The cell theory was formulated in the mid-19th century by three key scientists:

1. Matthias Schleiden (1838): A German botanist, Schleiden proposed that all plant tissues are composed of cells and that the cell is the basic unit of plant structure.
2. Theodor Schwann (1839): A German zoologist, Schwann extended Schleiden’s ideas to animals, proposing that all animal tissues are also composed of cells. Together with Schleiden, Schwann helped establish the concept that cells are the fundamental unit of life for all living organisms.
3. Rudolf Virchow (1855): A German physician, Virchow contributed the idea that all cells arise from pre-existing cells, emphasizing the continuity of life through cell division. This addition completed the cell theory by including the principle of cell lineage.

The cell theory, as formulated by these scientists, has three main principles:

1. All living organisms are composed of one or more cells.
2. The cell is the basic unit of structure and organization in organisms.
3. All cells arise from pre-existing cells.