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In the 1830s Matthias Schleiden, botanist, developed the first cell theory in which defined the cell as the basic unit of plant structure. The first part states that all organisms are made of cells. The second part states that cells are the basic units of life. These parts were based on a conclusion made by Schwann and Matthias Schleiden in 1838, after comparing their observations of plant and animal cells.

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First, in 1838, Theodor Schwann, a zoologist, concluded that both plants and animals were composed of cells. Second, these cells have independent lives. Third, these independent lives are subject to the organism's life.

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Around 1600s the firsts compounds microscopes were inverted, creating much higher resolving power than that of a single lens or a simple microscope.

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The Dutch spectacle maker Zacharias Janssen (b. 1585) is credited with making one of the earliest compound microscopes (ones that used two lenses) .The compound microscope is typically used for observing objects at the cellular level.

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Robert Hooke used a compound microscope in 1665 to observe a thin layer of cork. While looking at cork, Hooke observed box-shaped structures, which he called “cells” as they reminded him of the cells, or rooms, in monasteries.

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Hooke also reported seeing similar structures in wood and in other plants.
He wrote a book Micrographia and coined the term Cellula which was later on changed into cells.Term cell was derived from latin word cellula = a hollow space.

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First Robert Hooke's cell

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Paramecia seen through a compound microscope

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In the 1860s, Louis Pasteur explained that all the cells come from pre-existing cells, and with this, the cell theory was complete. And with that he contributed to cell theory by disproving the notion of spontaneous regeneration. Also the germ theory of disease was devised by Louis Pasteur.

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The theory explains that the cell is the structural (the living things are made up of cells), functional (are able to carry out all vital functions) and reproductive unit of life (cells duplicate their contents and then divide to yield multiple cells with similar, if not duplicate, contents).

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animal cell in microscope, Schwann

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plant cell in microscope, Schleiden