One of the difficulties in discussing the morality of medical technology is that in order to understand the ethics, we first need to understand the technology and the science that underlies it. In this chapter, I am providing a biology lesson to remind us how life begins at fertilization. I will also examine arguments that suggest human life begins after fertilization.

Human development

Human conception begins with fertilization of an egg[1] by a sperm. Cells in the human body have 46 chromosomes, made up of 23 pairs. Chromosomes carry the genetic material, or DNA (deoxyribonucleic acid), that guides our individual growth and development. Both the egg and sperm carry half the usual number of chromosomes, so their union creates a single cell with the full complement of chromosomes. This single cell is called a zygote and has its own unique genetic code.[2] Both the sperm and the egg cease to exist individually at this point. It is not a ‘fertilized egg’ so much as the first cell of the new human, physically representing the ‘one flesh’ (Gen 2:24) of the father and mother. All the genetic material required for full maturity of the human being is present in this single cell, and from this point on it will direct its own growth. From this point, development of the individual will be a continuum through pregnancy and childhood to adulthood. We therefore have in the human zygote a member of the species homo sapiens.

A human being is conceived when a sperm penetrates the wall of a human egg, which normally happens in a woman’s fallopian tube.

Female reproductive organs
Early embryo development

The first cell division occurs within 24 hours of conception, and cellular division continues while the embryo travels down the fallopian tube towards the uterus.[3] At day 5 a blastocyst is formed, at which point the cells have divided into those which will become the baby and those which will become the placenta; an inner cell mass surrounded by a hollow ball of cells. The blastocyst will normally be floating in the uterus at the end of the first week, when implantation begins. The blastocyst attaches to the uterine wall and the mother’s blood supply starts to nourish it. Sadly this doesn’t always happen successfully and instead an early miscarriage occurs.

At around 14 days, the mother will notice that she has missed her menstrual period—the first outward sign of the pregnancy. Embryonic development from this point is quite rapid. Note that babies can vary in their development and the information below is a rough guide only, counting weeks from fertilization.[4]

Table 1: Human embryological development

Age (weeks)

Length (mm)[5]




Future spinal cord begins to develop and heart tubes begin to fuse. Blood cell production begins.



The embryo’s own heart begins to beat regularly. Early development of the brain, thyroid, eyes and ears, arms and legs. Embryo begins to curve into typical C shape.



Continued development of eyes and mouth, arms and legs. Nose, sinuses, lungs, and hands begin to grow.



Beginning of formation of feet, ears, nipples and bones. Continued development of face and brain. Fingers are growing on hands and toes on feet.



Trunk lengthens and straightens. Upper limbs are longer and bent at elbow. Hands approach each other, feet likewise. Kidneys and tastebuds start to develop. Hormones are beginning to be produced by the embryo.



Eyelids and external ear more developed. Limbs longer and more developed. Beginnings of all essential external and internal structures are present.

You can see that even while still at the embryonic stage (that is, from 0-8 weeks), an enormous amount of development has taken place. After 8 weeks, the fetal period begins. At 3 months, the fetus is fully formed and all organs are beginning to function. The remainder of the pregnancy is the time during which the fetus will mature.

Table 2: Human fetal development

Age (weeks)

Crown-rump length (mm)[6]




Eyes closing or closed. Head more rounded. Intestines are in the umbilical cord.



Intestines in abdomen. Early fingernail and bone development. The fetus can move spontaneously when seen on ultrasound. Fetus begins to swallow amniotic fluid.



External genitalia distinguishable as male or female. Well-defined neck. Tastebuds mature. Kidneys start to make urine. Fetal chest wall movements are starting.



Head erect and lower limbs well developed.



Ears stand out from head.



Early toenail development.



Head and body (lanugo) hair visible.



Skin wrinkled and red. Eyebrows and eyelashes usually recognizable.



Fingernails present. Lean body. May be able to suck and hear.



Eyes partially open. Eyelashes present.



Eyes open and sensitive to light. Good head of hair. Skin slightly wrinkled.



Toenails present. Body filling out. Testes descending.



Fingernails reach fingertips. Skin pink and smooth.



Body usually plump. Lanugo hair is disappearing. Toenails reach tips of toes.



Prominent chest; breasts protrude. Fingernails extend beyond fingertips.


