Is Twinning an Argument Against the Humanity of the Embryo?


The other night I ran into a friend at a an event.  Knowing I’m pro-life, he asked me about the problem of twins when discussing the humanity of the embryo.  Since I had not considered this argument before, I did some research.  First of all what is the issue?  In an Internet article  in the Journal Ethika Politika, John Paul Nunez defines the issue like this:  “The twinning argument goes something like this: an embryo can split in two up until it’s fourteen days old, so it can’t be an individual human organism yet.  Instead, it can only be a clump of cells that will soon become an individual organism (once twinning is no longer possible).  As a result, proponents of this argument can claim that, at the very least, embryos don’t have a right to life before day fourteen.”  Click here to read the full article refuting this argument.

Professor Robert P. George, an expert on bio-ethics at Princeton University,  has written a brilliant defense of the humanity of the embryo in his 2008 book called Embryo.  Dr. George, in Chapter six of the book, points out that some have denied the humanity of the embryo on the basis that the embryo at its earliest stages is not sufficiently unified; others deny the humanity of the embryo because of the possibility of monozygotic twinning.

On page 149 of Embryo, George states it this way: “the phenomenon of twinning is still not perfectly understood.  But it is clear enough that in the early stages of embryonic life – possibly up to as late as the fourteenth day prior to beginning of gastrulation – an embryo may divide into two distinct organisms, each with the potential to develop to maturity.  Why should this capacity be troubling?  Proponents of the argument from twinning against the humanity of the embryo assert that the potential for division indicates that the embryo does not, while it still can so divide, possess the intrinsic unity characteristic of a whole distinct organism…so the suggestion is that as long as twinning is still possible, what exists is not yet a unitary human being but only a mass of cells, each one at first totipotent, then pluripotent, but each allegedly independent of the other.. This conceptual question of whether an entity that is genuinely one could be split so as to become two has a ready answer.  Consider the parallel case of division of a flatworm.  Parts of a flatworm have the potential to become a whole flatworm when isolated from the present whole of which they are a part.  Yet no one would suggest that prior to the division of a flatworm to produce two whole flatworms, the original flatworm was not a unitary individual.”

Professor George further states “Thus, prior to an extrinsic division of the cells of the embryo, these cells together do constitute a single organisms.  So the fact of twinning does not show that the embryo is a mere incidental mass of cells; and the evidence against this claim likewise serves to refute the first argument, which claimed that the embryo lacked the unity of a single living being.  Rather, the evidence, clearly indicates that the human embryo, from the zygote state forward is a unitary human organism.”

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