The Early Human Embryo: 'Scientific' Myths and Scientific Facts: Implications for Ethics and Public Policy
A One-Act Drama


1 But see, e.g., Dianne N. Irving, "Philosophical and scientific expertise: An analysis of the arguments on 'personhood'", in Linacre Quarterly (February 1993), 60:1:18-46. [Back]

2 See, e.g., Richard McCormick, S.J., "Who or what is the preembryo?", Kennedy Institute of Ethics Journal 1:1 (1991). In this paper McCormick draws heavily on the work of frog embryologist Clifford Grobstein, as well as from "an unpublished study of a research group of the Catholic Health Association entitled 'The Status and Use of the Human Preembryo', (p. 14).

The influence of the McCormick/Grobstein term "pre-embryo" was (and still is) widespread even among Catholic scholars. In addition to the works of McCormick and Grobstein, see acceptance of the term "pre-embryo" also in: Andre E. Hellegers, Fetal development, in Thomas A. Mappes and Jane S. Zembatty (eds.), Biomedical Ethics, (New York: Macmillan, 1981); Hellegers, "Fetal development", Theological Studies (1970), 31:3-9; Charles E. Curran, "Abortion: Contemporary debate in philosophical and religious ethics", in W. T. Reich (ed.), Encyclopedia of Bioethics 1 (London: The Free Press, 1978), pp. 17-26; Kevin Wildes, "Book Review: Human Life: Its Beginning and Development" (L'Harmattan, Paris: International Federation of Catholic Universities, 1988); Carlos Bedate and Robert Cefalo, "The zygote: To be or not be a person", Journal of Medicine and Philosophy (1989), 14:6:641; Robert C. Cefalo, "Book Review: Embryo Experimentation, Peter Singer et al (eds.); 'Eggs, embryos and ethics'", Hastings Center Report (1991), 21:5:41; Mario Moussa and Thomas A. Shannon, "The search for the new pineal gland: Brain life and personhood", The Hastings Center Report (1992), 22:3:30-37; Carol Tauer, The Moral Status of the Prenatal Human (Doctoral Dissertation in Philosophy; Kennedy Institute of Ethics, Georgetown University, Washington, D.C.: Georgetown University, 1981) (Sister Tauer's dissertation mentor was Richard McCormick; she later went on to become the ethics co-chair of the NIH Human Embryo Research Panel 1994); C. Tauer, "The tradition of probabilism and the moral status of the early embryo", in Patricia B. Jung and Thomas A. Shannon, Abortion and Catholicism (New York: Crossroad, 1988), pp. 54-84; Lisa S. Cahill, "Abortion, autonomy, and community", in Jung and Shannon, Abortion and Catholicism (1988), pp. 85-98; Joseph F. Donceel, "A liberal Catholic's view", in Jung and Shannon, Abortion and Catholicism (1988), pp. 48-53; H. Tristram Engelhardt, The Foundations of Bioethics (New York: Oxford University Press, 1985), p. 111; William A. Wallace, "Nature and human nature as the norm in medical ethics", in Edmund D. Pellegrino, John P. Langan and John Collins Harvey (eds.), Catholic Perspectives on Medical Morals (Dordrecht: Kluwer Academic Publishing, 1989), pp. 23-53; Norman Ford, When Did I Begin? (New York: Cambridge University Press, 1988), p. 298; Antoine Suarez, "Hydatidiform moles and teratomas confirm the human identity of the preimplantation embryo", Journal of Medicine and Philosophy (1990), 15:627-635; Thomas J. Bole, III, "Metaphysical accounts of the zygote as a person and the veto power of facts", Journal of Medicine and Philosophy (1989), 14:647-653; Bole, "Zygotes, souls, substances, and persons", Journal of Medicine and Philosophy (1990), 15:637-652.