I have noticed in my research that language is often used to confuse the debate regarding when life begins. Depending on your purpose, early embryos can be called many things, including ‘pre-embryos’, ‘fertilized eggs’, ‘pre-syngamy eggs’, ‘little clusters of cells’, and ‘genetic material (which is going to be thrown away)’. And the problem isn’t just euphemisms for early embryos; I have also seen the term ‘conception’ used to describe the beginning of fertilization (fusion of sperm and egg), the end of fertilization (syngamy), full genetic expression (around the 8-cell stage) and implantation (around 7-10 days). Then there are various options for the term ‘cloning’: we have ‘therapeutic cloning’ (which sounds downright good for you), ‘cloning for research’ (which also sounds fairly harmless), and ‘somatic cell nuclear transfer’ (which for most people means nothing at all). Although the technical terms can be difficult for those not used to them, there is a lot to be said for clarity. Any terms that obscure the truth instead of increasing transparency should be avoided.

Common objections to the argument that human life begins at fertilization

In public debate, no educated person questions the humanity of the human embryo any more. The argument now focuses on when the embryonic human deserves protection. Nonetheless, despite the straightforward embryology, you may still hear arguments that suggest the embryo is not human.[7] I have listed the most common of these below, along with a response to each one.[8]

1. Twinning and the problem of individuality

This objection states that, because it is possible for an embryo to divide into two identical twins, or indeed (rarely) for two embryos to meld into one, and because the early embryo is totipotent (each individual cell retains the capacity to develop into a separate embryo), it is not possible to say that a single human individual exists from zygote stage (because you could end up with two). It is argued that it is only at around 14 days, when totipotency and the possibility of twinning no longer exist, that the human individual exists.

Response: This argument confuses individuality with indivisibility. While most individual humans are indivisible, it is not necessary for them to be indivisible in order to be human individuals. Consider conjoined (Siamese) twins. Despite the fact that they are permanently joined, we talk about them as two persons rather than one. In any case of fertilization, an individual embryo exists from day one. It may or may not divide during the next two weeks. At present we don’t know enough about the twinning process to be able to know from day one whether or not the embryo will divide. However, the possibility of division does not remove the fact that the individual embryo exists. Should it divide, then two individuals exist. The existence of one has begun in a way that is unusual for a human being (i.e. twinning), but this does not alter the fact that it has begun to exist as an individual. The other embryo continues to exist, as it has since day one, maintaining its identity (ontological continuity) for the rest of its life. The unusual process of generation does not change these facts, and our inability to identify which is the ‘new’ embryo and which is the ‘continuing’ embryo also does not change these facts. The objection that two embryos can become one (mosaicism) can be responded to in the same way.[9]

2. The problem of destiny

This objection notes that most of the substance of the early embryo does not contribute to the future fetus. The developing cells of the embryo do not separate into those that will become the embryo and those that will become the extraembryonic membranes (such as the placenta) until the blastocyst stage, and all tissues supporting the development of the fetus are discarded at birth. How can we call the early embryo a human being when this excess tissue makes up more than half of it?

Response: Our inability to distinguish those cells that will become the body from those cells that will become the placenta after the first week does not change the fact that a human individual is represented in an early embryo. It is just a human individual with extra tissue. The lack of clarity should make us more careful in our handling of it—not less.

3. The problem of wastage

It is estimated that roughly up to 75% of all embryos created naturally will fail to implant after fertilization, often without the woman ever being aware that she was pregnant. It has therefore been suggested that the destruction of human embryos which is sometimes part of the reproductive technology process is the equivalent of this normal ‘wastage’, and therefore of no moral concern. This argument suggests that the sheer number of lost embryos in either process reduces the significance of each individual one.

Response: It is true that there is a high rate of loss of human embryos before natural implantation (the range is 30%-70%). This is frequently due to a genetic abnormality, such as the wrong number of chromosomes or missing/extra bits of chromosomes, which can be fatal for embryos. Other reasons that some embryos don’t make it include local endometrial (womb lining) factors affecting receptivity to the blastocyst; or a lack of energy in the embryo due to mitochondria problems, especially if they were made from an older woman’s eggs (the mitochondria is the ‘power house’ of the cell). However, this is not an expression of the unimportance of early embryos so much as a problem of living in a fallen world. In countries where many children die before 12 months of age, you would not consider each child less important, but as a casualty of disease in a fallen world. You certainly would not say they weren’t human. Furthermore, how can a statistical argument (which speaks in percentages) give us a sufficient indication of discontinuity in the individual embryo to justify destruction?[10] As we do not know in advance which embryos will live and which will die, we need to treat them all carefully.

