mid nineteenth

“Some of the earliest seventeenth-century microscopic work showed that plant tissues were built of little units with distinct borders—cells.8 It wasn’t until the mid nineteenth century, however, that the German scientists Matthias Schleiden and Theodor Schwann hypothesized that the cell was actually the basic unit of life, that it was in some sense an independent system, and that all living things were composed of cells and their secretions.”

Simon & Schuster: The Edge of Evolution