The Histologic Basis of Regeneration and Reassociation in Lower Invertebrates

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Abstract
Histological factors are emphasizedas no less important, although less known, than organismal factors in regeneration. Recent conclusions by various investigators relating to the histological changes and problems of regeneration in sponges, coelenterates. nemerteans, turbellarians and annelids are summarized briefly and discussed, notably the problem of formative cells and results from the technique of irradiation as applied in the study of regeneration. No observations not previously published are included, except a preliminary account of the author''s study upon regeneration of the tip of the pharynx in planarians. The cells forming the new tip arise not by de-differentiation of pharyngeal cells in this region but by migration of formative cells from the body of the planarian through the pharynx to the region of regeneration. Migration of formative cells and the absence of any dedifferentiation of pharyngeal cells seem clearly established in this instance.