John’s quick initial reply to my posting relied on a non-sequitur. A high rate of “natural” embryo loss does not entail that human embryos are not human beings. A number of people have pointed this out and I think that everyone (including John, who is obviously a thoughtful guy) will see the error in John’s logic here if they reflect on it a bit. Now, however, John has raised the question of monozygotic twinning. That’s a much more interesting matter. In its strongest form, the argument is that as long as twinning can occur what exists is not yet a unitary human being, but only a mass of cells—each cell is totipotent and allegedly independent of the others.
It is true that if a cell or group of cells is detached from the whole at an early stage of embryonic development then what is detached can sometimes become a distinct organism and has the potential to develop to maturity as distinct from the embryo from which it was detached. But this does nothing to show that before detachment the cells within the human embryo constituted only an incidental mass. Consider the parallel case (discussed by Aristotle) 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 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. Likewise, at the early stages of human embryonic development, before specialization by the cells has progressed very far, the cells or groups of cells can become whole organisms if they are divided and have an appropriate environment after the division. But that fact does not indicate that prior to such an extrinsic division the embryo is other than a unitary, self-integrating, actively developing human organism.
In the first two weeks, the cells of the developing embryonic human being already manifest a degree of specialization or differentiation. From the very beginning, even at the two-cell stage, the cells differ in the cytoplasm received from the original ovum. Also they are differentiated by their position within the embryo. In mammals, even in the unfertilized ovum, there is already an “animal” pole (from which the nervous system and eyes develop) and a “vegetal” pole (from which the future “lower” organs and the gut develop). After the initial cleavage, the cell coming from the “animal” pole is probably the primordium of the nervous system and the other senses, and the cell coming from the “vegetal” pole is probably the primordium of the digestive system. Moreover, the relative position of a cell from the very beginning (that is, from the first cleavage) has an impact on its functioning. Monozygotic twinning usually occurs at the blastocyst stage, in which there clearly is a differentiation of the inner cell mass and the trophoblast that surrounds it (from which the placenta develops). The orientation and timing of the cleavages are species specific, and are therefore genetically determined, that is, determined from within. Even at the two-cell stage, the embryo begins synthesizing a glycoprotein called “E-cadherin” or “uvomorulin,” which will be instrumental in the compaction process at the 8-cell stage, the process in which the blastomeres (individual cells of the embryo at the blastocyst stage) join tightly together, flattening and developing an inside-outside polarity. And there is still more evidence, but the point is that from the zygote stage forward, the embryo is not only maintaining homeostasis, but is internally integrating various processes to direct them in an overall growth pattern toward maturity.
However, the clearest evidence that the embryo in the first two weeks is not a mere mass of cells but is a unitary organism is this: if the individual cells within the embryo before twinning were each independent of the others, there would be no reason why each would not regularly develop on its own. Instead, these allegedly independent, non-communicating cells regularly function together to develop into a single, more mature member of the human species. This fact shows that interaction is taking place between the cells from the very beginning (even within the zona pellucida, before implantation), restraining them from individually developing as whole organisms and directing each of them to function as a relevant part of a single, whole organism continuous with the zygote. Thus, prior to an extrinsic division of the cells of the embryo, these cells together do constitute a single organism. So, the fact of twinning does not show that the embryo is a mere incidental mass of cells. Rather the evidence clearly indicates that the human embryo, from the zygote stage forward, is a distinct, unitary human organism–a human being.