Origin of the Regeneration Blastema in Polychaete Annelids

Since the early part of this century, controversy has surroundedthe origin of the cells ot the regeneration blastema in annelids.Cells of the epidermis and reserve mesodermal cells have eachbeen thought to form the entire blastema. With increasing frequency,it is being suggested that in numerous polychaetes each layerof tissue in the regenerate arises from the corresponding oldgerm layer. Evidence presented here from experiments utilizingtechniques of radioautography, histochemistry, X-irradiation,and electron microscopy in addition to histological observationlargely supports the last hypothesis. In the light of recentexperiments, it appears that the new epidermis and gut arisefrom the corresponding old tissue. The formation of the mesodermcannot yet be generalized, but reports indicate that it mayarise from dedifferentiated muscle cells, cells of the peritonealepithelium, and free coelomocytes.