The scientists who developed the theory that all living things are made up of tiny cells are Theodor Schwann and Matthias Schleiden.

Here's a breakdown:

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

* Theodor Schwann (1839), a zoologist, extended this observation to animals, realizing that they too are made of cells.

Together, their work led to the formulation of the Cell Theory, which states:

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 they didn't work in complete isolation, their combined efforts were crucial in establishing this foundational principle of biology.