Robert Hooke first observed cells in 1665 when examining slices of cork under a microscope. He described the structures as "little boxes". Anton van Leeuwenhoek later discovered living cells. The Cell Theory states that all living things are made of cells, cells are the basic unit of structure and function in organisms, and new cells are produced from existing cells. Cells vary in size from bacteria at 0.1-0.5 micrometers to ostrich egg cells at 170mm by 130mm. They also vary in shape from round to elongated to branched. Cells are either eukaryotic, containing a nucleus and organelles, or prokaryotic, lacking these structures. The basic parts of a cell are the cell