They did not mergewith you, or give you any cell membranes or nutrients just an infinitesimally small packet of DNA, less than one-millionth of your mass.
Then there's a completely new idea, which is to generate a kind of a cell that you would mergewith a kind of an egg, and you'd get something that was kind of an embryo but it wasn't really an embryo, and it wouldn't have the potential to grow into anything, and maybe those would give rise to stem cells.