Pregnancy and Human Development
Check Your Understanding
What is the function of the amnion?
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
in which the
primary germ layers
—are present (Figure 28.7c).
Tis process, called
shun), involves cel-
lular rearrangements and migrations.
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
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
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
, which helps to form
the placenta (Figure 28.7c). As the outermost membrane, the
chorion encloses the embryonic body and all other membranes.
Head end of bilayered
(e) Bilayered embryonic disc, superior view
(c) 3-D view
view in (e)
(f) 14-15 days
(g) 16 days
Formation of the three
primary germ layers. (a–d)
Surface view of an embryonic
disc, amnion and yolk sac removed.
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
endoderm. Those that follow
mesoderm. The epiblast surface is now called