The Origin of Humans

Stone J. Why Potentiality Matters. Canadian Journal of Philosophy 1987; 17: 824–25.

Wade F. Potentiality in the Abortion Discussion. Review of Metaphysics 1975; 29: 239–255. See also Stone, op. cit, and Watt, H. Potential and the Early Human, forthcoming.

Tonti-Filippini N. Further Comments on the Beginning of Life. Linacre Quarterly 1992; 59: 77–81.

Buckle S. , Dawson K. , and Singer P. The Syngamy Debate - When Precisely Does a Human Life Begin? In Embryo Experimentation, ed. Singer P. Cambridge: Cambridge University Press, 1989.

It is this penetration, not the earlier penetration of the zona pellucida, which creates a diploid zygote from a haploid ovum. Even if the zona is thought of as part of the embryo (see note 7) the presence of one or more sperm in the perivitelline space does not suffice to form a zygote. If more than one sperm is present within the perivitelline space, it will not be possible to date the formation of the zygote from the entry of either sperm into the perivitelline space, since in principle it may be the other sperm which penetrates the inner membrane - an event which makes the penetration of the zona by the first sperm superfluous. In contrast, if one sperm penetrates the inner membrane, releasing its genetic material, it is then that the zygote is formed with developmental powers. Any subsequent penetration by a second sperm and release of its genetic material will be responsible not for the formation of a zygote (unless, for example, a polar body is fertilized) but for damage to an existing zygote.

Ford N. When did I begin? Cambridge: Cambridge University Press, 1988.

Germain Grisez describes the zona pellucida as an organic part of the embryo (When do People Begin? In Abortion: A New Generation of Catholic Responses, ed. Heaney S.J. Braintree: The Pope John Center, 1992. p. 25 n. 50). Whether or not the zona is a part of the embryo, it appears that the zona may not be essentail for early human development (Martin Evans, personal communication), although it is useful in preventing the formation of chimeras where there is more than one embryo present. If the zona is not essential for early human development, it would appear that activated cells from an early embryo are themselves embryos with active developmental potential - whether or not they are put in surrogate zonas, as in the recent “cloning” experiment (see Kolberg. R. Human Embryo Cloning Reported. Science 1993; 262: 652-653), and whether a zona is a (natural or synthetic) part of an embryo, or a part of the embryo's environment.

McLean J. The Early Human Embryo. In The Human Embryo: Science, Law, Ethics and Public Policy, ed. Gormally L. , and Watt H. , Forthcoming.

Hill D. , Strain A. , and Milner R. Growth Factors in Embryogensis. In Oxford Reviews of Reproductive Biology, ed. Clarke J. Oxford: Clarendon, 1987, pp. 403–404, 411.

Ford, op cit, p. 162.

Holland Alan A Fortnight of My Life is Missing: a discussion of the status of the human “pre-embryo”. Journal of Applied Philosophy 1990; 7: 27.

Grisez , op cit, p. 17

Sutton A. Prenatal Diagnosis: Confronting the Ethical Issues. London: The Linacre Centre, 1990. p. 108.

See Braine D. The Human Person: Animal and Spirit. London: Duckworth, 1993. pp. 480–545.

The organization of the oocyte and embryogenesis in twinning and fusion malformations. Acta Geneticae Medicae et Gemellologiae Roma 1987; 36: 421–431, cit. McLean, op cit.

In the case of chimeras, where a change takes place through aggregation rather than division, it seems most likely that one individual survives while the other is radically disrupted -“eaten” - by the survivor. More elaborate explanations would be that both are simultaneously destroyed, or that one “takes over” the other and is then taken over by a newly formed third. It may not always be easy to determine which individual if any, survives; however, a non-dualist approach to the human individual will see death or survival as an outcome of physical changes to that individual: changes which are discernible in principle, if not in practice.

Ford, op cit, pp. 119–120.

By “active tendency to twin” I mean an active tendency to initiate the twinning process. Since an active tendency refers to continuity between not doing and doing on the part of an individual, it cannot extend to the end of a process, such as symmetric twinning, which involves the destruction of that individual.

In the case of an active tendency to twin asymmetrically, the problems seen here would not arise, since one of the individuals present after twinning would be the original individual. An active tendency to reproduce asexually would be no more a threat to human identity than the normal active tendency of adults to reproduce sexually.

It may be asked what the difference would be between a genetic propensity to twin in some environment and a genetic program to twin in some environment. We may feel some reluctance to define the difference in terms of a distinction between teleological and accidental developments. In other contexts, however, we are happy enough in practice to use teleological terms to describe a selection of natural phenomena (for example, “blueprint”, “function”, “health”, “repair”, etc.).

Quinn, Abortion W. Identity and Loss. Philosophy and Public Affairs 1984; 13: 27.

Sutton , op cit, pp. 109–110.

Holland , op cit, p. 33.

See Suarez A. Hydatiform Moles and Teratomas Confirm the Human Identity of the Preimplantation Embryo. Journal of Medicine and Philosophy 1990; 15: 627–635.

Grisez , op cit, p. 11.

McCullagh P. Brain Dead. Brain Absent, Brain Donors. Chichester: John Wiley, 1993. pp. 112–114.

See Braine , op cit, pp. 480–545.

If there are other such organisms, these too will be moral subjects of particular importance. However, human persons - the kind with which we are familiar - have conditions of existence specific to themselves.

On the implications with regard to abortion see Finnis John (The Rights and Wrongs of Abortion. Philosophy and Public Afairs 1973; 2: 117–145) and Stephen Schwarz ((The Moral Question of Abortion. Chicago: Loyola University Press, 1990). I discuss these implications briefly in Watt, H. Abortion and Shelter: Beyond Thomson's Violinist. Ethics and Medicine 1992; 8: 9-10.

Fisher A. IVF: The Critical Issues. Melbourne: Collins Dove, 1991. p. 138.

See, e.g. Grisez G. , Boyle J. , and Finnis J. Practical Principles, Moral Truth and Ultimate Ends. American Journal of Jurisprudence 1987; 32: 99–151.