The term cell was first used by an English cytologist, Robert Hooke (1665) not for unit protoplasmic masses, but for the well defined and empty compartments, he observed in a thin slice of cork under his self invented microscope. This finding of Hooke was cork is a dead and dry plant material. Obviously, what Hooke actually observed were empty cells with cellulose walls originally produced by the living cells they surrounded, yet the term cell has since been retained and universally adopted for unit protoplasmic masses of live organisms.
Leeuwenhoek (1674-76) discovered free living cells. He also observed the presence of a distinct central structure in certain unicellular organisms and some cells of multicellular organisms. Robert brown (1831) discovered that a central structure is an essential is an essential part of all living cells. He named it as nucleus first observed the jelly like protoplasm in living cells and termed it arcade. When observation upon varied types of cells and unicellular organisms many scientists during17th and 18th centuries were leading to the realization that plants and animals were made up of component cells. Two German biology Matthias j Schleiden and Theodor Schwann (1938-39) separately formulated. Famous cell theory stating that cells are the basic structural and functional unit or building blocks of the bodies of all organisms, like the bricks of house. Thus this cell theory came as the most fundamental biological generalization implying a common pattern of body architecture in all living beings.