Here's a breakdown:

* Matthias Schleiden (1838) was a botanist who observed that all plants are made up of cells.

* Theodor Schwann (1839), a zoologist, extended this idea to animals, concluding that all living things are composed of cells.

Together, their work laid the foundation for the cell theory.

While Schwann and Schleiden are credited with the core ideas, it's important to remember that Robert Hooke (1665) was the first to observe and name cells (though he did not understand their significance).

Additionally, Rudolf Virchow (1855) added another key principle to the cell theory: "Omnis cellula e cellula" (all cells arise from pre-existing cells).

So, while Schwann and Schleiden are the main figures, the development of cell theory involved several scientists and their contributions over time.