Overview of human embryology
Ethical writing about abortion frequently makes references to various periods of
development, some of which are considered morally significant by some writers1. This
is a rough overview of the process of human embryonic development, included to
clarify some of the terminology used.
By definition, fertilisation occurs at day 0, when sperm and egg contact, usually in the
uterine/Fallopian tube. When the genetic material intermingles and the first mitotic
division occurs (within hours) and the zygote is formed.
The zygote undergoes repeated mitotic division into blastomeres, and when 12-15
blastomere are present the developing human is called a morula. This occurs around
3 days after fertilisation. When the morula enters the uterus, uterine fluid enters
between the cells and a hollow ball of cells is formed, called the blastocyst (around 45 days after fertilisation). Blastocyst implantation into the uterine wall normally
begins at around day 6-7.
Some writers attach significance to the development of the ‘primitive streak’, which
occurs around 14-16 days after fertilisation2. The primitive streak is the first visible
sign of a process called gastrulation, which is the process of developing bodily form.
It marks, for instance, the point at which the earliest sign of neural development and
head/foot orientation can be identified.
The term embryo is used a little loosely by some writers in reference to this stage but
normally covers the period from fertilisation to 8 weeks3. All major body structures
are present at the end of this period, and the heart and circulation functions
independently of the maternal system. From 8 weeks to birth the developing human is
called a foetus.
(Embryological definitions largely referenced from Moore KL, Persaud TVN. “The
developing human: clinically oriented embryology” 7e, published Elsevier, 2002)
1
Most commonly, ethicists have argued for the moral significance of conception, implantation,
individuation or birth. ‘Viability’ is also commonly argued; it is not discussed here because it does not
refer to a particular stage of development but to a fluid point heavily dependant on medical technology.
2
Some consider this period significant because, before this time, the developing embryo can split into
two and develop as twins. This is known as ‘individuation’, and one line of argument has been
advanced that no individual human being can be said to be present before this time. Some Christian
writers have suggested this as a basis for the acceptability of the morning after pill, while still opposing
abortion.
3
I have noticed a tendency for writers to use ‘embryo’ for first trimester and ‘foetus’ thereafter, but I
think this is more ideological than anatomical.