The Cell Theory, which states that all living organisms are composed of cells and that all cells arise from pre-existing cells, was developed over time by several scientists. It's not fair to attribute it to just one person.

Here are some of the key figures and their contributions:

* Robert Hooke (1665): First to observe and name "cells" while looking at thin slices of cork under a microscope. However, he was observing empty cell walls.

* Anton van Leeuwenhoek (1674): Observed living cells (bacteria, protozoa) using improved microscopes, making more detailed observations than Hooke.

* Matthias Schleiden (1838): Concluded that all plants are made of cells.

* Theodor Schwann (1839): Extended Schleiden's idea, declaring that all animals are also made of cells. He also proposed that cells are the basic unit of life.

* Rudolf Virchow (1855): Formulated the idea that all cells come from pre-existing cells.

So, while Hooke's observations were crucial, it was the combined work of Schleiden, Schwann, and Virchow that led to the complete formulation of the Cell Theory as we know it today.