The Cell Theory is a fundamental concept in Biology that explains the existence and functions of living organisms. The theory states that all living things are composed of one or more cells, and that cells are the basic structural and functional units of life. It also states that all cells arise from pre-existing cells through the process of cell division.

The development of the Cell Theory is credited to three scientists: Matthias Schleiden, Theodor Schwann, and Rudolf Virchow. In 1838, Schleiden observed plant tissues under a microscope and concluded that all plants are made up of cells.

Two years later, Schwann observed animal tissues and came to the same conclusion. Together, they proposed the first two tenets of the Cell Theory: that all living things are composed of cells and that cells are the basic units of life.

In 1855, Virchow added to the theory by proposing that all cells arise from pre-existing cells through cell division. He based this on his observations of disease processes in humans where he noted that new cells were formed from existing ones as part of normal tissue repair.

These three scientists contributed significantly to our understanding of the fundamental building blocks of life as we know it today. Their work paved the way for further research into cellular biology and has been instrumental in advancing modern medicine.

To summarize, The Cell Theory states that all living things are made up of one or more cells, which are the basic structural and functional units of life. It also affirms that new cells arise only from pre-existing ones through cell division. This theory was developed by Matthias Schleiden, Theodor Schwann, and Rudolf Virchow in the mid-19th century and has since become a cornerstone concept in Biology.