteriormost segment. As this gap narrows, the growth rate slows down. During caudal regeneration, first of all a new pygidium with an adjacent proliferation zone is formed and the original positional value of the posteriormost part of the body is reestablished. Segment regeneration follows the same rules as segment growth. The results presented here also demonstrate that the VNC plays an important role, not only in segment proliferation, but also in signalling positional information to the newly formed segments.