Now, if we’re to follow the embryo itself,
it begins to fold. And looking at it from
the side, what happens is that as the body
begins to curve into a C-shape, so the whole
embryo begins to rise into the amniotic cavity
and the amniotic cavity is swinging around,
front and back. And also, as we’ll see from
side to side, so that gradually, the embryo
is going to be suspended as it were in the
amniotic fluid which fills the amniotic cavity.
So that’s the view seen from the side. And
if we looked at a solid view of the embryo,
you can see the same structures as it curves
around to form this C-shape. At this point,
you can begin to get the idea that this might
be a vertebrate embryo. You might even begin
to guess that it might be like a human embryo.
It might be looking a bit like a human embryo
from later on. Everyone see the same
thing seen in transverse
section, a cross-section across the long axis
of the embryo. Again, we’ve marked the amniotic
cavity, and you can see how it’s sweeping
around from the sides as well. So the embryo
rising up as the amniotic cavity filled with
this fluid begins to sweep around the embryo,
and pinch-off the yolk sac underneath. Little
later still, that sweeping around process
is continued, and here, we can see how the
embryo is rising up and projecting into the
amniotic cavity; and the yolk sac is contributed
towards formation of the gut.
Now, let’s look at essentially the same
process again, but we’re going to step back
a level. Now we’re looking at in the context
of the whole of the cavity of the chorionic
cavity, which is the large space that you
can see here. You can see the amniotic cavity
is marked again, the body of the embryo and
the yolk sac. What happens is that mesoderm
will cover all of those internal and external
surfaces covering over the amniotic cavity,
covering off the yolk sac, and
lining the extra-embryonic, chorionic cavity.
On the right-hand side of the picture, you
can see our amniotic cavity again. And again,
I emphasize that that’s the thing to focus
on. That’s expanding as the embryo folds
up into the amniotic cavity, and the yolk
sac will steadily diminish in size and relative
importance. You can see on the outside of
the trophoblast cells, the villi have become
more complicated, and they’re beginning
in to develop a branched structure as they
continue to increase their absorptive area.
And of course, that is surrounded by syncytiotrophoblast
with maternal blood running through these
spaces in the syncytiotrophoblast.
Little later still, again, focus on the amniotic
cavity. And you can see as you move from three
and a half weeks up to about five and a half
weeks, the amniotic cavity is expanding, and
gradually, the chorionic cavity is diminishing
in size. So go on expanding in that way.
The section view through the embryo, you can see
here, shows the body of the embryo as it rises
up into the amniotic cavity. Now, let’s move
from a diagram to some actual
specimens. Here’s a human embryo at roughly
the same kind of stage of development.
Here, you’re looking at the chorionic cavity.
You can see the embryo inside its embryonic
sac. So it’s lying inside the amniotic cavity
and the little structure, just to the side
of the embryo, is the diminishing yolk sac.
Then surrounding all around the yolk side
are the thorns of the villi, which are in
contact with syncytiotrophoblast and through
which maternal blood is flowing. You can
already see that perhaps, they’re
a little thicker towards the top of the picture
than they are down at the bottom of the picture.
What’s happening is the villi are beginning
to become concentrated in one particular area.
So looking at that in the diagrammatic view,
now you can see that the amniotic cavity has
almost completely excluded all of the previous
chorionic cavities, and the embryo is floating
freely within that. And it’s during the
embryonic period that the embryo first becomes
capable of moving. So it will show spontaneous
movements although these are still too small
and gentle for the mother to be aware of them.
But you can see how the villi are concentrating
at one side of the external structure.