The third week:
the embryo gets direction
In the second week the embryo grew explosively in the periphery. The
tissue between the outer trophoblast and the inner embryoblast tore
away and resulted in the chorion cavity. If the process of strong
growth in the periphery and little growth inside
would continue, a growing and ultimately unbridgeable distance would
arise between the nutritive trophoblast and the embryonic disc. To stop
that process a reversal takes place in the third week. The periphery
will develop further, but the focus of the development will now be on
the embryonic disc.
Several developments occur more or less simultaneously. For easy
reference, they will be discussed separately in the order in which they
formation of mesoderm tissue
The first process that takes place in the third week is
(gaster = stomach). For simple animals gastrulation means the formation
of a stomach and a digestive tract (see the page: plant and animal).
At the same time the body cavity
is formed in which the mesoderm develops. For humans,
gastrulation is the process whereby only the mesoderm is formed.
The digestive tract develops later in a process in which
the amnion is involved.
At the beginning of the third week the tissue of the epiblast thickens
and a small indentation is formed in the
embryonic disc near the connective stalk. This occurs at the
caudal end of the embryonic disc (cauda = tail, caudal
= at the tail end; Fig. 20 and 21A). This formation is called
streak. The primitive streak grows in cranial direction (=
side of the head), i.e. away from the connective stalk. From the
primitive streak mesoderm cells are ligated, which migrate to
the space between the epiblast and the hypoblast. In this process
the cells of the hypoblast are replaced by these newly formed cells,
The primitive streak grows to the middle of the embryo (Fig.
21B). There it makes the
primitive knot. This is the place where most mesoderm cells are
ligated. These cells migrate to any place between the epi- and
hypoblast, except for
the prochordale plate and the places where the mouth and anus
will arise, because epi- and hypoblast stick together at these
places. The epiblast is now
called ectoderm and the hypoblast entoderm.
Then a tube is formed that grows from the primitive knot between the
epi- and hypoblast to the prochordale plate. This
tube is called the notochord (= chorda dorsalis; Fig. 21C).
This flexible rod gives the embryonic disc solidity and creates a
symmetry-axis in the embryo. The notochord also forms the directions left
the mesoderm gives the embryonic disc content and thickness.
The notochord is the formation around which the
vertebrae will grow. In an adult small remnants are
visible in the intervertebral discs.
At the same time, the embryo grows
rapidly in length and not so much in width, and faster at the
cranial side than at the caudal side. Thereby the form changes
from round to oval. Because the embryo grows faster at the cranial end,
the importance of the primitive streak diminishes and it disappears
The neural tube and
The genesis of the neural tube is related to
the growth and development of the
notochord. From the 19th day, the ectoderm above the notochord gets
(Fig. 22). This formation, about half of the ectoderm, is called the
neural plate. The central
nervous system will develop from this tissue. The rest of the ectoderm
does not thicken and will become the skin.
The embryonic disc is growing fast: in five days it doubles its length.
On the cranial side it grows much faster than on the caudal side.
the neural plate at the head longer and wider than at the side
of the tail. The thick neural plate (Fig. 23 and 24) bends in, first in
the middle and from there to the cranial and caudal ends. The
indentation becomes deeper and the
neural groove is formed (Fig. 23 and 24). The connection with the
ectoderm is broken and the neural groove and the ectoderm
close. The neural tube is created. From the middle of the
embryonic disc, this
process goes to both ends. The neural tube closes at the cranial end
on the 29th day, one day earlier than at the caudal end (30th day).
In the neural tube there is amniotic fluid. From the neural crest
ganglia will arise, that lie next to the spinal cord and spread out
over the body. Left and right of the neural tube paired clusters
cells develop, called somites (soma = body; Fig. 24), about 40
pairs in total. The vertebrae and muscles will eventually develop from
At the beginning of the third week the yolk sac grows out
finger-like into the connective stalk (Fig. 21 C2). This small
organ is called the
allantois (allantos = sausage). It plays a role in the development
of blood and the circulatory system. Later it participates in the
formation of the bladder. In humans it remains small, in embryonic
birds the allantois is the organ for breathing.
The blood and its
In the second week, the embryonic disc is so small that nutrition and
from the mother can reach the cells by diffusion in the intercellular
Disposal of waste products takes place in the same way. The growth of
the embryonic disc in the
third week causes this to be no longer sufficient and a
new transport-system is needed. This is created from the
middle of the third week on.
At first, the blood vessels and the blood develop in
the mesoderm on the outside of the yolk sac and the allantois. There,
clusters of cells are formed, called blood islands (Fig. 25). In the
blood islands cavities arise.
The blood islands grow and several blood islands connect with each
other. Then the cavities
merge, so that capillaries are formed. The capillaries become longer
and find their way to the embryonic disc. Some cells in the
wall are transformed into blood cells that will flow in the liquid in
Blood is first formed outside the embryonic disc. In
the embryo itself it is not formed until the fifth week.
The blood starts to flow because in the embryonic disc some substances
are required and others are expelled. The blood flows in capillaries in
the embryo on the ventral side towards the head, where its flow is
stopped by firmer tissue. The blood must turn around to be able to flow
back on the dorsal side of
the embryonic disc to the connective stalk and the
trophoblast. In this movement, congestion occurs and the stream of
blood is stopped for a short while. This is why the blood starts to
pulsate. In this place (i.e. on the head side of the neural tube; above
the tissues that will become the head) the heart is created (Fig. 26).
