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Chapter 2

PSY 302 Chapter Notes - Chapter 2: Amniotic Sac, Fetal Movement, Embryonic Stem Cell

Course Code
PSY 302
Lixia Yang

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Chapter 2
-looking back at history, major differences in how we thought about prenatal development
-fourth century B.C.E., Aristotle posed a question that underlined Western thought about it for
next 15 centuries: Does prenatal life start with the new individual already preformed,
composed of a full set of tiny parts, or do the many parts of the human body develop in
-Aristotle rejected idea of preformation in favour of what he termed epigenesis — the
emergence of new structures and functions during development
-sperm from father and egg from our mother — these gametes are unique in function and that
they only contain half of the genetic material found in other cells
-gametes are produced by meiosis — special type of cell division in which eggs and sperm
receive one member from each of the 23 chromosome pairs
-process of reproduction starts with launching egg from one of woman’s ovaries into adjoining
fallopian tube
-egg moves through tube to uterus, emits chemical substance acting like a beacon to signal
sperm toward it
-if sex takes place near time egg is released, conception, the union of sperm and egg, will be
Developmental Processes
-four major developmental processes underlie transformation of a zygote into an embryo and
then a fetus
I. first is cell division, known as mitosis
within 12 hrs after fertilization, zygote divides into two parts, then into four, and etc.
into full newborn
II. second major process, occurring during embryonic period, is cell migration — movement
of newly formed cells away from their point of origin
III. third process is cell differentiation
initially all embryo’s cells — known as embryonic stem cells, are equivalent and
interchangeable: no fixed fate or function
can develop into ~350 different types of cells
IV. fourth developmental process — death
selective death of certain cells — known as apoptosis, is readily apparent in hand
formation of fingers depends on death of cells between ridges in hand plate
-in addition to four developmental processes, there are influences of hormones on
i.e. hormone play role in sexual differentiation
presence of absence of androgens — class of hormone incl. testosterone, cause
development to proceed one way or other
if present, male sex organs develop

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Chapter 2
Early development
-by 4th day of conception, cells arrange themselves into hollow sphere with a bulge of cells,
called the inner cell mass, on one side
this is stage at which identical twins originate — result from splitting in half of the inner
cell mass
-fraternal twins result when two eggs happen to be released from ovary and both were
because arise from two diff. eggs and two diff. sperm, same alikeness with non-twin
-after implantation, embedded ball of cells in uterine wall starts to differentiate
-inner cell mass becomes embryo, rest of cells become elaborate support system — incl.
amniotic sac and placenta
-inner cell layer 1 layer thick initially
after 2nd week, folds itself into three layers, each with diff. developmental destiny
top layer nervous system, teeth, nails, inner ear, lens of eyes, outer surface of skin
middle layer muscles, bones, circulatory system, inner layers of skin, other internal
bottom layer digestive system, lungs, urinary tract, and glands
-few days after three layers, U-shaped groove forms down center of top layer
folds at top of groove move together and fuse, creating neural tube — one end swells
into brain, rest spinal cord
-key element of support system for embryo development is amniotic sac — a membrane
filled with clear, watery fluid in which fetus floats
operates as protective buffer for developing fetus, providing even temp. and cushioning
it against jostling
-second key element, the placenta — allows for exchange of materials carried in
bloodstreams of fetus and its mother
highly vascular
blood vessels running from placenta to embryo and back again are contained in
umbilical cord
placenta prevents exchange of blood but allows for essential nutrients, minerals, oxygen,
and some antibodies; waste products cross from fetus to mother
placental membrane serves as defensive barrier against host of dangerous toxins and
infectious agents inhabiting mother
placenta also produces hormones, incl. estrogen, which incr. flow of maternal blood to
uterus and progesterone, which suppresses uterine contractions that could lead to
premature birth
box 2.1: a closer look
phylogenetic continuity
-using non-human animals, we are subscribing to the principle of
phylogenetic continuity — idea that because of common evolutionary
history, humans share many characteristics and developmental
processes with other living things

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Chapter 2
An Illustrated Summary of Prenatal Development
-early development takes place rapidly than later development
-areas near head develop earlier than those farther away — cephalocaudal development
4th week
-several facial features have their origin set
-primitive heart visible — already circulating blood and beating
-arm bud and leg bud can be seen
5 1/2 week
-nose, mouth, palate beginning to differentiate into separate structures
-3 weeks later, mouth and nose almost fully formed
-risk of cleft palate between 5 1/2 & 8 weeks
9th week
-head overwhelms rest of structure
-bulging forehead — extremely rapid brain growth
-rudimentary eyes and ears
-all internal organs present — development still req.
-sexual differentiation started
-ribs, fingers, and nails growing
-can make spontaneous movement
11th week
-heart, basic adult structure
-developing spine and ribs
-major divisions of brain
last 5 months
-growth of lower part
-movements incr. dramatically: chest making breathing movements, some reflexes — grasping
swallowing sucking
-16 weeks fetus capable of intense kicks , mother feels them as ‘flutter
-external genitalia substantially developed
18th week
-covered in fine hair
-greasy coating protecting skin from long immersion in liquid
20th week
-more time in head-down position
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