Schwann, Theodor (tāˈōdōr shvän) [key], 1810–82, German physiologist and histologist. He was a student of J. P. Müller and professor at the universities of Louvain (1838–48) and Liège (from 1848). A cofounder (with Matthias Schleiden) of the cell theory, Schwann extended the work of Schleiden and demonstrated that the cell is the basis of animal as well as of plant tissue, and because he recognized the physiological and the morphological significance of the cell in advance of other 19th-century biologists he may be called the father of cytology. He described the nerve sheath known by his name and demonstrated the living nature of yeasts. Of great influence was his Microscopical Researches … in the Structure and Growth of Animals and Plants (1839, tr. 1847).