Theodor Schwann, along with fellow scientist Matthias Schleiden, developed the cell theory, which is basically a description of the various parts and functions of plant and animal cells. Schwann and Schleiden developed the cell theory in 1838. Schwann went on to publish their findings in 1839 and, as a result, received much of the notoriety for their work.
Schwann's ideas about cell theory were essential to expanding the idea of cellular structure into the animal world. Cells were originally discovered in the mid-16th century by Robert Hooke, who identified them by studying pieces of cork under a microscope. Although there was some minor advancement in cellular theory between Hooke's discoveries and the presentation of Schwann's and Schleiden's theory, it was Theodor Schwann that first introduced the idea that living things are composed of cells. This varied significantly from previous theories that animals and plants had different compositions because of their extreme differences. Schwann postulated that cells are present in plants and animals as well as non-living things and that, although there are many different types of cells, they are structurally similar. This postulation still heavily influences the modern cellular theory that the cell is life in its simplest form and also that cells breed other cells to comprise living beings.