The second week:
growing out in a new shelter
This week the embryo nestles in the wall of the uterus. The tissue on
the periphery, the nutritive trophoblast, grows fast. From the inner
cell mass, the embryoblast, the embryonic disc will be formed. The
embryonic disc becomes largely disconnected from the trophoblast.
Differences in growthrate are responsible for this process.
The embryo (the blastula) is out of its rigid zona pellucida and has
arrived in the uterus. It can now grow and it implants itself on the
side of the embryoblast into the wall of the uterus. Enzymes of
the embryo digest the maternal uterus tissue. The embryo invades; it
eats into the wall of the uterus. The embryo behaves aggressively.
The trophoblast grows fast, so fast that it causes a proliferative
tissue with many nuclei and without cell membranes (called
syncytiotrophoblast (syn = together, cyto = cell). A layer of 'normal'
(cyto)trophoblast cells (= nutritive tissue with cell membranes)
remains present between the syncytiotrophoblast and the embryoblast.
In the syncytiotrophoblast gaps arise, called lacunae, through
which maternal blood starts to flow. Only one membrane exists between
maternal blood and embryonic tissue, and there is just one
barrier for the exchange of substances. Embryonic tissue also surrounds
capillaries and maternal glands. In this way, the embryo can be
supplied with oxygen and nutrients and waste products can be disposed
of. However, substances that are bad for the embryo can also get
The embryo eats into the maternal tissue. On the other hand, the
mother gives room to the embryo in her own tissue. She allows a strange
creature to grow in her own body. This is a wonderful process, because
strange creatures (which is what the embryo is to the mother because of
the fusion of egg and sperm) normally are fought against. A hormone of
the embryo (HCG) ensures that the mother accepts the embryo.
On day 10 the embryo is completely inside the maternal tissue and a
ball or wad is formed to close the wall of the uterus. Now this wall is
completely closed. Around the embryo is the trophoblast, later called
chorion (= skin), uterine tissue and the uterine wall.
Development of the
Flat, square cells are ligated from the embryoblast on the side of the
blastocoele (Fig. 12 to 15). These cells are called the hypoblast (hypo
= under). The hypoblast expands on the inside of the trophoblast, too,
and covers it. The blastocoele is now called yolk sac.
In the embryoblast a small cavity develops, called
the amniotic cavity, its roof is called the amnion (= sheep skin). The
cells adjacent to the hypoblast become cylinder - or elongated
cube-shaped; these cells form the epiblast (epi = upper).
Through the formation of the yolk sac and the amniotic cavity a round,
flat embryonic - or germ disc is formed, existing of two layers (epi-
and hypoblast). At the end of the week the prochordale plate (a spot of
different shaped cells) develops in the embryonic disc.
Origin of the chorion cavity and the connective stalk
From day 9 on, the tissue between trophoblast and embryonic disc,
amnion and yolk sac
thickens (Fig. 13). This tissue is called extra-embryonic mesoderm
(extra = outside). A confusing name, because the tissue lies within the
embryo. The name only indicates that the tissue lies outside the
embryonic disc. Because the syncytiotrophoblast and the cytotrofoblast
grow much faster than the germ disc, in this mesoderm tissue crevices
arise (called the extra-embryonic coelom (= cavity)). From day 12 on
they unite and form the chorionic cavity. The trophoblast is now
called chorion (= skin or leather).
First, the germ disc with amnion and yolk sac is connected to the
chorion (12 days) on the posterior side. This attachment moves to
the tail (or better:
the place where the tail will develop) and becomes narrower. This
attachment is called the connective stalk.
round, flat germ disc, amniotic cavity and yolk sac are attached to the
connective stalk, like two halves of a ball, and they hang freely in a
large spherical space (the chorionic cavity) with the chorion as a
The embryo grows. At the beginning of the second week its size was
0.3 mm, at the end 3 - to 3.5 mm. In a one week period it grew to 10
times its size. The germ disc is still small: 0.5 mm.
About one in 90 pregnancies is a twin. Twins may grow from one or two
eggs. Dizygotic twins result from two fertilized eggs, do not look the
same and can either be of the same - or of different sex. They cover
2/3 of all twins. The two embryos develop as described above and both
have their own amnion and chorion (Fig. 16).
Monozygotic twins are born from one fertilized egg, usually because in
the first week two embryoblasts develop in the blastula. These embryos
have their own amnion and share the chorion and placenta (Fig. 17).
A small number of twins develops in the second week, after the
the amnion, by cleavage of the embryonic disc. They are in the same
amnion together and also share the same chorion and placenta (Fig. 18).
