Cleavage of Frog and Chick Eggs
Chapter VI
 Total Development = cleavage,
morphogenesis, differentiation, growth
 Cleavage-immediately after fertilization
 Two kinds of divisions to convert egg to
mass of cells
– mitosis and cytokinesis
 First cleavage in frog egg results in two
equal size cells
 2nd cleavage division in frog is also
complete and equal
 3rd division in frog is complete and
unequal. Cells are different sizes
 After many divisions the animal pole has
more and smaller cells
 Egg yolk has effect on cleavage divisions.
Slows them down
 Blastocoel-internal cavity forms near the
animal pole after many cleavage divisions
 Blastula--The developmental stage when
the blastocoel forms
 Bird eggs--99% of egg is yolk, yolk
segregated from the cytoplasm
 Most of the cytoplasm is in the cytoplasmic
cap on top of the yolk
 Cleavage happens in the cytoplasmic cap
 Don’t have cleavage in entire egg--
Incomplete
 The cytoplasmic cap is called the
blastoderm
 Under the blastoderm is a space called the
subgerminal space
 Blastoderm is not a blastula because there is
no cavity
 Cleavage in chicken egg produces a
blastoderm located at the animal pole
 Incomplete cleavage occurs
Morphogenesis in Frog and
Chick Development
Chapter VII
Morphogenesis
 Animals acquire form and structure during
this developmental stage
 Movement of cells to specific locations is
characteristic of morphogenesis
 Organism takes on a specific shape
 Gastrulation--happens in morphogenesis
and results in three germinal layers.
 Also results in the formation of gastrula
 Three layers in gastrula are ectoderm,
mesoderm and endoderm
Frog Gastrulation
 Begins with the blastula
 First part of gastrulation is the formation of
a blastopore on surface of blastula
 The cells begin to fold inward
 Further folding of the blastopore results in a
cavity called the archenteron. The future gut
 Blastopore becomes anus
 The blastocoel is being displaced
 Continued morphogenic movements result
in enlarging the archenteron, reducing the
blastocoel and forming a yolk plug over the
blastopore
 In mature frog gastrula there are three germ
layers
 Ectoderm which are the outer cells,
Endoderm which line the archenteron, and
mesoderm which is in between endo and
ecto
 Mature gastrula is foundation for more
development to occur
 Doesn’t stop morphogenesis
 Others follow to form cell masses called
primary organ rudiments
 Neurula-stage where the neural plate
develops and forms the neural tube which
will develop into the future nerve cord
Chick Gastrulation
 Begins with the blastoderm
 Forms a new cavity below the blastoderm
 When cavity is formed its called the
blastocoel.
 The subgerminal space is now called the
archenteron or future gut
 Stage is not yet a gastrula because no germ
layers
 Primitive groove forms on surface of
blastoderm due to cells moving into the
blastocoel
 Related to the blastopore in frog
development
 After cells move into the blastocoel there
are three distinct germ layers and can now
be called the gastrula
 Continuation of morphogenesis results in
the neural plate and development of other
primary organ rudiments
 In both the frog and chick the mature
gastrula provides the structural foundation
for forming more organs
Differentiation in Animal
Development
Chapter VIII
 Differentiation--where cells take on specific
structures and functions
 Experiment--Future eye region was
transplanted into different areas and a salt
water solution.
– The regions differentiated into different
structures when transplanted
– Salt water-did not differentiate
 Potency-the ability of an embryonic part to
develop into different kinds of structures
 Eye region of frog has many potencies
 Future eye region of frog was
undifferentiated
 Differentiation of eye depends on its
location based on experiment
 Eye regions in neurula were placed at
different locations in a later embryo
 All developed into an eye
 Eye region of neurula has only one potency
 In neurula stage the future eye region has
become differentiated and can only form an
eye
 The primary organ rudiment in neurula is
more determined than in the gastrula
 As the embryo ages the number of potencies
possible for an area becomes less
 Morphogenesis continues along with the
processes of differentiation and growth
Factors Controlling Animal
Development
Chapter IX
Nuclear Factors
 Experiments are presented that demonstrate
possible control mechanisms of the events
of animal development
 Experiment with needle into frog eggs.