See also: See Richard McCormick's testimony in The National Commission for the Protection of Human Subjects of Biomedical and Behavioral Research; Report and Recommendations; Research on the Fetus; U.S. Department of Health, Education and Welfare, 1975, pp. 34-35; McCormick, How Brave a New World? (Washington, D.C.: Georgetown University Press), p. 76; McCormick, "Proxy consent in the experimentation situation", Perspectives in Biology and Medicine (1974), 18:2-20; Paul Ramsey's testimony in The National Commission for the Protection of Human Subjects of Biomedical and Behavioral Research; Report and Recommendations; Research on the Fetus; U.S. Department of Health, Education and Welfare, 1975, pp. 35-36.

The use of the term "pre-embryo" has been quite widespread for decades -- nationally and internationally. In addition to the Catholic scholars who accepted the use of the term "pre-embryo" as noted above, a partial list of secular bioethics writers who also accepted the use of the term in these debates includes: Paul Ramsey, "Reference points in deciding about abortion" in J.T. Noonan (ed.), The Morality of Abortion (Cambridge, MA: Harvard University Press, 1970), pp. 60-100, esp. p. 75; John Robertson, "Extracorporeal embryos and the abortion debate", Journal of Contemporary Health Law and Policy (1986), 2;53;53-70; Robertson, "Symbolic issues in embryo research", The Hastings Center Report (1995, Jan./Feb.), 37-38; Robertson, "The case of the switched embryos", The Hastings Center Report (1995), 25:6:13-24; Howard W. Jones, "And just what is a preembryo?", Fertility and Sterility 52:189-91; Jones and C. Schroder, "The process of human fertilization: Implications for moral status", Fertility and Sterility (August 1987), 48:2:192; Clifford Grobstein, "The early development of human embryos", Journal of Medicine and Philosophy (1985), 10:213-236; also, Science and the Unborn (New York: Basic Books, 1988), p. 61; Michael Tooley, "Abortion and infanticide", in The Rights and Wrongs of Abortion, M. Cohen et al (eds.) (New Jersey: Princeton University Press, 1974), pp. 59 and 64; Peter Singer and Helga Kuhse, "The ethics of embryo research", Law, Medicine and Health Care (1987),14:13-14; Kuhse and Singer, "For sometimes letting - and helping - die", Law, Medicine and Health Care (1986), 3:40:149-153; Kuhse and Singer, Should The Baby Live? The Problem of Handicapped Infants (Oxford University Press, 1985), p.138; Singer, "Taking life: Abortion", in Practical Ethics (London: Cambridge University Press, 1981), pp. 122-123; Peter Singer, Helga Kuhse, Stephen Buckle, Karen Dawson, Pascal Kasimba (eds.), Embryo Experimentation (New York: Cambridge University Press, 1990); R.M. Hare, "When does potentiality count? A comment on Lockwood," Bioethics (1988), 2:3:214; Michael Lockwood, "When does life begin?", in Michael Lockwood (ed.), Moral Dilemma's in Modern Medicine (New York: Oxford University Press, 1985), p. 10; Hans-Martin Sass, "Brain life and brain death: A proposal for normative agreement," Journal of Medicine and Philosophy (1989), 14:45-59; Michael Lockwood, "Warnock versus Powell (and Harradine): When does potentiality count?" Bioethics (1988), 2:3:187-213.

See also the use of the term "pre-embryo" in many national and international documents (a small sample): Ethics Advisory Board (1979) Report and Conclusions: HEW Support of Research Involving Human In Vitro Fertilization and Embryo Transfer, Washington, D.C.: United States Department of Health, Education and Welfare, p. 101; National Institutes of Health Human Embryo Research Panel Meetings (Washington, D.C.: NIH, 1994), Feb. 2 meeting, pp. 27, 31, 50-80, 85-87, 104-106; in the Feb. 3, 1994 meeting, pp. 6-55; April 11 meeting, pp. 23-41, 9-22. See also, Dame Mary Warnock, Report of the Committee of Inquiry into Human Fertilization and Embryology, (London: Her Majesty's Stationary Office, 1984), pp. 27 and 63; British House of Lords, "Human Fertilisation and Embryology (Research Purposes) Regulations 2001"; Commonwealth of Australia, Select Senate Committee on the Human Embryo Experimentation Bill, (Canberra, Australia: Official Hansard Report, Commonwealth Government Printer, 1986); Parliamentary Assembly of the Council of Europe, On the Use of Human Embryos and Foetuses for Diagnostic, Therapeutic, Scientific, Industrial and Commercial Purposes, Recommendation 1046, 1986; and On the Use of Human Embryos and Foetuses in Scientific Research, Recommendation 1000, 1989; Ethics Committee of the American Fertility Society (AFS), "Ethical Considerations of the New Reproductive Technologies", Fertility and Sterility (1986), 46:27S. See also Jonsen, esp. Chapters 4 and 12. [Back]

