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But today, everything's gonna be A-OK!

The modern-day cell theory was formulated only after about 200 years

since the introduction of microscopy.

In 1665, English scientist Robert Hooke observed a piece of quark under a simple microscope.

He observed little boxes which he called cells

because he thought these structures resembled tiny rooms or cells occupied by moths.

He also reported that in living plants, these cells are filled with juices.

In 1673, Anton van Leeuwenhoek observed microscopic organisms from pond water

which he called animalcules.

Later, these organisms were found to be bacteria and protozoans.

He also reported his observations of blood cells and sperm cells.

In 1805, German naturalist Lorenz Ocken postulated that all organisms originated from

and consist of cells.

This became the initial statement of the modern cell theory.

In 1833, Robert Brown discovered the nucleus in plant cells

which were later also observed in animal cells.

As the microscope became more powerful, scientists Matthias Leiden and Rudolf Virchow

were able to see structures inside the cells.

They also observed that cells would grow larger and divide into smaller but still living cells.

Furthermore, they noted that some organisms are unicellular

while others are multicellular.

Rudolf Virchow also concluded that the cell is the basic structural unit of life

and that every cell is formed from pre-existing cells.

Based on these observations, the modern day cell theory evolved.

The three principles of the modern cell theory state that

first, every living organism is made up of one or more cells.

Some organisms like the bacteria and some protozoans are unicellular

while some organisms like plants and animals are made up of many cells.

Second, cells are the basic unit of organization of all organisms.

Third, cells arise only by division of a previous cell.

Thank you for watching!