Download 1. Contrast the theories of preformation and

Chapter 47 Reading Quiz
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5.
Which reaction acts as a “fast block” to
polyspermy?
Which reaction acts as a “slow block” to
polyspermy?
Name the series of cell divisions that
transforms a zygote into a blastula.
Which vertebrate germ layer becomes
the skin?
Which extraembryonic avian membrane
acts as a disposal sac?
1. Contrast the theories of preformation and
epigenesis.
• Preformation  suggests that the embryo
contains all of its descendants as a series
of successively smaller embryos within
embryos (popular until the 18th century)
• Epigenesis  proposes that the form of an
embryo gradually emerges from a formless
egg
- organism’s development is determined by
the zygote’s genome and cytoplasmic
determinants 
2. Describe the acrosomal reaction.
• Acrosomal reaction  the discharge of
hydrolytic enzymes from a vesicle in the
acrosome of a sperm cell
- upon contacting the egg’s jelly coat, the
enzymes enable the sperm’s membrane to
reach the egg’s membrane and cause
depolarization of the plasma membrane to
prevent polyspermy 
3. Describe the cortical reaction.
• The fusion of the egg and sperm
membranes stimulates a series of changes
in the egg’s cortex known as a cortical
reaction
- stimulates transduction pathway that
releases Ca+ from egg ER
- eventually through the pathway a
hardened “fertilization membrane”
prevents the entry of more sperm by a
slow block to polyspermy 
4. What happens when the egg is activated?
• The sharp rise in cytoplasmic Ca2+ concentration
incites metabolic changes and activates the egg
cell
• Cellular respiration & protein synthesis rates
increase
• Cytoplasmic pH changes from slightly acidic to
mildly alkaline due to H+ extrusion
• Activation can be artificially induced by injection
of Ca2+
• The sperm nucleus fuses with the egg nucleus to
form zygote
• DNA replication begins and the first division
occurs in about 90 minutes 
5. Define cleavage, and explain what happens
as differentiation takes place.
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Cleavage  a succession of rapid mitotic cell
divisions following fertilization that produce a
multicellular embryo, the blastula
Zygote is composed of two hemispheres, the
vegetal and animal
The first two cleavage divisions are vertical and
divide the embryo into four cells
The third cleavage plane is horizontal &
produces an 8 cell embryo 
6. Distinguish between meroblastic and
holoblastic cleavage.
• Meroblastic  an incomplete division of
the yolk-rich egg; cleavage is restricted to
a small disc of yolk-free cytoplasm at the
animal pole of the egg cell (birds, reptiles)
• Holoblastic  the complete division of
eggs having a moderate amount or a little
yolk (sea urchins, frogs) 
7. Describe gastrulation and the process,
including the layers which result.
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Gastrulation  a dramatic process that
rearranges the blastula (a hollow ball of cells)
into a gastrula (a three-layered embryo)
Ectoderm
Endoderm
Mesoderm
Where the opening occurs determines if the
animal is a protostome or deuterostome 
Sea Urchin
gastrulation
Frog gastrulation
8. Define organogenesis, and describe what
changes occur in the developing animal.
• Organogenesis  the development of the organs
• Folds, splits, and condensation of cells are the
first steps to building organs
• The neural tube and notochord are the first
organs to develop in chordates
• Ectoderm  skin, inner ear, eye lens
• Mesoderm  notochord, coelom lining, muscles,
skeleton, gonads, kidneys, circulatory system
• Endoderm  digestive tract linings, liver,
pancreas, and lungs 
Chicken
organogenesis
9. Briefly overview avian development,
including key terminology.
• The yellow yolk is a large food reserve for the
embryo
• The egg white is protein-rich and provides additional
nutrients during development
• After fertilization, meroblastic cleavage occurs at
the animal pole, which creates the blastodisc
• Similar to the blastula, the blastomeres sort into
upper and lower layers, with a blastocoel in between
• Gastrulation occurs and the three-layered embryo is
formed
• The primary germ layers also form 4 extraembryonic
membranes  yolk sac, amnion, chorion, and allantois

10. Briefly overview mammalian development.
• Fertilization occurs in oviducts, early development
occurs as the zygote travels to the uterus
• Egg stores little nutrients and shows holoblastic
cleavage, which is slow
• At 7 days, the embryo consists of 100 cells
forming the blastocyst
• The inner cell mass protrudes into one end of the
cavity and will develop into the embryo and some
extraembryonic membranes
• The trophoblast is the outer epithelium
surrounding the cavity which will form the fetal
part of the placenta
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10 continued…
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The blastocyst reaches the uterus and begins to implant
(enzymes, projections)
Chorion  forms the trophoblast and surrounds embryo
and all extraembryonic membranes
Amnion  forms as a dome and encloses the embryo in a
fluid-filled cavity
Yolk sac  encloses a fluid-filled cavity but no yolk, its
membrane is the site of early blood cell formation
Allantois  develops from an outpocketing of the
rudimentary gut and is incorporated into the umbilical
cord (nutrients, gases, wastes) 
11. Describe what morphogenesis entails.
• Morphogenesis in animals involves specific changes
in cell shape, position, and adhesion
• Cell extension, contraction, and adhesion are all
involved in these movements
• Changes in shape involves the reorganization of
the cytoskeleton
• Cell adhesion molecules (CAMs) exist on the cell
surface and contribute to the selective association
of certain cells with each other 
12. Describe what effects cytoplasmic
determinants have on the fate of cells.
• The heterogeneous distribution of
cytoplasmic determinants leads to regional
differences in early embryo development
• Different substances (mRNA, proteins) are
partitioned differently among the cells
• The local differences in distribution of
these cytoplasmic determinants influences
gene expression and the developmental
fate of the cells 
13. Briefly describe how polarity is
determined.
• The eggs of most vertebrates have cytoplasmic
determinants that help establish the body axes
• The first step to development is determining the
anterior-posterior, dorsal-ventral, and right-left
axes
• Polarity is not determined until after cleavage
(mammals)
• Some species the polarity is determined much
earlier (frogs) with the locations of the vegetal
and animal hemispheres (determines anteriorposterior) 
14. Describe how inductive signals drive
differentiation and pattern formation in
vertebrates.
• Induction  the ability of one cell group to
influence the development of another
- usually the switching on of a set of genes
that makes the receiving cells
differentiate into a specific tissue
- one part of the early gastrula plays a role
in where the neural tube and notochord
develop 
15. Define pattern formation and how
positional information is relayed.
• Pattern formation  the development of an
animal’s spatial organization with organs and
tissues in their characteristic places in the three
dimensions of the animal
• Is controlled by positional information, which is a
set of molecular cues that indicate a cell’s location
relative to other cells in an embryonic structure
and that help to determine how the cell and its
descendants respond to future molecular signals
The End!