While both played a crucial role, here's a breakdown:

* Matthias Schleiden (1838) observed plant cells and concluded that all plants are made up of cells.

* Theodor Schwann (1839) extended this idea to animals, proposing that all living organisms are made up of cells.

Together, their work laid the foundation for the cell theory, which states that:

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 credited with the foundation of cell theory, Rudolf Virchow later contributed significantly by stating the third tenet of the theory.