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Origin of the regeneration blastema in polychaete annelids.

Since the early part of this century, controversy has surrounded the origin of the cells ot the regeneration blastema in annelids. Cells of the epidermis and reserve mesodermal cells have each been thought to form the entire blastema. With increasing frequency, it is being suggested that in numerous polychaetes each layer of tissue in the regenerate arises from the corresponding old germ layer. Evidence presented here from experiments utilizing techniques of radioautography, histochemistry, X-irradiation, and electron microscopy in addition to histological observation largely supports the last hypothesis. In the light of recent experiments, it appears that the new epidermis and gut arise from the corresponding old tissue. The formation of the mesoderm cannot yet be generalized, but reports indicate that it may arise from dedifferentiated muscle cells, cells of the peritoneal epithelium, and free coelomocytes.