4. The problem of environment

This is really an extension of the ‘wastage’ argument. It is suggested that until implantation, the embryo is not in a secure environment where nurture is ensured. In the 10 days or so before implantation, the embryo could still be flushed out of the uterus with the next menstrual flow and there is no guarantee that it will remain in an environment where it will be able to flourish.

Response: Location is not a biologically significant factor when deciding what an embryo is. It is either a human being or not, regardless of where it is. None of us would survive long if we were not in an environment conducive to our survival.

5. The problem of syngamy

The continuity of the human being from fertilization is linked to its unique genetic code (DNA), which results from the fusion of egg and sperm. However, as we have discovered more about the embryo we have realized that fertilization is not so much an event as a process. We now know that there is a gap of about 20 hours between the penetration of the egg by the sperm and the total fusion of male and female DNA (syngamy). It is suggested that if personhood is linked to the genetic continuity of the individual from embryo to live birth, then the human embryo should be protected only once syngamy (and therefore fertilization) is complete.

This may seem petty—arguing over a few hours time difference—but it is significant because some scientists want to conduct research into aspects of fertilization that occur before syngamy (for example, the microinjection of a single sperm into the egg). This has led to a change in the definition of an embryo in some jurisdictions so that it is said to exist only after syngamy has occurred.

Also, a few hours before syngamy, the genetic material from the sperm and the egg are visible as separate vacuoles (storage bubbles) called pronuclei. Some ART[11] clinics offer the service of freezing embryos at this stage (they may call the embryos ‘fertilized eggs that have not yet become embryos’) to avoid producing more ‘embryos’ than will be used, yet still have the benefits of freezing.

Those who believe that legislation should protect the human embryo only after syngamy is complete are not necessarily arguing against the view that a new human life begins at fertilization. Rather, they may be attempting to explain the traditional view more precisely.

Response: There are many points at which the embryonic human life is said to have begun: on penetration of the egg by the sperm; at syngamy; on implantation; at viability; and at birth. Certainly there are significant milestones reached at each of these points, and each milestone is necessary for the ongoing development of the human being involved. However, the first ‘significant moment’ is when a particular sperm penetrates the egg so that the sperm and the egg individually no longer exist. At this time the structure of the egg wall changes so that no other sperm can enter. This is the moment when the unique combination of genetic material of the new individual is first together within one cell, and all other genetic combinations (had a different sperm won the race) are no longer possible. The gender of the embryo is decided. To choose any later ‘significant’ point is arbitrary.

Although it is possible for the normal sequence of events in fertilization to fail in some way—for example, more than one sperm may enter the egg—this is unusual and leads to significant abnormalities in the embryo. I would not change my definition of the embryo on this basis. We create definitions based on the common manifestation rather than the exception. So while it is possible for a man to have one leg, he usually has two. We would not say he was not a man because of this difference, nor would we refrain from defining ‘man’ as a two-legged creature because an exception is possible.[12]

We also know that the beginning of fertilization (prior to syngamy) is significant in other ways. The organization (orientation) of the embryo’s development seems to be present from the start, and may be related to the sperm’s point of penetration.[13] In addition, the embryo is a separate organism that will direct its own growth and development from that point on.

Furthermore, the argument from syngamy seems to me to be trying consciously to follow the letter of the law while avoiding the spirit of the law. We all know that a human being comes from the joining of the sperm and the egg. If it is not a nascent human being prior to syngamy then what is it?

6. The problem of potential

This argument concedes that the early human embryo has the potential to develop into a fully conscious human being, but denies that this potential means it deserves to be given moral significance. This argument does not grant moral significance on merely biological grounds.[14]

Response: The words used in ethical debates can make a difference to how the community thinks. One problem of discussing embryos in terms of ‘potential’ is that it gives us the impression that the embryo is not fully human, when what is meant is that it is not fully developed. It would be more helpful to describe an embryo as a ‘human with potential’ than as a ‘potential human’.

However, this helps us clarify some of the confusion. If we mean that the embryo will become a fully conscious human, it would be fair to say we imply that the embryo is not fully a human being yet. But that is not what I mean when I say an embryo deserves protection because of its potential. I am indicating that there is continuity between the embryonic human and the fetal human and the child human and the adult human. This is referred to as ontological continuity. We were all embryos once. And when we were, we looked exactly like an embryonic human is supposed to look.

