Cell is the smallest, structural and functional unit of life.
Anything less than a compete structure of a cell doesn't ensure independent existence.
Unicellular organisms are capable of,

1. Independent existence and
2. Performing the essential functions of life.

Anton Von Leeuwenhoek, first saw and described a live cell. Robert Brown, Discovered nucleus.

NOTE: G.N. RAMACHANDRAN, Discovered the triple helical structure of collagen published in 'naturae' in 1954 and his analysis of the allowed conformations of proteins through the use of the 'Ramachandran's plot' rank among the most outstanding contributions in structural biology.

Matthias Schleiden, a German botanist told that ''All plants composed of different kinds of cells.
Theodore Schwann, a British zoologist told that,

1. animal cells had a thin outer layer now called as plasma membrane
2. Plant cells have an unique character i.e., presence of cell wall

On this basis, Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and product of cells.
Schleiden and Schwann together formulated the cell theory but they couldn't explain how new cells were formed.

Rudolf Virchow modified the cell theory and first explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula-e cellula).
Cell Theory understood today as:

1. All living organisms are composed of cells and products of cells.
2. All cells arise from pre-existing cells.