Cell theory is based upon the following evidence:

-The use of an early microscope by Robert Hooke to look at a piece of cork and called what he saw (dead cells) as Cells (1663)
-In 1673, Anton van Leeuwenhoek saw bacteria and protoza through his microscope
-In 1833, Robert Brown discovered the nucleus
-In 1838, Dr. Schleiden published a paper stating that the nucleus was the most important part of the cell and the basic structure from which the rest of the cell developped.
-In 1939, Dr Theodor Schwann published a book in which he stated that there was a link between animal and plant cells: the nucleus. This spawned what is now known as the cell theory
-The final addition was from Rudolph Virchow. he stated that all cells come from other cells.
http://fig.cox.miami.edu/~cmallery/150/unity/cell.text.htm