We should also remember that in biblical terms, no-one reaches their potential in this life. Philippians 1:6 tells us that God is still working in each one of us. In this sense, we are all ‘humans with potential’ until we are face to face with God.

7. The problem of appearance

According to this argument, early embryos are not human because they do not look human. For one thing, they are very small (smaller than a full stop). They are also a different shape, and they can’t do anything.

Response: It is obviously true that embryos in the first month of development look different from the way we do now. But you also look different now from the way you did one month after birth. You may not like it, but that’s what you looked like at that stage of development. It does not help us determine what your moral value is. Appearance is not morally relevant.

8. The problem of detection

This argument notes that there is no test for the mother to identify that an embryo has been created until after implantation when her human chorionic gonadotropin (hCG) level is checked. Therefore, it is claimed, the pregnancy (and by implication, human life) cannot start until then. This argument is sometimes expanded to include the idea that ‘conception’ is a process that begins with fertilization and ends with implantation, and cannot be said to have definitely occurred until the embryo is implanted in the mother’s womb.

Response: This argument seems to be saying that something cannot exist if you can’t confirm it with a test. Indwelling Holy Spirit aside, years of scientific research should have taught us by now that just because you can’t detect something, it doesn’t mean it’s not there. It would make sense that there are no biochemical markers detectable in maternal blood before the embryo physically attaches to the wall of the mother’s uterus and makes contact with her blood supply. (In much the same way, there is no evidence I have taken medication until it is absorbed into my system, but that doesn’t mean the tablet is not in my tummy.)

Perhaps this argument supports the idea that pregnancy begins with implantation, but our inability to test for the embryo’s existence prior to this does not mean that human life begins at implantation. There was a time when English law did not confidently extend protection to an unborn child until the mother felt it move (known as ‘quickening’) because that was how they knew it was definitely there. Times change. Perhaps we should simply say that we cannot routinely detect the presence of an embryo in the womb prior to implantation yet.

However, it so happens that studies have demonstrated that hCG in the mother’s blood is not the earliest signal of pregnancy. Although the test has only been done in research laboratories, Early Pregnancy Factor (EPF) has been detected in maternal blood within 24 hours of fertilization. It is thought that the embryo releases EPF to prepare the nearby endometrium for implantation.[15] This may yet provide us with an earlier test for pregnancy, but technically it is quite difficult to do.

It is also important to realize that the existence of embryos fertilized outside of the body (as in the case of IVF) is not questioned just because implantation has not occurred. Their importance may be questioned, but not their existence as embryological human beings. If confirmation of existence is the requirement for humanity, on these grounds embryos are human beings before implantation.

9. Difference in kind

This is more of a moral significance issue, discussed by philosopher Michael Sandel in the prestigious New England Journal of Medicine.[16] Using the analogy of an acorn and an oak tree, he argues that just as we do not value an acorn as much as an oak tree, so we do not need to value embryos as we would adult human beings. He dismisses the developmental continuity, saying that embryos and adult humans (like acorns and oak trees) are different kinds of things and so do not have the equivalent moral value.

Response: It is important to note that human beings and oak trees are not moral equivalents. Indeed, it is because of the kind of thing it is that we value an adult human more than an oak tree. In the same way, it is because of the kind of thing it is that we value a human being at all stages of development. Made in the image of God, our value lies in what we are rather than in what we can do. It is our essential nature that gives us moral value. In contrast, the reason we value an oak tree over an acorn is because of what RP George and C Tollefsen call its “accidental characteristics”—the shade it provides, its magnificence and perhaps its sentimental value.[17] These ‘accidental characteristics’, and not its essential nature, explain why an oak tree might be valuable to us—indeed, why a large, beautiful, oak tree would be highly valued while a small, ugly one would not.

The oak tree analogy does not work, although it does help us understand that when we grieve the loss of an adult more than an embryo, it is because of the ‘accidental characteristics’ of the human adult with which we have become familiar as we have been in relationship with them. Just because the embryo and adult human have equivalent moral value does not mean they are identical. I would suggest it is not true that loss of a human embryo is never mourned,[18] but it is certainly mourned less, on the whole, than loss of a more mature human.