3 D.N. Irving, "The woman and the physician facing abortion: The role of correct science in the formation of conscience and the moral decision making process", presented at "The Scientific Congress, The Guadalupan Appeal: The dignity and status of the human embryo", Mexico City, October 28-29, 1999; published in Un Appello Per La Vita: The Guadalupan Appeal: Dignita E Statuto Dell'embryione Umano (Libreria Editrice Vaticana (2000), pp. 203-223, also in, Linacre Quarterly Nov./Dec. 2000); D.N. Irving, "The impact of scientific 'misinformation' on other fields: Philosophy, theology, biomedical ethics and public policy", Accountability in Research April 1993, 2(4):243-272. [Back]

4 Aristotle, " … the least initial deviation from the truth is multiplied later a thousand fold.", De Coelo, I, 1.5.27(1)b8-13, in Richard McKeon (ed.), The Basic Works of Aristotle (New York: Random House, 1941); St. Thomas Aquinas, De Ente et Essentia, Armand Mauer (trans.) (Toronto: The Pontifical Institute of Mediaeval Studies, 1983), p. 28. [Back]

5 D. N. Irving, "When does a human being begin? 'Scientific' myths and scientific facts", International Journal of Sociology and Social Policy, 1999, 19:3/4:22-47. [Back]

6 Ronan O'Rahilly and Fabiola Muller, Human Embryology &Teratology (New York: Wiley-Liss, 2001), p. ix. [Back]

7 O'Rahilly and Muller 2001, p. 12. [Back]

8 For a current textbook on clinical and research studies in in vitro fertilization, see Peter R. Brinsden (ed.), A Textbook of In Vitro Fertilization and Assisted Reproduction, 2nd ed. (New York: The Parthenon Publishing Group, 1999); see also, Geoffrey Sher, Virginia Marriage Davis, and Jean Stoess, In Vitro Fertilization: The A.R.T. of Making Babies (New York: Fact On File, 1998). [Back]

9 Wilhelm His, Anatomie menschlicher Embryonen (Leipzig: Vogel, 1880-1885); O"Rahilly and Muller 1994, p. 3; Keith L. Moore and T.V.N. Persaud, The Developing Human: Clinically Oriented Embryology (use 6th ed. only) (Philadelphia: W.B. Saunders Company, 1998), p. 12. [Back]

10 FULL REFERENCES: "Although life is a continuous process, fertilization … is a critical landmark because, under ordinary circumstances, a new, genetically distinct human organism is formed when the chromosomes of the male and female pronuclei blend in the oocyte. This remains true even though the embryonic genome is not actually activated until 2-8 cells are present at about 2-3 days. … Fertilization is the procession of events that begins when a spermatozoon makes contact with a secondary oocyte or its investments, and ends with the intermingling of maternal and paternal chromosomes at metaphase of the first mitotic division of the zygote. … Fertilization takes place normally in the ampulla (lateral end) of the uterine tube. (p. 31); … Coalescence of homologous chromosomes results in a one-cell embryo. …The zygote is characteristic of the last phase of fertilization and is identified by the first cleavage spindle. It is a unicellular embryo and is a highly specialized cell. The combination of 23 chromosomes present in each pronucleus results in 46 chromosomes in the zygote. Thus the diploid number is restored and the embryonic genome is formed. The embryo now exists as a genetic unity." (p. 33); "… [I]t is now accepted that the word embryo, as currently used in human embryology, means 'an unborn human in the first 8 weeks' from fertilization'. Embryonic life begins with the formation of a new embryonic genome (slightly prior to its activation)." (p. 87) [O'Rahilly and Muller, 2001]

