PSYC 2450 Chapter Notes - Chapter 4: Yolk Sac, Blastocyst, Embryonic Disc
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16 Nov 2017
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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
embryo
Implantation
• 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
functioning
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