10. Confusion with gametes

I have heard many arguments where embryos are confused with gametes (sex cells; sperm and eggs). It is argued that if sperm and eggs are alive but are not treated as if they are human, why should embryos be treated as if they are human? The reverse is also argued: that if we treat an embryo as if it is morally significant, why would we not have to treat sperm and eggs just as carefully?

Response: As mentioned above, sperm and eggs each consist of a single cell that is different from other types of cells in the body. Each has a half complement of DNA (genetic information)—that is, they have 23 chromosomes each rather than the usual 46. As such, they cannot grow individually as they do not contain all the required genetic material for maturation. Therefore, neither is a human being at an early stage of development, and so neither has moral significance. When the egg and sperm combine to make a zygote, however, a cell is created with a new set of the full 46 chromosomes—a unique individual with its own unique genetic code, combining the mother and father’s DNA in a new way. This cell also has the ability to continue to divide and direct its own development from that point until it is a fully grown human. As a genetically distinct human being even at this early embryonic stage, it has moral value.

11. Confusion with somatic cells

Following on from the previous argument, it has been suggested that if the other cells in the body apart from the sex cells (called somatic cells) each have 46 chromosomes just like an embryo, and we now know that they can each grow into an embryo through the cloning process, then it follows that every cell in the human body has as much potential for development as any human embryo. Therefore it is suggested that embryos have no greater significance than ordinary somatic cells. And since we obviously don’t treat every cell in our body as morally significant, it is argued, we shouldn’t give this status to an embryo.

Response: This is, once again, a mistake in biology. Although an embryo can be generated from a somatic cell through the cloning process, the somatic cell of itself is not a distinct organism, and is only able to change into an organism with the introduction of other factors.[19] In contrast, a human embryo is of itself a “unified, unique, dynamic, self-directed whole”,[20] distinct from other organisms, as soon as it is created. A similar argument to this—that stem cells are equivalent to embryos because they can also be used to create an embryo—can be refuted on the grounds that, once again, a stem cell cannot of itself develop into an individual organism.

Having said that, if a human embryo were to be created through any of the cloning techniques, it would deserve to be treated with respect similar to that of a human embryo that was created by fertilization, just as we treat other human beings whose beginnings were atypical with similar respect. But by that stage it is no longer a somatic cell or a stem cell.

The strange case of the clone

How can a cloned embryo have the same moral status as a fertilized embryo? Isn’t the definition of a human embryo based on sexual reproduction—the joining of a sperm and an egg? Now that we can make ‘embryos’ from single cells (with a bit of extra help—asexual production), what does that mean in terms of moral significance? Are they the same kind of thing?

Yes. Despite their different origins, once you have created an embryo that continues to promote and direct its own growth, the two types of embryo are indistinguishable. You could only identify the clone genetically by showing that its DNA was the same as another person’s (this is the definition of a clone). Furthermore, the development of the two types of embryos will be a continuum through pregnancy to birth and further growth. Should any children come to birth through asexual production they will be fully human, made in the image of God, and morally valuable.

12. The problem of Christian apathy

According to this argument, human embryos should not be treated as morally significant human beings today because they have been routinely destroyed for years through the use of certain contraceptives and the development of assisted reproduction, and the church has not made any significant objection to this in the past.

Response: Ouch. It is true that we have failed to protect human embryos, those most vulnerable of human beings, in the past. This reflects not so much on the nature or value of those embryos as on our own indifference. Maybe we should change. We will address this issue below—but for now, I will end this section by directing you to the real experts.

The view of embryologists

Embryologists are the experts in this field. They are quite clear about what fertilization represents. There are many references I could quote to make the point; here are just some:

Almost all higher animals start their lives from a single cell, the fertilized ovum (zygote)… The time of fertilization represents the starting point in the life history, or ontogeny, of the individual.[21]

Embryologist Ronan O’Rahilly originated the international Carnegie Stages of human embryological development, used for many decades now by the international Terminologica Embryologica committee, which determines the scientifically correct terms to be used in human embryology around the world. This internationally pre-eminent human embryologist has no doubt that in biological terms we are dealing with a human being from the time of fertilization:

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 [egg]. This remains true even though the embryonic genome is not actually activated until 2-8 cells are present, at about 2-3 days…

During the embryonic period proper, milestones include fertilization, activation of the embryonic genome, segregation of embryonic from extra-embryonic cells, implantation, and the appearance of the primitive streak and bilateral symmetry.