In the first week the embryo developed in the shell of the zona
pellucida. The single-celled zygote was divided into
many small and similar cells, creating the morula and blastula. At
the end, the trophoblast and the embryoblast arose. Because
the embryo breaks out of the zona pellucida at the end of the
week, we see a reversal in the second week: the
embryo starts to grow. The cells are not getting any smaller, they
can grow now. In particular, the trophoblast is growing very fast, so
fast that no cell membranes are made in the syncytiotrophoblast.
The focus of growth lies at the periphery, the trophoblast. The growth
of the centre, of the embryoblast, stays behind, as is shown by the
tearing of the extra-embryonic mesoderm and the development of the
chorionic cavity. In the centre we see a
differentiation in amnion and yolk sac, in epi- and hypoblast.
There is now interaction with the environment, the isolation through
the zona pellucida of the first week is gone. The embryo gives off
hormones and enzymes, takes up nutrients, etc. Therefore it
can grow and gain a place in the uterine
wall. The shelter of the zona pellucida is
replaced by the shelter of the nourishing uterine wall.
The centre, the embryonic disc, is a flat round disc of two
cell layers, which has an upper and a bottom side, but no left and
right. The disc has no
content, as the two layers are both on the outside. An early
orientation of directions occurs only at the end of the week with
the formation of the connective stalk.
Monozygotic twins can develop in the first week, and at the beginning
the second week. Thereafter the embryonic disc is no longer divisible.
The embryo has then become in-dividual (= not-divisible).
The development of the first week is similar in shape and
time in all mammals. Now that's no longer the case, both form and
duration are species-dependent.
The characteristics of this week are a reversal of the characteristics
of the first week and are that of the plant:
- There is a huge growth on the outside.
- As the embryonic disc is flat and round, there
is no left and right in the centre and there is no content.
Therefore, it can be
said that the embryo of the second week has the characteristics of the
plant and can be called the "plant-man".
Rudolf Steiner said that in the second week man is not in the
embryo yet, but floats around it. The spirit lives in the
periphery, in the tissue of the trophoblast.
|communication with the environment
Implantation of the embryo in the
Left: on day 7-8. The embryo lies against the uterine
wall on the
side of the embryoblast. The syncytiotrophoblast expands into
hypoblast is ligated
from cells of the
Right: on day 8-9. The embryo eats further
into the uterine wall, the
hypoblast (flat cells) has extended all the
way down and forms, together with the cytotrophoblast, the membrane of
the blastocoele. In the embryoblast the amnion arises. The epiblast
(high cells) is located above the hypoblast and
the amnion is formed from
epiblast and cytotrofoblast cells. The syncytiotrophoblast lies against
a maternal blood vessel (capillary).
Figure 13. Implantation, continued
Left on day 9. The embryo eats
further into the uterine wall. The syncytiotrophoblast proliferates
the maternal tissue and lies around capillaries and makes holes
(lacunae) where maternal blood can flow. Maternal blood and tissues
remain separated from embryonic tissue. The amniotic cavity develops,
by which the two-cells-thick embryonic disc arises. The tissue of the
hypoblast covers the blastocoel membrane, now called yolk
Between the trophoblast and
the membrane of the yolk sac a thick tissue develops: the
day 12. The embryo is completely enclosed by the tissue of the uterine
wall. The syncytiotrophoblast is still rapidly expanding. The
mesoderm is thicker and holes in the extra embryonic coelom are
Figure 14. The embryo on day 13 (simplified)
The holes in the
extra-embryonic mesoderm have joined together to form
the chorionic cavity, its membrane is
the chorion. The syncytiotrophoblast
is growing all around the
embryo, it is thicker on
the inside of the uterine tissue than near the
epithelium. The embryonic disc is on the backside attached to the
chorion. All around the chorion cavity lies the syncytiotrophoblast
with the lacunae
containing maternal blood. (The primary
yolk sac which is now separated from the now called
secondary yolk sac has been omitted.)
Figure 15. The embryonic disc on day 9 (left) and day 14 (right)
Left: The embryonic disc consists of two cell layers and is
round and flat.
Right: at the tail-side the embryo is attached to
cyto- and syncytiotrophoblast by
the connective stalk. It hangs free in the fluid-filled chorion cavity.
The prochordale plate is formed in the
embryonic disc at the side where the head will develop. Epiblast cells
(up) are high, hypoblast
cells (under) are flat.
Figure 16. Dizygotic twins
There are two zygotes, which implant. They
both have their own shell (amnion, chorion) and trophoblast.
Figure 17. Monozygotic twins
There is one zygote with two embryoblasts
that implant. Both embryos have their own amnion, and share the same
Figure 18. Monozygotic twins from the second week The embryonic disc
divides. The embryos are together in the same amnion and chorion.
Differences of the embryological processes of the first and the second
Figure 19. The
plant or a picture for the "plant-man" (from van der Wal
With its growth the plant is committed to the periphery; it grows from
its seasonal sprout in all directions into space.