The ectoderm (EK-to-derm) is the primary germ layer
that gives rise to the nervous system and the epidermis
of skin and its derivatives.
The mesoderm (MËS-o-derm) is the middle primary
germ layer that gives rise to connective tissues, blood
and blood vessels, and muscles.
The endoderm (EN-do-derm) is the primary
germ layer of the developing embryo, which
gives
rise
to
the
gastrointestinal
tract,
urinary bladder, urethra, and respiratory
tract.
Following gastrulation, at about 22 to 24
days, cells of the mesoderm form a cylinder
called a
n o to c h o r d
. The notochord induces the
overlying ectoderm to form a neural tube that
eventually becomes the nervous system; this
process is called
n e u r u la tio n
.
As the embryo develops and changes into a fetus, this
maternal-fetal connection becomes more specialized as the
placenta and umbilical cord (Figure 16.13b). The pla-
centa is the site of the exchange of nutrients between the
mother and fetus. As with the chorionic villi, nutrients and
oxygen diffuse across the capillaries; the blood of
the mother and fetus does not mix. The actual
connection between the placenta and embryo
(and later the fetus) is the
u m bilica l cord.
Once the placenta has formed, the embry-
onic body is surrounded by a thin membrane
called the
a m n io n .
Eventually, the amnion sur-
rounds the entire embryo and is filled with
a m n io tic flu id ,
which serves as a shock absorber
for the fetus, helps regulate body temperature,
helps prevent drying out, and prevents adhe-
sions between the skin of the fetus and sur-
rounding tissues.
n o to c h o r d
(NO-to-
cord) A flexible rod of
mesodermal tissue
that helps form part
of the backbone and
intervertebral discs.
n e u r u la tio n
(noor-
oo-LA-shun) The
process by which the
neural plate, neural
folds, and neural tube
form.
b. Placenta and umbilical cord
Chorionic villi
Yolk sac
Amniotic
fluid in
amniotic
cavity
Umbilical
cord
Chorion
Amnion
Maternal portion
Fetal portion
of placenta
of placenta
Chorionic
villi
Maternal
blood
vessels
Fetal
blood
vessels
The placenta is formed from the chorionic villi and the endometrium. The
chorion portion of the placenta exchanges gases, nutrients, and wastes by
diffusion between the adjacent blood vessels of the mother and the fetus.
When fully developed, the chorion is pancake-shaped and connects to the
fetus through the umbilical cord. The amnion portion of the placenta forms
a protective, fluid-filled sac around the baby. The placenta is shed as the
afterbirth once the baby is born.
Details of placenta
and umbilical cord
Pregnancy Lasts from Fertilization to Delivery
491
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