PSYC 2450 Chapter Notes - Chapter 4: Yolk Sac, Blastocyst, Embryonic Disc

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Developmental Psychology Textbook Notes
Chapter 4 Prenatal Development
From Conception to Birth
Prenatal development is often divided into three major phases:
o Period of the Zygote: lasts from conception through implantation, when
the developing zygote becomes firmly attached to the wall of the uterus
Normally lasts about 10-14 days
o Period of the Embryo: lasts from the beginning of the third week though
the end of the eighth; where all the major organs are formed and the heart
begins to beat
o Period of the Fetus: lasts from the ninth week of pregnancy until the child
is born; all major organ systems begin to function and the developing
organism grows rapidly
The Period of the Zygote
As the zygote moves down the fallopian tub toward the uterus it divides into two
cells (mitosis)
The two cells and all the resulting cells continue to divide, forming a ball-shaped
structure or blastocyst; that will contain 60 to 80 cells within 4 days
The inner layer of the blastocyst, or embryonic disk, becomes the embryo,
whereas the outer layer will develop into tissues that protect and nourish the
As the approach to the uterus, small tendrils emerge from its outer surface
These tendrils burrow inward tapping the mother’s blood supply implantation:
the burrowing of the blastocyst into the lining of the uterus
“window of implantation”: where the blastocyst must communicate with the
uterine wall, position itself, attach, and invade
Whole process takes 10-14 days after ovulation; takes about 48 hours 7-10 days
after ovulation
Only about half of all fertilized ova are firmly implanted, as many as half of all
such implants are either genetically abnormal and fail to develop, or burrow into a
site incapable of sustaining them and are miscarried
o 3 out of 4 zygotes fail to survive the initial phase of prenatal development
Development of Support Systems
amnion: a watertight membrane that surrounds the developing embryo, serving
to regulate its temperature and to cushion it against injuries; amniotic fluid
Floating is a balloon-shaped yolk sac that produces blood cells until the embryo
is capable of producing its own
Chorion: a membrane that becomes attached to the uterine tissues to father
nourishment for the embryo
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Placenta: an organ, formed from the lining of the uterus and the chorion, that
provides for respiration and nourishment of the unborn and the elimination of it’s
metabolic wastes
Umbilical Cord: a soft tube containing blood vessels that connects the embryo
to the placenta
Purpose of the Placenta
The placenta is fed by blood vessels from the mother and the embryo, although
its hair like villi act as a barrier that prevents these two bloodstreams from mixing
Semipermeable allows some substances (oxygen, carbon dioxide, salts,
sugars, proteins & fats) that are small enough to pass the barrier; but blood cells
are too large
Maternal blood flowing into placenta delivers oxygen and nutrients into the
embryo’s bloodstream through the cord
Site of all metabolic transactions that must sustain the embryo
The Period of the Embryo
The outer layer (ectoderm) will become the nervous system, skin, and hair
Middle layer (mesoderm) will become the muscles, bones, and circulatory system
The inner layer (endoderm) will become the digestive systems, lungs, urinary
tract, and other vital organs, such as the pancreas and liver
Neural tube: the primitive spinal cord that develops from the ectoderm and
becomes the central nervous system
at no future time in the future will this organism ever grow as rapidly or change as
much as it has during the first prenatal month
The Second Month
becomes much more human in appearance
A primitive tail appears, which later becomes the coccyx
Middle of the 5th week, eyes have cornea and lenses
7th week, the ears are well formed and embryo has a rudimentary skeleton
brain develops rapidly; directs the organism’s first muscular contractions by the
end of the embryonic period
During the 7th and 8th weeks, the sexual development begins with the
appearance of a genital ridge called the indifferent gonad
Circulatory system now functions on its own; can now produce blood cells
Slightly more than 2.5cm and weighs less than 7 g
The Period of the Fetus
Period of rapid growth and refinement of all organ systems
Fetus begins to move, sense, and behave
Individuality emerges as different fetuses develop unique characteristics (facial
expressions, movements)
7.5 cm and 28 g, all major organ systems have formed and several are
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