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Literally "all cells from preexisting cells". This saying summarizes what has become called the cell theory. Today when we discuss this theory we understand the theory as having a at least three parts:
1. A living things are made up of cells.
2. All cells come from pre-existing cells.
3. There is no spontaneous generation under current conditions.
Today these seem self evident, but this theory actually only dates from the mid 19th century. For example, before the mid 19th century many people believed in spontaneous generation. This is the idea that living things can develop from non-living things. People, for instance, used to believe that flies actually developed from rotten meat or that bacteria developed from stagnant water, or that frogs developed from the mud at the bottom of ponds, or that a horse hair put in water would turn into a worm. Slowly, though scientists cleared up these beliefs until now we accept the notion that under current conditions, life does not arise from non life.
Cell theory is really important because it provided, and still provides one
of the great unifying theories in biology. One that says in spite of all the
vast diversity of organisms, they are are united at a very fundamental level,
namely the presence of cells.
referring links:VBS Home page, Cells navigator