"Human pregnancy begins with the fusion of an egg and a sperm, but a great deal of preparation precedes this event. First both male and female sex cells must pass through a long series of changes (gametogenesis) that convert them genetically and phenotypically into mature gametes, which are capable of participating in the process of fertilization. Next, the gametes must be released from the gonads and make their way to the upper part of the uterine tube [fallopian tube], where fertilization normally takes place. … Finally, the fertilized egg, now properly called an embryo, must make its way into the uterus ….." (p. 2); … "'Fertilization age' dates the age of the embryo from the time of fertilization." (p. 23) " … In the female, sperm transport begins in the upper vagina and ends in the ampulla of the uterine tube [fallopian tube] where the spermatozoa make contact with the ovulated egg." (p. 27) [Bruce M. Carlson, Human Embryology &Developmental Biology (St. Louis: Mosby, 1999)].

"In this text, we begin our description of the developing human with the formation and differentiation of the male and female sex cells or gametes, which will unite at fertilization to initiate the embryonic development of a new individual. … Fertilization takes place in the oviduct [not the uterus]… resulting in the formation of a zygote containing a single diploid nucleus. Embryonic development is considered to begin at this point." (p. 1); " … These pronuclei fuse with each other to produce the single, diploid, 2N nucleus of the fertilized zygote. This moment of zygote formation may be taken as the beginning or zero time point of embryonic development." (p. 17). [William J. Larson, Essentials of Human Embryology (New York: Churchill Livingstone, 1997)]

"Human development is a continuous process that begins when an oocyte (ovum) from a female is fertilized by a sperm (or spermatozoon) from a male." (p. 2); " … but the embryo begins to develop as soon as the oocyte is fertilized. " (p. 2); " … Zygote: this cell results from the union of an oocyte and a sperm. A zygote is the beginning of a new human being (i.e., an embryo). " (p. 2); "… Human development begins at fertilization, the process during which a male gamete or sperm … unites with a female gamete or oocyte … to form a single cell called a zygote. This highly specialized, totipotent cell marks the beginning of each of us as a unique individual.". (p. 18) "… The usual site of fertilization is the ampulla of the uterine tube [fallopian tube], its longest and widest part. If the oocyte is not fertilized here, it slowly passes along the tube to the uterus, where it degenerates and is reabsorbed. Although fertilization may occur in other parts of the tube, it does not occur in the uterus. … Human development begins when a oocyte is fertilized. Fertilization … begins with contact between a sperm and a oocyte and ends with the intermingling of maternal and paternal chromosomes of the zygote, a unicellular embryo." (p. 34) [Keith L. Moore and T.V.N. Persaud, The Developing Human: Clinically Oriented Embryology (use 6th ed. only) (Philadelphia: W.B. Saunders Company, 1998)]

"Of verified pregnancies that have survived the first 4 postovulatory weeks, it is generally maintained that 15-20% are lost through spontaneous abortion. Under 4 weeks, however, the number is far larger and may be as high as 40%. Many fertilized oocytes fail to become implanted, and as many as one-third of those implanted may be lost without being recognized. The total loss of conceptuses from fertilization to birth is believed to be considerable, perhaps even as high as 50% to nearly 80%. A high percentage of abortuses (30-80%, depending on the study) are structurally abnormal, and it is maintained that all abortuses under 4 postovulatory weeks have abnormally formed embryonic tissue. Most malformed conceptuses (more than 90%) are spontaneously aborted, compared with the normal 18%. Thus, spontaneous abortion greatly reduces the number of malformed fetuses born." (O'Rahilly and Muller 2001, pp. 92-93).

