At around the same time, Theodor Schwann noted a weird resmeblance of embryonic cord material to vegetable cells, and recognized cell-like character of certain animal tissue. Schwann, collaborating with Schleiden and his work, believed this similarity was more than a coincidence. He correctly concluded that if plants and animals share cells, and the nucleus plays a role in plant cells that Schwann described, animal cells must ultimately have a nucleues. Schwann also found nuclei in definitive parts of tissue, due to regular intervals of measurement involving the nucleus. Schwann soon produced from this info part of the famous cell theory, stating the unification of animal and plant tissues, in arounc 1838.
Unfortunately, Schleiden and Schwann could not agree on one aspect of cell theory. Schwann believed cells formed spontaneously, through a crystalization process simlar to that of crystals. Schleiden believed that cells formed from preexisting cells. Rudolf Virchow finally settled this argument in 1858, stating the third part of cell theory, "Omnis cellua e cellua" (all cells arise only from pre-existing cells).
Finally, the cell theory was complete. The final product consisted of these three parts: the cell is the unit of structure, physiology, and organization in living things; the cell retains a dual existence as a distinct entity and a building block in the construction of organisms; and all cells form from preexisting cells. Over the next couple hundred years, the cell theory would change humanity forever by giving us a much better understanding of life itself and access to scientific fields and discoveries we would've never known about.