Obviously a key question for cancer, with unchecked cell replication. Is there a factor from outside the cell that enters the cell and triggers division (ie an envioronmental factor), or is it a factor that is already inside the cell.
This assumes that one factor is sufficient, and that there are not multiple factors, any of which could be sufficient. But the issue is not number of factors as much as whether or not the cell wall needs to be crossed by them. Again, the "factor" is something that would be different between normal cells and cancer cells, and would cause that difference.
So, for example, testosterone would not be a candidate answer to this question, because blood contains testosterone, and testosterone goes into toboth both normal prostate cells and into cancer prostate cells.
I think the assumption is that the difference is something that must be "in" the cell, and is passed on to "both?" daughter cells "when" the cancer cell divides.
[Most terminally differentiated cells don't divide, though, I thought.]