All cells come from pre-existing cells by cell division. Schwann also proposed “Free Cell Formation” or spontaneous generation of cells — this was before Pasteur's definitive experiments. Modern Cell Theory Also States: Life's chemical processes, such as metabolism, occur inside of cells.

Introduction:
The famous generalization "All cells are derived from pre-existing cells" is attributed to a scientist named Rudolf Virchow. This principle is known as the cell theory and is a fundamental concept in biology.
Rudolf Virchow and the Cell Theory:
- Rudolf Virchow was a German physician and pathologist who lived in the 19th century.
- He is considered one of the founders of modern pathology and is known for his significant contributions to medical science.
- In 1855, Virchow proposed the cell theory, which states that all living organisms are composed of cells and that cells only arise from pre-existing cells.
- This principle contradicted the prevailing belief at the time, which suggested that cells could arise spontaneously.
Supporting Evidence:
- Virchow's cell theory was supported by his observations and experiments.
- He studied cellular pathology and found that diseases were caused by abnormalities in cells.
- Virchow also examined cell division and observed that new cells formed through the replication of pre-existing cells.
- His work provided strong evidence for the idea that cells can only arise from pre-existing cells.
Significance of the Cell Theory:
- The cell theory revolutionized our understanding of biology and laid the foundation for modern cell biology.
- It provided a unifying concept that explained the organization and functioning of living organisms.
- The cell theory also contributed to advancements in medical science, as it highlighted the importance of studying cells in understanding diseases.
Conclusion:
Rudolf Virchow's generalization that "All cells are derived from pre-existing cells" has become a fundamental principle in biology. His observations and experiments led to the development of the cell theory, which revolutionized our understanding of living organisms.

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