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Microscopic Researches Concerning the Unity in the Structure and Growth of Animals and Plants. Schwann's book became a scientific classic almost from the moment of its publication. It was Schwann, too, who, simultaneously with Cagniard la Tour, discovered the active principle of gastric juice to be the substance which he named pepsin. The cell theory was for some time combated by the most eminent German men of science. Thus Liebig, in apparent agreement with Helmholtz, took a firm stand against the new doctrine with his famous