Asexual reproduction and regeneration ofCatenula (Turbellaria,Archoophora)

Summary Studies were made onCatenula, a turbellarian of the order Catenulida, which had been cultured for 6 years in our laboratory. Fission begins inCatenula when the animal exceeds a specifically defined length. Neoblasts accumulate where the body wall narrows, near the subepithelial nerve cell. These cells have a large nucleus of condensed chromatin and a large active nucleolus. They have little cytoplasm, which in addition to free ribosomes, contains a small number of rough endoplastic reticular cisternae and a few mitochondria. Stem cells of epithelium were also found. These cells are similar to neoblasts, having additionally a bundle of centrioles in the cytoplasm.Differentiation of tissues and cells during regeneration proceeds in a manner identical to that during paratomy. After injury the neoblasts collect in two primordia of the brain, but do not form blastemae, as occurs in Tricladida. It is likely that dedifferentiation plays some role in each of the processes examined. A theoretical model of the mechanisms controlling paratomy and regeration is presented. The factors controlling these processes include the inductor formed by the subepithelial nerve cells and the inhibitor blocking it, formed by the brain. The inductor is probably a neurosecretion that combines with a competent receptor on the surface of cells capable of dedifferentiation.