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.