Between 1830 and 1930, improvements in microscopes made it possible for scientists to describe the nature and behavior of cells. Although Robert Hooke had seen cells more than 150 years earlier, new cultural stresses on individuality made nineteenth-century Western society especially receptive to cell and germ theory and encouraged the very technologies that made cells visible. Both scientists and nonscientists used images of cell structure, interaction, reproduction, infection, and disease as potent social and political metaphors. In particular, the cell membrane - and the possibility of its penetration - informed the thinking of liberals and conservatives alike. In Membranes, Laura Otis examines how the image of the biological cell became one of the reigning metaphors of the nineteenth century. Exploring a wide range of scientific, political, and literary writing, Otis uncovers surprising connections among subjects as varied as germ theory, colonialism, and Sherlock Holmes's adventures. At the heart of her story is the rise of a fundamental assumption about human identity: the idea that selfhood requires boundaries showing where the individual ends and the rest of the world begins.