Schleiden and Schwann proposed spontaneous generation as the method for cell origination, but spontaneous generation (also called abiogenesis) was later disproven. Rudolf Virchow famously stated "Omnis cellula e cellula"... "All cells only arise from pre-existing cells. "The parts of the theory that did not have to do with the origin of cells, however, held up to scientific scrutiny and are widely agreed upon by the scientific community today. The generally accepted portions of the modern Cell Theory are as follows:
The cell is the fundamental unit of structure and function in living things.
All organisms are made up of one or more cells.
Cells arise from other cells through cellular division.
The expanded version of the cell theory can also include:
Cells carry genetic material passed to daughter cells during cellular division
All cells are essentially the same in chemical composition
Energy flow (metabolism and biochemistry) occurs within cells