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time up to about 14 days after fertilisation - and that is longer
than human embryos have so far been kept alive outside the
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Taking Life: The Embryo and the Fetus
body - the embryo can split into two or more genetically identical
embryos. This happens naturally and leads to the formation
of identical twins. When we have an embryo prior to this point,
we cannot be sure if what we are looking at is the precursor of
one or two individuals.
This poses a problem for those who stress the continuity of
our existence from conception to adulthood. Suppose we have
an embryo in a dish on a laboratory bench. If we think of this
embryo as the first stage of an individual human being, we might
call it Mary. But now suppose the embryo divides into two
identical embryos. Is one of them still Mary, and the other Jane?
If so, which one is Mary? There is nothing to distinguish the
two, no way of saying that the one we call Jane split off from
the one we call Mary, rather than vice versa. So should we say
that Mary is no longer with us, and instead we have Jane and
Helen? But what happened to Mary? Did she die? Should we
grieve for her? There is something absurd about these speculations.
The absurdity stems from thinking of the embryo as an
individual at a time at which it is only a cluster of cells. So,
until the possibility of twinning is past, it is even more difficult
to maintain that the embryo is a human being, in any morally
significant sense, than it is to maintain that the fetus is a human
being in a morally significant sense. This provides some basis
for the laws and guidelines in Britain and various other countries
that allow experimentation on the embryo up to 14 days after
fertilisation. But for reasons already given, and others that we
are about to discuss, this is still an unnecessarily restrictive limit.
What of the argument from potential? Can the familiar claims
about the potential of the embryo in the uterus be applied to
the embryo in a dish in the laboratory? Before Robert Edwards
began the research that led to the IVF procedure, no-one had
observed a viable human embryo prior to the stage at which it
implants in the wall of the uterus. In the normal process of
reproduction inside the body, the embryo, or 'pre-embryo' as
it is now sometimes called, remains unattached for the first
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Practical Ethics
seven to fourteen days. As long as such embryos existed only
inside the woman's body, there was no way of observing them
during that period. The very existence of the embryo could not
be established until after implantation. Under these circumstances,
once the existence of an embryo was known, that embryo
had a good chance of becoming a person, unless its
development was deliberately interrupted. The probability of
such an embryo becoming a person was therefore very much
greater than the probability of an egg in a fertile woman uniting
with sperm from that woman's partner and leading to a child.
There was also, in those pre-IVF days, a further important
distinction between the embryo and the egg and sperm.
Whereas the embryo inside the female body has some definite
chance (we shall consider later how great a chance) of developing
into a child unless a deliberate human act interrupts its
growth, the egg and sperm can only develop into a child if there
is a deliberate human act. So in the one case, all that is needed
for the embryo to have a prospect of realising its potential is for
those involved to refrain from stopping it; in the other case, they
have to carry out a positive act. The development of the embryo
inside the female body can therefore be seen as a mere unfolding
of a potential that is inherent in it. (Admittedly, this is an oversimplification,
for it takes no account of the positive acts involved
in childbirth; but it is close enough.) The development
of the separated egg and sperm is more difficult to regard in this
way, because no further development will take place unless the
couple have sexual intercourse or use artificial insemination.
Now consider what has happened as a result of the success
ofIVF. The procedure involves removing one or more eggs from
a woman's ovary, placing them in the appropriate fluid in a
glass dish, and then adding sperm to the dish. In the more
proficient laboratories, this leads to fertilisation in about 80%
of the eggs thus treated. The embryo can then be kept in the
dish for two to three days, while it grows and divides into two,
four, and then eight cells. At about this stage the embryo is
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Taking Life: The Embryo and the Fetus
usually transferred to a woman's uterus. Although the transfer
itself is a simple procedure, it is after the transfer that things are
most likely to go wrong: for reasons that are not fully understood,
with even the most successful IVF teams, the probability
of a given embryo that has been transferred to the uterus actually
implanting there, and leading to a continuing pregnancy, is