Dear Sirs or Ladies:

An argument that is sometimes raised that did not appear in the George/Baltimore exchange attempts to deny that the embryo is a human individual because of the possibility of (identical) twinning. While some people have been impressed by this argument, it is completely without merit. The argument is that as long as twinning can occur (up to about the third week) what exists is not yet an individual, but only a mass of cells—each cell is totipotent and (the argument runs) independent of the others.

It is true that if a cell or group of cells are detached from the whole at this early stage then what is detached becomes a distinct organism and has the potential to develop to maturity as distinct from the embryo from which it was detached (this is the meaning of "totipotent"). But this does nothing to show that before detachment the cells within the human embryo, surrounded by a zona pellucida, constituted only an incidental mass. Rather, just as the fact that dividing a flatworm results in two whole flatworms does not show that prior to that division the flatworm was not a unitary individual, just so with the human embryo. Parts of a flatworm have the potential to become a whole flatworm when isolated from the present whole of which they are part. Likewise, at the early stages of development of the human embryo the degree of specialization by the cells has not progressed very far, therefore 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 in the least indicate that prior to such an extrinsic division the embryo is a mere mass of cells rather than a single, complex, actively developing human organism.

In the first few weeks, the cells of the developing embryo do have specialization or differentiation. From the very beginning, even at the zygote stage, the cells of this new organism are cytoplasmically and positionally differentiated. 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 first 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) does make a difference in how it functions. Again, most (identical) twinning 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).

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 within the zona pellucida, 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, and twinning is a phenomenon biologically equivalent to cloning.

Finally, the regular, orderly, predictable growth of the embryo, namely its active self-development by the normal process of directed, sequential cell division, from the zygote stage on day one through to the gastrulation stage in week three, is cogent evidence (evidence that was never set aside before it became socially convenient to do so) that the embryo is a unitary self-developing organism rather than a mere mass of cells. Thus, a distinct, living human individual comes to be with the fertilization of the ovum by the sperm; another distinct, living human individual may be generated from the cells of a single living human individual either by twinning or by cloning.

Patrick Lee, Ph.D

Franciscan University of Steubenville

Steubenville, Ohio