Despite the various embryological milestones, however, development is a continuous rather than a saltatory process, and hence the selection of prenatal events would seem to be largely arbitrary.[22]


Prenatal life is conveniently divided into two phases: the embryonic and the fetal…

…it 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).[23]


The embryo, from the time it is created, is a unified, unique, dynamic, self-directed whole, not just a collection of cells. There is evidence that organization exists from the first cell division.[24]

So if there is no doubt that, biologically, the human embryo is indeed a human being at an early stage of development, why is there confusion about how it should be treated? We shall consider this question in the next chapter.

  1. The correct term for the human female gamete is ‘oocyte’. However, even though it is a more culinary term, ‘egg’ is used here for familiarity. 
  2. For a more detailed explanation of human genetics, see appendix III. 
  3. The first week of embryo development is covered in more detail in chapter 12. 
  4. Working out the duration of pregnancy can be confusing, as some people date it from the first day of the last menstrual period (this is known as gestational age, which actually starts counting before fertilization takes place). This was traditionally used because most women know this date. Embryologists describe development in ovulation age (time from ovulation) or postconceptional age, which is used here. You can translate this number into gestational age by adding two weeks to the postconceptional age. For greater detail of embryology see R O’Rahilly and F Müller, Human Embryology and Teratology, 3rd edn, Wiley-Liss, New York, 2001; for fetal development see F Cunningham, K Leveno, S Bloom, J Hauth, D Rouse and C Spong, Williams Obstetrics, 23rd edn, McGraw-Hill, New York, 2010, chapter 4. 
  5. From 3-5 weeks greatest length is given; from 6 weeks crown-heel length is given. 
  6. Average measurements are given, variation increases with age. 
  7. Some of these arguments are also used to deny the moral significance of the unborn human. 
  8. I have heard these arguments from many sources and have not listed them all. This topic is treated in great detail in RP George and C Tollefsen, Embryo: A Defense of Human Life, Doubleday, New York, 2008. 
  9. Further discussion is found in JJ Davis, ‘Human embryos, “twinning”, and public policy’, Ethics and Medicine, vol. 20, no. 2, Summer 2004, pp. 35-46; and J Finnis, ‘Abortion and Health Care Ethics’, in R Gillon (ed.), Principles of Health Care Ethics, Wiley, Chichester, 1994, pp. 547-57. 
  10. See O O’Donovan, Begotten or Made?, OUP, Oxford, 1984, p. 57. 
  11. Assisted reproductive technology. 
  12. This point is challenged in S Buckle, K Dawson and P Singer, ‘The syngamy debate: When precisely does a human life begin?’, Law, Medicine and Health Care, vol. 17, 1989, pp. 174-81. 
  13. The very early development of the embryo is still not fully understood. See T Hiiragi, VB Alarcon, T Fujimori, S Louvet-Vallée, M Maleszewski, Y Marikawa, B Maro and D Solter, ‘Where do we stand now? Mouse early embryo patterning meeting in Freiburg, Germany (2005)’, International Journal of Developmental Biology, vol. 50, no. 7, 2006, pp. 581-88. 
  14. This argument is expanded in S Buckle, ‘Arguing from potential’, Bioethics, vol. 2, no. 3, July 1988, pp. 227-53. 
  15. H Morton, AC Cavanagh, S Athanasas-Platsis, KA Quinn and BE Rolfe, ‘Early pregnancy factor has immunosuppressive and growth factor properties’, Reproduction, Fertility and Development, vol. 4, no. 4, 1992, pp. 411-22. 
  16. MJ Sandel, ‘Embryo ethics: The moral logic of stem-cell research’, New England Journal of Medicine, vol. 351, no. 3, 15 July 2004, pp. 207-9. 
  17. Full discussion of this argument can be found in George and Tollefsen, op. cit., pp. 176-84. 
  18. See ‘Moving on’ at the end of chapter 12. 
  19. For further discussion of cloning, see chapter 15. 
  20. H Pearson, ‘Developmental biology: Your destiny, from day one’, Nature, vol. 418, no. 6893, 4 July 2002, pp. 14-15. 
  21. BM Carlson, Patten’s Foundations of Embryology, 6th edn, McGraw-Hill, New York, 1996, p. 3. 
  22. O’Rahilly and Müller, op. cit., p. 8. 
  23. ibid., p. 87. 
  24. Pearson, loc. cit. 

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