In the 1830s, the Germans Matthias Schleiden (with plants) and Theodor Schwann (with animals) declared that plants and animals are all made of cells. They argued that each cell begins with a nucleus, and the cell develops around that nucleus. Cells then join together to make up complex organisms like us.
They argued about whether cells arise by crystallization from surrounding material, through spontaneous generation, by cell division, or some other process.