Dear student, all the cells arise from pre-existing cells as the cell divides and so does the nucleus which gets incorporated in the daughter cells by the process of cell division and thus the new cells are formed. Cell is the basic fundamental unit of the life it is considered as the structural and functional unit in living organisms. Also according to the Meselson-Stahl experiment which suggests that cells undergoes semi conservative replication which means that a strand from parent DNA goes into the daughter cell thus it also suggests that all cells arises from the pre existing cells. Regards