Cell Theory was discovered by multiple different scientist. They all played important parts in giving us the 3 different parts to the theory.
Robert Hooke was the man who created the word for cell. He first noticed cells when looking at the individual components under a microscope.
Robert Brown was the first man to Robert Brown was known for the discovery of the nucleus, a very important organelle. He was able to find a black dot in each cell which he began to call the nucleus.
Matthias Schleiden is one of the men responsible for discovering that all plants are made of cells. He along with another fellow scientist found this true with animals as well.
Theodor Schwann was the man that discovered that all animals are made of cells. He worked with Schleiden and together they did multiple labs looking at animal and plant cells.
Lastly, Rudolf Virchow is the last man involved with cell theory. He discovered the theory that all cells come from pre-existing cells. This was the last piece to complete the three parts of cell theory.