German physiologist who served as an assistant to Johannes Müller. He discovered the digestive enzyme pepsin in 1836. He showed that yeast were tiny plant-like organisms, and suggested that fermentation was a biological process. Schwann was a master microscopist who examined animal tissue, specifically working on notochord development in tadpoles. In "Mikroskopische Untersuchungen über die Übereinstimmung in der Struktur und dem Wachstum der Thiere und Pflanzen" ("Microscopic researches on the Conformity in Structure and Growth Between Animals and Plants," 1839), he recognized nuclear structures similar to what Schleiden had observed in plants. In 1839, he extended Schleiden's cell theory to animals, stating that all living things are composed of cells. He believed that new cells form principally outside pre-existing cells, and wanted to draw an analogy to crystal formation.

Müller (Johannes), Schleiden