Around the 20th day it starts to beat.
In the beginning we see a flow of liquid, and because the stream meets
with firmer tissue, causing congestion and stagnation, the beating
heart comes into being. This phenomenon is also seen when a stream
collides with a hard material, like waves breaking on the coast, water
meeting stones in streams and flow-forms.
The heart arises from flow and stagnation, it is not the
cause of the
flow of blood. The heart is the only place where the flowing
blood comes to a standstill for a short time.
The circulatory system is the first organ in operation.
The chorion and the
The rampant growth of the trophoblast decreases towards the end of the
third week. The
syncytiotrophoblast develops cell membranes and will now be called
cytotrophoblast. It constitutes the binding layer between the chorion
and the uterine
tissue. Blood vessels develop in the chorion which run from the
connective stalk to the villi (= flakes), where the
exchange of substances with the blood of the mother takes place from
the end of this week.
The focus of the development of the embryo in the third week is on the
embryonic disc. There is still much growth in the periphery, but it is
longer a proliferating growth and the tissue gets cell membranes. There
now a relative calmness in the periphery. If the growth-movement of the
second week would continue, man would stay in the periphery (or his
enclosing organs) and would not develop a body.
the periphery (yolk sac, connective stalk) blood circulation
arises, finding its centre in the heart. The heart arises
from the flow of blood. Pulsation of
the heart arises from standstill and flow.
Simultaneously, thickness is created in the embryonic disc by the
ligation of mesoderm cells from the primitive streak and -knot. This
gives the embryonic disc content. Left and right are created by the
formation of the notochord, later reinforced by the formation of the
neural tube. The embryonic disc has become a spatial, three-dimensional
An important point in the embryonic development of man is around the
17th day, when circulation and heart are created. If not, the
development cannot go any further and stops.
Gastrulation in humans (similar to Fig. 21-A1)
through the embryonic disc in which the ligation of mesoderm cells
from the primitive streak in the
epiblast (cubic cells)
is visible. The
mesoderm cells migrate to the space between the
epiblast and the
and replace the flat
cells of the hypoblast, too.
The formation of the notochord is here only given in outlines. The
actual process is more complicated. See: www.embryology.ch
Gastrulation or the formation of the mesoderm
A, B and C Dorsal
A1, B1 and C1 are
the corresponding cross-sections
C2 is a longitudinal section of C
A: at the caudal
end the primitive streak grows toward the centre of
disc. From the ectoderm mesoderm cells are
ligated to the space between
the epi- and
B: In the middle
of the embryonic disc at the end of the primitive
streak arises the primitive knot. This is the place where most
mesoderm cells arise, that migrate to all parts between
ecto- and the entoderm.
From the primitive knot the notochord is created, a tube that
runs between ecto-
C: The notochord
runs until the prochordale plate. Between ecto-
is everywhere mesoderm.
C2: a longitudinal
section showing how the notochord runs. At the same
time as the notochord the
allantois is formed. This is a protrusion
the yolk sac
into the connective stalk.
Figure 22. Dorsal
view of the embryonic disc. Development of the
neural plate (dating and interpretation prochordale plate:
shown, the uncoloured tissue
would be blue, too. For clarity, only the neural
plate is coloured. The
becomes the skin.
A. On day 17
caudally the primitive streak appears, that
grows to the centre
of the embryo and ends in the primitive knot.
B. Here one can
see that the notochord grows from primitive knot
to the prochordale
plate. Above the notochord the ectoderm tissue thickens and
the neural plate. The primitive streak and notochord grow from the
caudal to the cranial side.
In C. the
embryonic disc becomes elongated as growth occurs mainly at
cranial side. The notochord has grown until the prochordale plate.
D. The neural
plate folds itself from the middle
into the neural groove. The
primitive streak barely grows, the notochord does.
Figure 23. The
development of the neural tube (cross-sections)
ectoderm bends into the neural groove. At the ends is the
neural crest. The neural groove deepens and the neural crest
expands (third image). On the second row the neural tube is
visible. From the neural crest ganglia arise. The neural tube is
between the skin and the notochord and will develop into the spinal
Figure 24. The
development of the neural tube; dorsal view
(www.embryology.ch). All tissue
the neural plate is coloured gray.
From left to right
days 25, 28 and 29.
The closure of the
begins in the middle. Next to the neural tube
develop, they are visible as bulges under the ectoderm.
The embryonic disc
grows from about
2 to 3.5 mm
in five days.
Figure 25. The
development of blood vessels
of cells arise, blood islands, in which cavities occur. These cavities
together and capillaries arise. From the same primitive tissue cells
blood cells emerge simultaneously, which will flow in the
stream of liquid.
Position of the heart and the circulation at the end of the third week
The heart lies cranially of the neural tissue and the
mouth-membrane (the thin spot behind the heart). The blood
flows ventrally (on the side of the stomach) to the head and dorsally
(the back side) back to the connective stalk (the arrows are drawn
outside the embryonic disc, however, the blood flows in it).
Figure 27. The
development of the chorion
and the placenta
The embryonic disc
and yolk sac
is attached to the connective stalk in the chorionic
cavity. In the chorion the blood vessels are
indicated that run from
the connective stalk to the villi. The syncytiotrophoblast has become
cell membranes and is now called cytotrofoblast.