Cell Theory: 1838

The Cell Theory
When Schleiden and Schwann proposed the cell theory in 1838, cell biology research was forever changed. The cell theory states that:

1. All life forms are made from one or more cells.
2. Cells only arise from pre-existing cells.
3. The cell is the smallest form of life.

The cell theory also provides us with an operational definition of "life." The tutorial on prokaryotes, eukaryotes, and viruses explores this concept in more detail.[1]