Chapter 28
Pregnancy and Human Development
1081
28
Events of Fetal Development
Indicate the duration of the fetal period, and note the
major events of fetal development.
During the fetal period, the developing fetus grows from a
crown-to-rump length of about 22 mm (slightly less than 1 inch)
and a weight of approximately 2 g (0.06 ounce) to about 360
mm (14 inches) and 3.2 kg (7 lb) or more. (Total body length at
birth is about 550 mm, or 22 inches.) As you might expect with
such tremendous growth, the changes in fetal appearance are
quite dramatic
(Figure 28.15)
. ±e fetal period is a time of
the aorta eventually reaches the umbilical arteries, which are
branches of the internal iliac arteries serving the pelvis. From
here the largely deoxygenated blood, laden with metabolic
wastes, is delivered back to the capillaries in the chorionic villi
of the placenta. ±e changes in the circulatory plan that occur at
birth are illustrated in Figure 28.14b.
Check Your Understanding
12.
The early embryo is flat like a three-layered pancake. What
event must occur before organogenesis can get going in
earnest?
13.
What germ layer gives rise to essentially all body tissues
except nervous tissue, the epidermis, and mucosae?
For answers, see Appendix H
Figure 28.15
Photographs of a developing fetus.
By birth, the
fetus is typically 36 cm long from crown to rump.
Yolk sac
Cut edge
of chorion
Amniotic sac
(a) Embryo at week 7, about 17 mm long.
Umbilical cord
Umbilical vein
Chorionic
villi
(b) Fetus in month 3, about 6 cm long.
(c) Fetus late in month 5, about 19 cm long.
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