The original cell theory, proposed by Schleiden and Schwann in the 1830s, stated that all living things are composed of cells and that cells are the basic unit of life. This theory has been modified and expanded over time due to new discoveries and experiments. Here are some key experiments that led to these changes:

1. Pasteur's Experiment (1860s): This experiment disproved the theory of spontaneous generation, which stated that living organisms could arise from non-living matter. Pasteur's experiment demonstrated that life only arises from pre-existing life, further solidifying the concept that cells are the basic unit of life.

2. Virchow's Observation (1855): Rudolf Virchow, building upon the work of Robert Remak, proposed the idea of "Omnis cellula e cellula" (all cells arise from pre-existing cells). This addition to the cell theory solidified the understanding that cells do not spontaneously form, but instead are generated through division of pre-existing cells.

3. The Discovery of Endosymbiosis (1960s): Lynn Margulis proposed the theory of endosymbiosis, which states that mitochondria and chloroplasts originated from free-living prokaryotes that were engulfed by larger cells. This discovery led to the understanding that cells are not simply self-contained units, but can be formed through the incorporation of other cells.

4. The Discovery of Viruses (late 19th century): Viruses are non-cellular entities that require a host cell to replicate. This discovery challenged the notion that all life is cellular and led to the expansion of the cell theory to include the concept of non-cellular life.

5. Advances in Microscopy: The development of electron microscopy in the 20th century allowed scientists to observe the internal structures of cells in much greater detail. This led to the discovery of new organelles and the understanding of the complex organization and functionality of cells.

These experiments, along with many other discoveries, have led to a more nuanced and comprehensive understanding of cells. The current cell theory states that:

* All living organisms are composed of one or more cells.

* Cells are the basic unit of life.

* All cells arise from pre-existing cells.

* Cells contain hereditary information (DNA) that is passed on during cell division.

* Cells are the functional unit of life, carrying out all the processes necessary for life.

This theory continues to be refined as new discoveries are made, demonstrating the ever-evolving nature of scientific understanding.