"Early spontaneous abortions occur for a variety of reasons, one being the presence of chromosomal abnormalities in the zygote. Carr and Gedeon (1977) estimated that about half of all known spontaneous abortions occur because of chromosomal abnormalities. Hertig et al. (1959), while examining blastocysts recovered from early pregnancies, found several clearly defective dividing zygotes … and blastocysts. Some were so abnormal that survival would not have been likely. The early loss of embryos, once called pregnancy wastage, appears to represent a disposal of abnormal conceptuses that could not have developed normally, i.e., there is a natural screening of embryos. Without this screening, about 12% instead of 2 to 3% of infants would likely be congenitally malformed (Warkany, 1981)." (p.p. 42 - 43) [Moore and Persaud 1998]. [Back]

11 For extensive scientific references for these processes of gametogenesis and fertilization, see D.N. Irving, "When does a human being begin? 'Scientific' myths and scientific facts", International Journal of Sociology and Social Policy 1999, 19:3/4:22-47. [Back]

12 "Gametogenesis is the production of germ cells (gametes), i.e., spermatozoa and oocytes. … The gametes are believed to arise by successive divisions from a distinct line of cells (the germ plasm), and the cells that are not directly concerned with gametogenesis are termed somatic. … The 46 human chromosomes consist of 44 autosomes and two sex chromosomes: X and Y. In the male the sex chromosomes are XY; in the female they are XX. Phenotypic sex is normally determined by the presence or absence of a Y chromosome. … During the differentiation of gametes, diploid cells are termed primary, and haploid cells are called secondary, e.g., secondary oocyte. Diploid refers to the presence of two sets of homologous chromosomes: 23 pairs, making a total of 46. This is characteristic of somatic and primordial germ cells alike. Haploid is used for a single set of 23 chromosomes, as in gametes." [O'Rahilly and Muller 2001, p. 19].

"A subset of the diploid body cells constitute the germ line. These give rise to specialized diploid cells in the ovary and testis that can divide by meiosis to produce haploid gametes (sperm and egg). … The other cells of the body, apart from the germ line, are known as somatic cells … most somatic cells are diploid … ." [Strachan and Read 1999, p. 28].

"Meiosis is a special type of cell division that involves two meiotic cell divisions; it takes place in germ cells only. Diploid germ cells give rise to haploid gametes (sperms and oocytes)." [Moore and Persaud 1998, p. 18].

"In a mitotic division, each germ cell produces two diploid progeny that are genetically equal." [Carlson 1999, p. 2].

"Like all normal somatic (i.e., non-germ cells), the primordial germ cells contain 23 pairs of chromosomes, or a total of 46. " [Larsen 1998, p. 4]. [Back]

13 "Future somatic cells thereby lose their totipotency and are liable to senescence, whereas germ cells regain their totipotency after meiosis and fertilization." [O'Rahilly and Muller 2001, p. 39]. "Early primordial germ cells are spared; their genomic DNA remains very largely unmethylated until after gonadal differentiation and as the germ cells develop whereupon widespread de novo methylation occurs." [Tom Strachan and Andrew Read, Human Molecular Genetics 2 (2nd ed.) (New York: Wiley-Liss, 1999), p. 191] See also notes 19, 20 and 22 for an explanation of the process of "regulation" involved in "twinning" when separated totipotent cells, such as human primitive germ line cells, and the cells of the inner cell mass of the 5-7-day old human blastocyst, are involved. Note too that because human germ line cells, even the more mature germ line cells, are still diploid, and therefore they too can be cloned. [Back]

14 "Cells differentiate by the switching off of large portions of their genome." [O'Rahilly and Mueller 2001, p. 39]. "Gene expression is associated with demethylation. Methylation of DNA is one of the parameters that controls transcription. This is one of several regulatory events that influence the activity of a promoter; like the other regulatory events, typically this will apply to both copies of the gene." [Benjamin Lewin, Genes VII (New York: Oxford University Press, Inc., 2000), p. 678; also p. 603]. "Gene regulation as the primary function for DNA methylation: DNA methylation in vertebrates has been viewed as a mechanism for silencing transcription and may constitute a default position." [Strachan and Read, pp. 193 ff] [Back]

