He, along with Matthias Schleiden, are credited with formulating the theory in the 1830s. Here's a breakdown:

* Matthias Schleiden focused on plant cells and concluded that all plants are made up of cells.

* Theodor Schwann expanded this idea to animals, finding that animal tissues are also composed of cells.

Together, they proposed the following key points of cell theory:

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.

While Schwann and Schleiden are often recognized for their contributions, Rudolf Virchow later solidified the theory with his famous phrase, "Omnis cellula e cellula" (all cells come from cells).