"Let x be the fetus in my past that grew into me. Here is a valid Aristotelian argument (though Aristotle himself would probably deny (4)).
1. (Premise) The identity of a bodily organ depends on the identity of the individual whose organ it is, so that if A is c's unshared heart (sharing occurs in the case of Siamese twins), and B is d's unshared heart, and c and d are distinct individuals, then A and B are distinct organs.
2. (Premise) x has exactly one heart, hx, and it is unshared.
3. (Premise) I have exactly one heart, hI, and it is unshared.
4. (Premise) hx=hI.
Therefore, I am x. (By 1-4) The controversial premises are (1) and (4)."
I think that if the identity of the organ (heart, in this case) depends on the identity of the individual of whose organ it is, then we must already know with whom the individual is identical. So for x and I, I must be identical with x in the first place for hx to = hI. Imagine that we decide that I am not identical with myself as a fetus; then my fetus' heart belongs to myself as a fetus, and my heart now belongs to me as I am now. Then, my fetus' heart is not identical with my heart. So the argument doesn't seem to prove anything not contained in the first premise.
Also, to whom the organ belongs can get tricky when talking about transplants. If at t1, hI belonged to me, and I decide to have a heart transplant from organ donor J, then at t2 I have hJ. Does hJ now belong to me and therefore is hI? Will hI then be identified with another person? How do we determine belonging with mobile organs?