* Robert Hooke: In 1665, Hooke observed thin slices of cork under a microscope and noticed small, box-like compartments. He called these compartments "cells" because they reminded him of the cells in a monastery. This was the first observation of cells, although he did not realize their biological significance.

* Theodor Schwann: In 1839, Schwann observed animal tissue under a microscope and concluded that all animal tissues were made up of cells. He then collaborated with Matthias Schleiden to develop the first part of cell theory, which stated that all living things are composed of cells.

* Matthias Schleiden: In 1838, Schleiden observed plant tissue under a microscope and reached the same conclusion as Schwann – that all plant tissues were made up of cells. He also proposed that cells were the basic units of life.

While these three men played a key role in the development of cell theory, it is important to note that others also contributed to its development. For example, Rudolf Virchow is credited with adding the third tenet of cell theory in 1855, which states that all cells arise from pre-existing cells.