By the mid-1600s, microscopes had enough magnifying power to allow scientists to observe cells for the first time. Robert Hooke, an English scientist, published an image of a slice of cork in 1665. He observed regular structures in the wood and gave them the name "cells" because he thought they resembled the small monastery rooms in which monks resided. Hooke did not understand the importance of his observation, although he did notice the wood contained a very large number of these cells. In the following decade Antonie van Leeuwenhoek, a Dutch scientist, observed living cells of protozoa and bacteria, sperm cells, and red blood cells, but he did not realize that they had similar structures. In the following century many scientists noted the large numbers of cells present in plants, animals, and single-celled organisms. However, it was not until biologist Theodor Schwann and botanist Matthias Schleiden compared their observations of animal and plant cells that they recognized the structural similarities and concluded that cells were the fundamental unit of both groups of organisms.
In 1838 Schleiden and Schwann proposed the first widely acknowledged version of modern cell theory. In their version of the cell theory, Schleiden and Schwann stated that the cell is the basic building block of life and that all organisms are made up of one or more cells. The last part of modern cell theory, that all cells arise from preexisting living cells, was proposed in the 1850s. This modern, unified cell theory helped scientists better understand their observations. Before the cell theory, some scientists thought that cells formed only one part of the structure of an organism and were connected to each other by fibers. Modern scientists understand that cells make up all of the tissues of a complex organism and that the connections between cells are an essential part of the cell structure.The second part of the cell theory, which states that all living things are made up of cells, helped scientists realize that cells in different organisms have similar essential structures and functions. For example, the development of either a plant cell or an animal cell is controlled by its nucleus, the organelle that houses the genetic material (DNA). Since all organisms are made up of cells, scientists can apply lessons learned from one cell type to understanding other cell types. The idea that all cells come only from other living cells provides a framework for all of modern biology. It helped establish the definition of life because it eliminated the earlier ideas that cells could arise spontaneously or from nonliving matter. Observations of genetic structures, such as the chromosome, which contains DNA, within the nucleus of the cell led to an understanding of the way cells could pass on traits through genetic inheritance.