Although different living things may be as unlike as a violet and an octopus, they are all built in essentially the same way. The most basic similarity is that all living things are composed of one or more cells. This is known as the Cell Theory. Our knowledge of cells is built on work done with microscopes. English scientist Robert Hooke in 1665 first described cells from his observations of cork slices. Hooke first used the word "cell". Dutch amateur scientist Antonie van Leeuwenhoek discovered microscopic animals in water. German scientists Schleiden and Schwann in 1830's were first to say that all organisms are made of one or more cells. German biologist Virchow in 1858 stated that all cells come from the division of pre-existing cells. Cells are the building blocks of life. A cell is chemical system that is able to maintain its structure and reproduce. Cells are the fundamental unit of life. All living things are cells or composed of cells. The Cell Theory can be summarized as: Cells are the fundamental unit of life - nothing less than a cell is alive. All organisms are constructed of and by cells. All cells arise from preexisting cells. Cells contain the information necessary for their own reproduction. No new cells are originating spontaneously on earth today. Cells are the functional units of life. All biochemical processes are carried out by cells. • Groups of cells can be organized and function as multicellular organisms Cells of multicellular organisms can become specialized in form and function to carry out subprocesses of the multicellular organism.