I mean I imagine there's research trying to do exactly that: you can identify beating heart cells very early on in embryos, and chimeric organisms are a thing.

I mean I imagine there's research trying to do exactly that: you can identify beating heart cells very early on in embryos, and chimeric organisms are a thing. I imagine someone somewhere is trying to swap out those early cells with human heart cells to see if you can literally grow a pig with a human heart.