HUMAN BIOLOGY

336 Chapter 17

allantois Extraembryonic
membrane that gives rise to
blood vessels of the umbili-
cal cord.

amnion Extraembryonic
membrane that encloses the
embryo in a sac that con-
tains amniotic fluid.

chorion The outer extra-
embryonic membrane
that protects the embryo
and releases HCG.

extraembryonic
membranes The amnion,
chorion, allantois, and yolk
sac.

umbilical cord A long tis-
sue containing blood ves-
sels that link an embryo to
the placenta.

yolk sac A short-lived extra-
embryonic membrane that
produces early blood cells,
germ cells, and parts of the
embryo’s digestive tube.

the Four extraembryonic membranes

As a blastocyst is implanting, a new cavity opens up

around the amnion and yolk sac (Figure 17.10B). The lin-

ing of this cavity becomes the chorion, a membrane

that is folded into fingerlike projections called chorionic

villi. While these changes take place, the erosion of the

endometrium that began with implantation continues. As

capillaries in the endometrium break down, spaces in the

disintegrating tissue fill with the mother’s blood. The cho-

rionic villi extend into these spaces. Inside each villus are

small blood vessels, as shown in Figure 17.11.

Eventually the chorion wraps around the embryo and

the other three membranes (Figure 17.10C). It contin-

ues the secretion of HCG that began when the embryo

implanted. HCG will prevent the lining of the uterus (the

endometrium) from breaking down until the placenta can

produce enough estrogen and progesterone to maintain

the lining.

n    during implantation and over the next few weeks,

specialized membranes form outside the embryo.

n    Links to Hormones of the hypothalamus and pituitary 15.3,

The female reproductive system 16.1

As described in Section 17.4, during implantation the

embryonic disk develops from the inner cell mass of

the blastocyst (Figure 17.10A). Some

cells of the disk will give rise to the

embryo. Others give rise to extra-

embryonic membranes that are

not part of the embryo.

One of the membranes, the yolk

sac, produces early blood cells and

germ cells that will become gam-

etes; then it disintegrates. Parts of it

also give rise to the embryo’s diges-

tive tube.

The amnion forms a fluid-filled

sac that encloses the embryo. The

amniotic fluid insulates the embryo,

absorbs shocks, and prevents the

embryo from drying out. Just out-

side it is the allantois, which gives

rise to blood vessels that will invade

the umbilical cord. These vessels

are the embryo’s contribution to

circulatory “plumbing” that will

link the embryo with its lifeline,

the placenta.

A Days 10–11. The yolk sac,

embryonic disk, and amniotic

cavity have started to form

from parts of the blastocyst.

B Day 12. Blood-filled

spaces form in maternal

tissue. The chorionic

cavity starts to form.

C Day 14. A connecting stalk

has formed between the embryonic

disk and chorion. Chorionic villi,

which will be features of a

placenta, start to form.

actual

size

actual

size

actual

size

yolk sac

connecting

stalk

amniotic

cavity

chorionic

villi

chorion chorionic

cavity

blood-filled spaces

start of chorionic cavity

start of

amniotic

cavity

start of

embryonic disk

start of

yolk sac

Figure 17.10 Extraembryonic membranes begin to form during the first 2 weeks of life. (© Cengage Learning)

1 7. 5

What are the extraembryonic membranes?

• The extraembryonic membranes are the yolk sac, the
  amnion, the allantois, and the chorion.


• The yolk sac mainly produces early blood cells and parts of
  the GI tract. The amnion forms a sac around the embryo and
  produces amniotic fluid.


• The allantois gives rise to blood vessels of the umbilical
  cord, while the chorion forms fingerlike villi that contain blood
  capillaries. The villi grow into the endometrium and become
  part of the placenta.

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