In 1665, Robert Hooke was able to observe in a piece of cork specimen structures which appear as tiny compartments similar to small rooms that are fitted to each other. Hooke coined the word "cell" to describe the chamber-like structures and later became famous. Hooke thought that only plants and fungi were the only ones made up of cells. 

In 1676, Anton van Leeuwenhoek published his observations on tiny living organisms which he named animalcules. It was believed that  Leeuwenhoek was the first to observe under his microscope the structure of a red blood cell of different animals as well as sperm cell. 

In 1831, Robert Brown,  one of the leading botanists in his time was able to compare diverse kinds of plant specimens under the microscope. He markedly indicated that there is one common thing about them- they are all composed of cells, and inside the cell is a dark dense spot which he termed as the nucleus. 

In 1838, Matthias Schleiden,  a German botanist concluded that all plant parts are made of cells. 

In 1839, Theodor Schwann,  also a botanist and a close friend of Schleiden,  stated that all animals tissues are composed of cells, too. 

In 1858, Rudolf Virchow concluded that all cells come from pre-existing cells. 

The discoveries made by Hooke, Leeuwenhoek, Schleiden, Schwann, Virchow, and others led to the formulation of the CELL THEORY. The cell theory is universal for all living things, no matter how simple or complex,  tiny or huge it is. This theory can be summed up into three basic components: (1) all living organisms are composed of one or more cells; (2) the cell is the basic unit of life in all living things; and (3) all cells come from pre-existing cells.