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During the last few years a whole literature has come into existence on the alleged parasite of carcinoma. In every laboratory this obscure branch of pathological histology has been studied with zeal and persistence. Cohnheim's theory that tumors are due to exaggerated growth of primary remnants of the embryonal folds, gone astray in various parts of the body, has been severely criticised by the enthusiastic believers in the parasitic origin of carcinoma; it has been characterized as purely speculative, as founded only upon probability. It has been shown that embryonal folds exist in all multi-cellular organisms and that consequently invertebrates, theoretically considered, would be just as much liable to various forms of tumors as the higher animals, but such a thing as a carcinoma has never been described in the inferior animals up to the present time although they are very liable to parasitic tumors of various kinds. Cohnheim stated
THE PARASITE OF CARCINOMA.. JAMA. 1893;XX(2):49-50. doi:10.1001/jama.1893.02420290023003