Unique cutting arrangements of 1H worms highlight the key role ventral nerve cords (VNC) play in directing regeneration outcomes.

Excised trunk fragments of planaria were cut with an additional incision to generate a long segment containing a VNC with backwards polarity (cuts shown as red dotted line on traced nerve map in A, and on synapsin stain in B), or with an inverted incision of the same pattern to generate a long, free segment containing a VNC with forwards polarity (red dotted line on traced nerve map in I, and on synapsin stain in J). Modeling-predicted morphogen gradients were consistent with the formation of a tail on the long, free segment for the backward-polarity VNC incision pattern (G, H), and head formation on the long, free segment for the forwards-polarity VNC cutting pattern (O, P). These predictions were a direct match to actual regeneration outcomes (N = 80/80). The backwards-polarity VNC (synapsin stains in C, D, E, and final body-plan in F) showed regeneration of a tail on the long, free segment, as predicted. The forwards-polarity VNC (synapsin stains in K, L, M, and final body-plan in N), showed regeneration of a head on the long, free segment, as predicted.