1. Joseph F. Donceel, ‘Immediate Animation and Delayed Hominization,’ Theological Studies 31, 1970, 76–105; James Diamond, ‘Abortion, Animation and Biological Hominization,’ Theological Studies 36,1975, 30–24; Thomas A. Shannon & A. B. Wolter, ‘Reflections on the Moral Status of the Pre-embryo,’ Theological Studies 51, 1990, 603–626; Jason T. Eberl, ‘The Beginning of Personhood: A “Thomistic Biological Analysis’” Bioethics 14, 2000,134–157; Barry Smith and Berit Brogaard, ‘Sixteen Days,’ Journal of Medicine and Philosophy 28, 2003, 45–78; David W. Shoemaker, ‘Embryos, Souls, and the Fourth Dimension,’ Social Theory and Practice 31,2005, 51–75.
2. This is a simplistic use of the term Membrane/ and is purely designed to succinctly illustrate the point that the twins at this stage exist within the same limited physical parameters. One might more accurately label this membrane the zona pellucida, but as the pre-implantation embryo(s) ‘hatches’ from the zona pellucida at some point prior to implantation (whereby the zona pellucida degenerates), and a new type of membrane forms the outer layer of the pre-implantation embryo, it is in the interests of overall clarity to simply make the point that the biological processes under discussion occur within the same ‘membrane’ (broadly denned).
3. The zygote may also develop into nothing more than a hydatidiform mole or teratoma. However, the implications of this phenomenon will not be pursued here. Furthermore, there is a strong argument that such organisms are not the result of‘normal, biologically complete, conceptions’ but are in fact ‘flawed or deficient fertilizations’ (Francis Beckwith, ‘The Explanatory Power of the Substance View of Persons,’ Christian Bioethics 10, 2004, 51). Thus, such organisms cannot really be said to originate in zygotes, and therefore they are not appropriate targets for the individuation argument canvassed in this paper — see Stephen Heaney, ‘The Human Soul in the Early Embryo,’ The Thomist 56,1991,46; and Lisa Sowle Cahill, ‘The Embryo and the Fetus: New Moral Contexts,’ Theological Studies 54, 1993, 136.
4. It should be noted that, as the number of cells constituting the pre-implantation human organism increases, biology provides different labels. For instance, the original single celled fertilized egg is the ‘zygote‘; the approximately 16 celled organism is a ‘morula‘; and the approximately 32+ celled organism is a ‘blastocyst.’ It is only upon implantation that the term ‘embryo’ is generally utilised. However, for present purposes (and in the interests of overall clarity) this article will refer to the ‘pre-implantation embryo’ for all stages of the human organism from immediately post-zygote stage (i.e., immediately post a single celled organism) to implantation stage.
5. See Anne McLaren, ‘Where to Draw the Line?’ Proceedings of the Royal Institution 56, 1984, 101–121; Louis M. Guenin, ‘The Nonindividuation Argument Against Zygotic Personhood,’ Philosophy 81, 2006, 463; Smith & Brogaard, ‘Sixteen Days,’ 54–58.