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Stimulation of germ cell proliferation in the planarian Dugesia tigrina (Girard).

Sexual strains of Dugesia tigrina display a seasonal development of sex organs in late summer or autumn and lay cocoons in the suceeding spring. This is followed by a period of asexual reproduction in mid-summer. Such a seasonal alternation of reproductive pathways makes this flatworm an ideal subject of inquiry into the inductive mechanisms controlling reproduction and its ultimate coupling to changes in the environment. Asexual strains of this species reproduce by fission only. Neurosecretory phenomena appear to be involved in the induction of sexual development of planarians, as was recently demonstrated by Lender (1964) and Ude (1964) using aldehyde fuchsin staining techniques. Supporting electronmicrographic evidence came from Morita & Best (1965), Oosaki & Ishii (1965) and Lentz (1967). Brain regeneration experiments, notably by Grasso (1959) and Ghirardelli (1965), point in the same direction.