A fine structural study of regeneration after fission in the planarian Dugesia japonica

We examined morphologically the process of regeneration before: and after f ission in a sexual strain of the freshwater planarian Dugesia japonica. Usu ally fission takes place in the post-pharyngeal region. Decapitation signif icantly accelerates the rate of fissioning. When decapitated worms were tre ated with substance P and neuropeptide K separately, the rate of fission ma rkedly decreased in both cases. Before the onset of fission, a presumptive region of fission was recognized in the post-pharyngeal portion where undif ferentiated cells, regenerative cells and newly differentiated cells were l ocalized. Moreover a functional network of fixed parenchyma cells was noted in this region. After fission, cell distribution in the blastema became qu ite different from that of artificially amputated worms. This difference se ems to be due to the process that occurs in the presumptive region of fissi on.