The Origin and Continuous Replacement of Epidermal Cells in the Planarian Polycelis Tenuis (iijima).

R A Y L A N K E S T E R (1873) coined the term 'Triploblastic' and supposed that the gut, parenchyma and epidermis of Turbellaria corresponded to the germ layers of contemporary dogma. This idea is still current, though neither the origin, nor the maintenance of the epidermis of planarians has been investigated in detail. Most embryological studies have been restricted to early development, but Bardeen (1902) worked on embryos of a wide range of ages and claimed that their epidermal cells divide amitotically. Both Mattiesen (1904) and Fulirisky (1916), however, denied that cell division occurs there, and since this has been confirmed for the epidermis of the adult (Skaer, 1961), the cells must be recruited from elsewhere. I suggested that the entire epidermis might be continuously renewed by migration of cells from the parenchyma to the periphery. The idea that cells from the parenchyma might enter the epidermis has been put forward several times. Hallez (1887) described motile cells, equivalent to neoblasts, that enter the epidermis throughout development. Both Mattiesen (1904) and Fulirisky (1916) mentioned (without further description) the presence of a second epithelial layer beneath the flattened blastomeres of the young embryo; they supposed that this layer was made up of replacement cells for the epidermis, although they found no evidence of replacement. Loman (1887) suggested that rhabdite-containing cells from the parenchyma might enter the epidermis of the land planarian Bipalium, but gave no evidence for this suggestion and did not even discuss it. Korotneff (1909) held that the epidermal cells of triclads were phagocytes of mesenchymal origin, and Sabussow (1911) claimed that free mesenchymal cells (recognizable as neoblasts) sometimes entered the epidermis of planarians from Lake Baikal. Epidermal replacement cells in Acoela from the Gulf of Finland were described by Luther (1912); he suggested that they arose in the parenchyma but he did not find them there. Dorey (1963),