Prior to the 1600s, organisms were perceived as being complete and inseparable into smaller parts. This was due in part to the inability to see smaller structures through optical instruments like the microscope.1 Robert Hooke, in 1665, put together a primitive compound microscope and with it, he tested its capabilities on a piece of cork wood. A honeycomb-like structure was picked up by Hooke, where he compared the spaces he saw in the cork to the small rooms of a monastery. The honey-comb structure became known as cells. Hooke was unable to see a cell’s nuclei as well as other organelles since cork consists of desiccated non-living cells. A living cell was viewed for the first time under a microscope in 1674 by Anton van Leeuwenhoek. It was later noticed that each cell was unique to another cell and that they could even by separated from each other. Further research indicated that tissues were made of cells, and two centuries later, Rudolph Virchow showed the scientific community that diseased cells can originate from regular tissue cells.
The three tenets of the cell theory are that cells composed all living things, the cell is the basic functional unit of life and pre-existing cells are where other cells arise only from. A fourth tenet has since been added to the theory, after much research in the area of Molecular Biology. The fourth tenet states that cells carry genetic information in the form of deoxyribonucleic acid (DNA) and that his genetic material is hereditary from parent to daughter cell.1
1) Villarreal, L. P. (2008, August 8). Are Viruses Alive. Scientific American.