888
UNIT SIX
FIGURE 23.12
±Embryo.±(
a
) A human embryo at three weeks, posterior view; (
b
) at three and one-half weeks, lateral view; (
c
) at about four weeks,
lateral view. (Figures are not to scale.)
(a)
(b)
(c)
(b)
Actual length
4 weeks
Actual
length
5 weeks
Actual
length
6 weeks
Actual
length
7 weeks
(a)
FIGURE 23.13
Development of an embryo. (
a
) In the ²
fth through
the seventh weeks of gestation, the embryonic body and face
become more distinct. (
b
) A human embryo after about six weeks of
development.
The embryonic portion of the placenta is composed
of parts of the chorion and its villi; the maternal portion is
composed of the area of the uterine wall (decidua basalis)
to which the villi are attached
(f g. 23.17)
. The fully formed
placenta is a reddish brown disc about 20 centimeters long
and 2.5 centimeters thick, weighing about 0.5 kilogram.
While the placenta is forming from the chorion, a second
membrane, called the
amnion
(am
ne-on), develops around
the embryo. It appears during the second week. Its margin
is attached around the edge of the embryonic disc, and fl uid
called
amniotic fl
uid
F lls the space between the amnion and
the embryonic disc. The amniotic fl uid provides a watery envi-
ronment in which the embryo can grow freely without being
compressed by surrounding tissues. The amniotic fl uid also
protects the embryo from being jarred by the movements of
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