15 "The expression of genes is determined by a regulatory network that probably takes the form of a cascade. Expression of the first set of genes at the start of embryonic development leads to expression of the genes involved in the next stage of development, which in turn leads to a further stage, and so on until all the tissues of the adult are functioning." [Lewin, p. 63; also pp. 914, 950]. [Back]

16 See, e.g., G. Kollias, J. Hurst, E. deBoer, and F. Grosveld, "The Human beta-globulin gene contains a downstream developmental specific enhancer", Nucleic Acids Research 15(14) (July 1987), 5739-47; R. K. Humphries et al, "Transfer of human and murine globin-gene sequences into transgenic mice", American Journal of Human Genetics 37(2) (1985), 295-310; A. Schnieke et al, "Introduction of the human pro alpha 1 (I) collagen gene into pro alpha 1 (I) - deficient Mov-13 mouse cells leads to formation of functional mouse-human hybrid type I collagen", Proceedings of the National Academy of Science - USA 84(3) (Feb. 1987), pp. 764-8. [Back]

17 D. Irving, "Testimony Before the U.S. House of Representatives' Hearing on Cloning: Legal, Medical, Ethical and Social Issues", Linacre Quarterly May 1999, 66:2:26-40. [Back]

18 "A variety of early experiments in mice were also unsuccessful before the landmark study of Wilmut et al (1997) reported successful cloning of an adult sheep. For the first time, an adult nucleus had been reprogrammed to become totipotent once more, just like the genetic material in the fertilized oocyte from which the donor cell had ultimately developed. … Successful cloning of adult animals has forced us to accept that genome modifications once considered irreversible can be reversed and that the genomes of adult cells can be reprogrammed by factors in the oocyte to make them totipotent once again. … Other more recent studies are now forcing us to reconsider the potency of other cells. … [A]nd so the developmental potential of stem cells is not restricted to the differentiated elements of the tissue in which they reside (Bjornson et al, 1999)." Tom Strachan &Andrew P. Read, Human Molecular Genetics 2 (New York: Wiley-Liss, 1999), p. 509. [Back]

19 "Early mammalian embryogenesis is considered to be a highly regulative process. Regulation is the ability of an embryo or an organ primordium to produce a normal structure if parts have been removed or added. At the cellular level, it means that the fates of cells in a regulative system are not irretrievably fixed and that the cells can still respond to environmental cues." (pp. 44-49). … Blastomere removal and addition experiments have convincingly demonstrated the regulative nature (i.e., the strong tendency for the system to be restored to wholeness) of early mammalian embryos. Such knowledge is important in understanding the reason exposure of early human embryos to unfavorable environmental influences typically results in either death or a normal embryo." (p. 46) [Carlson 1999] [Back]

20 "The embryo enters the uterine cavity after about half a week … Each cell (blastomere) is considered to be still totipotent (capable, on isolation, of forming a complete embryo), and separation of these early cells is believed to account for one-third of cases of monozygotic twinning." (p. 37) "… Biopsy of an embryo can be performed by removing one cell from a 4-cell, or two cells from an 8-cell, embryo. This does not seem to decrease the developmental capacity of the remaining cells." [O'Rahilly and Muller 2001, p.37]

"Of the experimental techniques used to demonstrate regulative properties of early embryos, the simplest is to separate the blastomeres of early cleavage-stage embryos and determine whether each one can give rise to an entire embryo. This method has been used to demonstrate that single blastomeres, from two- and sometimes four-cell embryos can form normal embryos, … " (p. 44); " … Some types of twinning represent a natural experiment that demonstrates the highly regulative nature of early human embryos, …" (p. 48); "… Monozygotic twins and some triplets, on the other hand, are the product of one fertilized egg. They arise by the subdivision and splitting of a single embryo. Although monozygotic twins could … arise by the splitting of a two-cell embryo, it is commonly accepted that most arise by the subdivision of the inner cell mass in a blastocyst. Because the majority of monozygotic twins are perfectly normal, the early human embryo can obviously be subdivided and each component regulated to form a normal embryo." (p. 49) [Carlson 1999]

