The cell theory was arrived at due to the contributions of many scientists. Anton van Leeuwenhoek, in the 1600's made simple microscopes. He is credited with discovering microorganisms in a drop of water and went on to observe over the next 50 years as many things as he could with his simple microscopes. Robert Hooke, another scientist in 1665 looked at thin slices of cork and observed a honeycomb shape. These empty spaces he named "cells" because they resembled the cells that monks lived in. They were actually cell walls from dead plant cells. By 1838, Matthias Schleiden, a German botanist stated that all plants were composed of cells. A German zoologist Theodore Schwann stated that all animals were made of cells, in the following year. Further observations by others and the invention of newer and better microscopes led to the cell theory. It states that all organisms are made of one or more cells. The cell is the basic unit of structure and function in living things and all cells come from preexisting cells.