(2) Mesoderm gives rise to muscles, bones, blood
vessels, lymphatic vessels, reproductive
organs, kidneys, and linings of body cavities.
(3) Endoderm gives rise to linings of the digestive
tract, respiratory tract, urinary bladder, and
e. Chorionic villi develop and are surrounded by
spaces F
lled with maternal blood.
f. The embryo develops a head, face, upper limbs, lower
limbs, and mouth, and appears more humanlike.
g. The placental membrane consists of the epithelium
of the chorionic villi and the endothelium of the
capillaries inside the chorionic villi.
(1) Oxygen and nutrients diffuse from the
maternal blood through the placental
membrane and into the fetal blood.
(2) Carbon dioxide and other wastes diffuse
from the fetal blood through the placental
membrane and into the maternal blood.
h. The placenta develops in the disc-shaped area
where the chorion contacts the uterine wall.
(1) The embryonic portion consists of the chorion
and its villi.
(2) The maternal portion consists of the
i. A fl uid-F
lled amnion develops around the embryo.
j. The umbilical cord is formed as the amnion
envelopes the tissues attached to the underside of
the embryo.
(1) The umbilical cord includes two arteries and a
(2) It suspends the embryo in the amniotic cavity.
k. The chorion and amnion fuse.
l. The yolk sac forms on the underside of the
embryonic disc.
(1) It gives rise to blood cells and cells that later
form sex cells.
(2) It helps form the digestive tube.
m. The allantois extends from the yolk sac into the
connecting stalk.
(1) It forms blood cells.
(2) It gives rise to the umbilical vessels.
n. By the beginning of the eighth week, the embryo is
recognizable as a human.
5. ±etal stage
a. This stage extends from the end of the eighth week
and continues until birth.
b. Existing structures grow and mature; only a few
new parts appear.
c. The body enlarges, upper and lower limbs reach
nal relative proportions, the skin is covered with
sebum and dead epidermal cells, the skeleton
continues to ossify, muscles contract, and fat is
deposited in subcutaneous tissue.
d. The fetus is full term at the end of the ninth
month, which equals approximately 266 days.
(1) It is about 50 centimeters long and weighs
2.7–3.6 kilograms.
(2) It is positioned with its head toward the cervix.
6. ±etal blood and circulation
a. Umbilical vessels carry blood between the placenta
and the fetus.
b. ±etal blood carries a greater concentration of
oxygen than does maternal blood because the
1. Period of cleavage
a. The zygote undergoes mitosis, and the newly
formed cells divide mitotically.
b. Each subsequent division produces smaller and
smaller cells.
c. A solid ball of cells (morula) forms, and it becomes
a hollow ball called a blastocyst.
d. The inner cell mass that gives rise to the embryo
proper forms within the blastocyst.
e. The blastocyst implants in the uterine wall.
(1) Enzymes digest the endometrium around the
(2) ±ingerlike processes from the blastocyst
penetrate the endometrium.
f. Cleavage lasts through the F
rst week of
g. The trophoblast secretes hCG, which helps
maintain the corpus luteum, helps protect the
blastocyst against being rejected, and stimulates
the developing placenta to secrete hormones.
2. Hormonal changes during pregnancy
a. Embryonic cells produce hCG that maintains
the corpus luteum, which continues to secrete
estrogens and progesterone.
b. Placental tissue produces high concentrations of
estrogens and progesterone.
(1) Estrogens and progesterone maintain the uterine
wall and inhibit secretion of ±SH and LH.
(2) Progesterone and relaxin inhibit contractions
of uterine muscles.
(3) Estrogens enlarge the vagina.
(4) Relaxin helps relax the ligaments of the pelvic
c. The placenta secretes placental lactogen that
stimulates the development of the breasts and
mammary glands.
d. During pregnancy, increasing secretion of
aldosterone promotes retention of sodium and
body fl
uid, and increasing secretion of parathyroid
hormone helps maintain a high concentration of
maternal blood calcium.
3. Other changes during pregnancy
a. The uterus greatly enlarges.
b. The woman’s blood volume, cardiac output,
breathing rate, and urine production increase.
c. The woman’s dietary needs increase, but if intake
is inadequate, fetal tissues have priority for use of
available nutrients.
4. Embryonic stage
a. The embryonic stage extends from the second
through the eighth weeks.
b. The placenta and main internal and external body
structures develop.
c. The embryonic disc becomes cylindrical and
attaches to the developing placenta by the
connecting stalk.
d. The cells of the inner cell mass fold inward,
forming a gastrula that has two and then three
primary germ layers.
(1) Ectoderm gives rise to the nervous system,
portions of the skin, the lining of the mouth,
and the lining of the anal canal.
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