Essentials of Anatomy and Physiology

in the procedure called amniocentesis (see Box 21–2:
Fetal Diagnosis). The rupture of the amnion (some-
times called the “bag of waters”) is usually an indica-
tion that labor has begun.
The chorionis the name given to the trophoblast
as it develops further. Once the embryo has become

implanted in the uterus, small projections called

chorionic villibegin to grow into the endometrium.

These will contain the fetal blood vessels that become

the fetal portion of the placenta.

At about 4 to 5 weeks of development, the embryo

shows definite form. The head is apparent, the heart is

Human Development and Genetics 479

Endometrium

Trophoblast

Amnion

Yolk sac

Embryo

Placenta

Chorion

Approximately 20 days

Approximately 14 days

Approximately 12 days

Approximately 4–5 weeks

Umbilical

cord

Chorionic

villi

Head of

embryo

Head of embryo

Limb buds

Embryonic

disc

Embryonic

disc

Figure 21–3. Embryonic development at 12 days (after fertilization), 14 days, 20 days,

and 4 to 5 weeks. By 5 weeks, the embryo has distinct parts but does not yet look definitely

human. See text for description of embryonic membranes.

QUESTION:At 14 days, what structures shown will become the fetal portion of the

placenta?