Chapter 28
Pregnancy and Human Development
1075
28
Check Your Understanding
10.
What is the function of the amnion?
11.
Which extraembryonic membrane provides a path for the
embryonic blood vessels to reach the placenta?
For answers, see Appendix H.
Gastrulation: Germ Layer Formation
Describe gastrulation and its consequence.
During week 3, the two-layered embryonic disc transforms into
a three-layered
embryo
in which the
primary germ layers
ectoderm, mesoderm
, and
endoderm
—are present (Figure 28.7c).
Tis process, called
gastrulation
(gas
0
troo-la
9
shun), involves cel-
lular rearrangements and migrations.
Figure 28.9
focuses on the changes that take place in the
days between Figure 28.7b (12-day embryo) and Figure 28.7c
eggs contain very little yolk and nutritive functions have been
taken over by the placenta. Nevertheless, the yolk sac is impor-
tant in humans because it (1) forms part of the gut (digestive
tube), and (2) is the source of the earliest blood cells and blood
vessels.
Te
allantois
(ah-lan
9
to-is) forms as a small outpocketing of
embryonic tissue at the caudal end of the yolk sac (Figure 28.7c).
In animals that develop in shelled eggs, the allantois is a disposal
site for solid metabolic wastes (excreta). In humans, the allantois
is the structural base for the
umbilical cord
that links the em-
bryo to the placenta, and ultimately it becomes part of the urinary
bladder. Te fully formed umbilical cord contains a core of em-
bryonic connective tissue (Wharton’s jelly), the umbilical arteries
and vein, and is covered externally by amniotic membrane.
We have already described the
chorion
, which helps to form
the placenta (Figure 28.7c). As the outermost membrane, the
chorion encloses the embryonic body and all other membranes.
Amnion
Bilayered
embryonic disc
Head end of bilayered
embryonic disc
Yolk sac
Yolk sac
(cut edge)
Cut edge
of amnion
Left
Right
Primitive
streak
Primitive streak
Epiblast
Hypoblast
Endoderm
Mesoderm
Endoderm
Ectoderm
Head end
Tail end
(b) Frontal
section
(a)
(e) Bilayered embryonic disc, superior view
(c) 3-D view
(d) Section
view in (e)
(f) 14-15 days
(g) 16 days
Figure 28.9
Formation of the three
primary germ layers. (a–d)
Orienting
diagrams.
(e)
Surface view of an embryonic
disc, amnion and yolk sac removed.
(f, g)
Cross sections of the embryonic disc,
showing the germ layers resulting from cell
migration. The first epiblast cells that migrate
medially into the primitive streak
(f)
become
endoderm. Those that follow
(g)
become
mesoderm. The epiblast surface is now called
ectoderm.
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