What does 'theory' really mean?
Plant cells and unicellular organisms had been observed in the 17th century, but another century was passing before they would be seen in animals. From about 1830 onwards, cell theory was developing - cells are the basic unit of life, and all living organisms are made up of them.
Sir Paul Nurse: By about 1800, it was becoming clear that both animals and plants were composed of cells, and it was the basic unit of life that was being seen here. By about the 1830’s it was accepted that thewas the basic unit of life. And that was well stated by Theodor Schwann and [Matthias] Schleiden. This is Theodor Schwann [in the picture]. And Schwann, quoted in German, said: “We have seen that all organisms are composed of essentially like parts, namely, of cells.”
Over the next 10 or 20 years it was proposed that these cells were also the basic functional unit of life. And that was well stated by Rudolph Virchow, who was the founder of pathology, worked in Germany. What he said was "every animal appears as a sum of vital units, each of which bares in itself the complete characteristics of life." This is a great sentence. I mean, it really sums up what cells are.