"If the splitting occurred during cleavage -- for example, if the two blastomeres produced by the first cleavage division become separated -- the monozygotic twin blastomeres will implant separately, like dizygotic twin blastomeres, and will not share fetal membranes. Alternatively, if the twins are formed by splitting of the inner cell mass within the blastocyst, they will occupy the same chorion but will be enclosed by separate amnions and will use separate placentae, each placenta developing around the connecting stalk of its respective embryo. Finally, if the twins are formed by splitting of a bilaminar germ disc, they will occupy the same amnion." (p. 325) [Larsen 1998] [Back]

21 "[O]ther events are possible after this time [segmentation -- 14 days] which indicate that the notion of "irreversible individuality" may need some review if it is to be considered as an important criterion in human life coming "to be the individual human being it is ever thereafter to be". There are two conditions which raise questions about the adequacy of this notion: conjoined twins, sometimes known as Siamese twins, and fetus-in-fetu. … Although conjoined twins and fetus-in-fetu have rarely been documented, the possibility of their occurring raises several points related to the notion of irreversible individuality. Conjoined twins arise from the twinning process occurring after the primitive streak has begun to form, that is, beyond 14 days after fertilization, or, in terms of the argument from segmentation, beyond the time at which irreversible individuality is said to exist. … Similar reasoning leads to the same confusion in the case of fetus-in-fetu. … One case recorded and studied in detail showed that the engulfed twin had developed to the equivalent of four months gestation and consisted of brain, bones, nerve tissue, muscle and some rudimentary organs. Microscopic study showed that engulfment had occurred at about four weeks after fertilization, in terms of the argument for segmentation long after the time when it is claimed that individuality is resolved." [Her reference is: Yasuda, Y., Mitomori, T., Matsurra, A. and Tanimura, T., "Fetus-in-fetu: report of a case", Teratology 31 (1985), 337-41.] [Karen Dawson, "Segmentation and moral status", in Peter Singer, Helga Kuhse, Stephen Buckle, Karen Dawson, and Pascal Kasimba, Embryo Experimentation (New York: Cambridge University Press, 1990), pp. 57-59].

"MZ [monozygotic] twinning usually begins in the blastocyst stage, around the end of the first week (before formation of the germ disc starting at 8 days)…. Uncommonly, early separation of embryonic blastomeres, (e.g., during the 2 - 8 cell stages) results in MZ twins with two amnions, two chorions, and two placentas that may or may not be fused. (p. 159); … About 35% of MZ twins result from early separation of the embryonic blastomeres; i.e., during the first 3 days of development. The other 65% of MZ twins originate at the end of the first week of development; i.e., right after the blastocyst has formed [5-7 days]. Late division of early embryonic cells, such as division of the embryonic disc during the second week, results in MZ twins that are in one amniotic sac and one chorionic sac." (p. 159); … If the embryonic disk does not divide completely, or adjacent embryonic discs fuse, various types of conjoined MZ twins may form. … the incidence of conjoined (Siamese) twins is 1 in 50,000- 100,000 births." [Moore and Persaud 1998, p. 161].

"Partial duplication at an early stage and attempted duplication from 2 weeks onward (when bilateral symmetry has become manifest) would result in conjoined twins." (p. 30); … Once the primitive streak has appeared at about 13 days, splitting that involves the longitudinal axis of the embryo would be incomplete and would result in conjoined twins." [O'Rahilly and Muller 1994, p. 30]. … Similarly, after the appearance of the primitive streak and notochordal process, any attempt at longitudinal division would be incomplete and would result in conjoined [Siamese] twins. " (ibid, 2001, p. 55) [Back]

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