with the uterine wall is restricted to a disc-shaped area that
becomes the placenta
separates embryonic blood
in the capillary of a chorionic villus from maternal blood
in a lacuna. This membrane is composed of the epithelium
of the chorionic villus and the endothelium of the capil-
lary inside the villus
. Through this membrane,
substances are exchanged between the maternal blood and
the embryo’s blood. Oxygen and nutrients diffuse from the
maternal blood into the embryo’s blood, and carbon dioxide
and other wastes diffuse from the embryo’s blood into the
maternal blood. Active transport and pinocytosis also move
substances through the placental membrane.
If a pregnant woman repeatedly ingests an addictive substance that
crosses the placenta, her newborn may suF
er from withdrawal symp-
toms when amounts of the chemical the fetus was accustomed to
receiving suddenly plummet. Newborn addiction occurs with certain
addictive drugs of abuse, such as heroin and with certain prescription
drugs used to treat anxiety.
Describe the major events of the embryonic stage of development.
Which tissues and structures develop from ectoderm? ±rom
mesoderm? ±rom endoderm?
Describe the structure of a chorionic villus.
What is the function of the placental membrane?
How are substances exchanged between the embryo’s blood and
the maternal blood?
forces blood through blood vessels, and tiny buds form, which
will give rise to the upper and lower limbs
(f g. 23.12)
During the fifth through the seventh weeks, as
shows, the head grows rapidly and becomes rounded
and erect. The face develops eyes, nose, and mouth. The
upper and lower limbs elongate, and F ngers and toes form
(f g. 23.14)
. By the end of the seventh week, all the main inter-
nal organs are established, and as these structures enlarge and
elaborate, the body takes on a humanlike appearance.
Until about the end of the eighth week, the chorionic villi
cover the entire surface of the former trophoblast. However,
as the embryo and the surrounding chorion enlarge, only
villi that contact the endometrium endure. The others degen-
erate, and the areas of the chorion where they were attached
become smooth. The region of the chorion still in contact
Early in the embryonic stage of development, the three primary germ layers form.
Germ layers of
Stages and Events of Early
Human Prenatal Development
Secondary oocyte fertilized,
meiosis is completed; zygote has
46 chromosomes and is genetically
30 hours to third
Mitosis increases cell number
Third to fourth day
Solid ball of cells
±ifth day through
Hollowed ball forms trophoblast
(outside) and inner cell mass, which
implants and ²
attens to form
End of second week
Primary germ layers form