They make use of the fact that a zygote can become two identical twins, and that two fraternal twin zygotes can combine to form parts of one human organism with different dna here and there.
I can't answer the second example at this time (in fact, I'm not sure of what it proves), but responding to the first is easy: if one can take the skin cell of an adult and use it for cloning (and the expectation is that some day it will be possible to do this with iPSCs), then the fact that this can be done with embryos does not demonstrate their non-humanity or non-personhood. The modem tollens would go like this: if clonability demonstrated non-humanity of embryos, then it would demonstrate the non-humanity of adults. But it doesn't. Therefore it doesn't demonstrate in the case of embryos.
I can't answer the second example at this time (in fact, I'm not sure of what it proves), but responding to the first is easy: if one can take the skin cell of an adult and use it for cloning (and the expectation is that some day it will be possible to do this with iPSCs), then the fact that this can be done with embryos does not demonstrate their non-humanity or non-personhood. The modem tollens would go like this: if clonability demonstrated non-humanity of embryos, then it would demonstrate the non-humanity of adults. But it doesn't. Therefore it doesn't demonstrate in the case of embryos.
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