– With nucleus---no effect on embryo
– Without nucleus--abnormal or no embryo
 Results indicate that nucleus controls
embryo development
 All cells result from mitosis and cytokinesis
Cytoplasmic Factors
 All cells should have the same set of genes
 They will differentiate into different
structures
 How can they develop differently if they
have the same set of instructions?
 Look at sea urchin development
 Experiment-pigment band develops into the
endoderm of gastrula
 Egg cut in half through the animal and
vegetal. Results animal forms blastula with
ectoderm only and dies-Vegetal forms
incomplete embryo with all 3 germs layers
and dies
 Results--endoderm and mesoderm
formation are contained in the vegetal
 Exp. 2--Sea urchin cut in half through both
animal and vegetal axis-length wise
– Forms two small complete embryos
 Results of both experiments--Germ
formation for three germ layers has
instructions in the cytoplasm
 Frog egg experiments-p120
– Hair used to constrict fertilized egg into two
halves
– Halves connected by bridge of cytoplasm
– Gray crescent area of cytoplasm equally
divided
– Half with nucleus divides first. Both halves
form normal embryos
 2nd frog experiment. Hair again only constricts
with part of grey crescent on one half with nucleus
 The side with only part of the grey crescent didn’t
develop properly while the half with the gray
crescent did
 These experiments indicate that the control of
nucleus is regulated by the cytoplasmic
environment
Extracellular Factors
 Experiments so far show what happens with
unequal distribution of materials within the
egg
 Are there other factors at play in the
development of the embryo?
 In the experiment given on p123 tissue was
transplanted into the blastocoel of a gastrula
stage embryo
 From this transplanted tissue a 2nd head formed
where belly tissue should have formed
 This means that transplanted tissue affects the
normal development of tissue
 Exp. On p125 ectoderm placed in culture = no
differentiation on tissues
– Ectoderm and mesoderm in culture = differentiation of
ectoderm but not mesoderm
– Why? Mesoderm stimulated differentiation in ectoderm
 Induction—name given to process where
one tissue induces the differentiation of
another
 This induction can explain how the
differentiation of different cells occur at
different times.
– As new tissue is formed it induces other tissues
to form
 Other experiments indicate that induction is not
the only influence on development
 When a frog gastrula was placed in culture with
pieces of adult heart-no normal heart produced in
embryo
 Differentiated adult heart produce certain
chemicals into culture that inhibited the
development of heart tissue in embryo
 Inhibition—differentiated cells produce substances
that inhibit cells from developing in the same way.
Don’t get two hearts this way
 Interactions occurring between groups of tissues
also help development of certain structures
– Exp. Limb bud of chick forms when ectoderm and
mesoderm work together.
– When they are grafted separately they don’t form the
limb properly
 Also has to be limb bud ectoderm and mesoderm.
Can’t be any ectoderm and mesoderm
 Also wing and leg bud experiments performed
 The mesoderm taken from wing or leg will
develop into wing or leg if combined with
ectoderm from the other limb
 Mesoderm controls development of limb
but may be affected by kind of ectoderm it
is in contact with
 Differential gene Action—not all genes are
turned on at the same time in the cells
nucleus
 Cells differentiate at different rates and in
different directions depending on which
genes are turned on when
The Operon Theory of Gene Control
Chapter X
 How are the genes controlled so that some
are on and some are off?
 Operon Theory—developed with research
on bacteria
 Enzyme-controls rate of chemical reaction
 Structural genes-control enzymes
 Operon is operator gene and structural
genes
 Operator gene controls the structural genes
 Regulatory gene also present to make repressor
which deactivates the operator gene and turns of
enzyme function of structural genes
 When enzymes are produced, if high enough
concentrations are reached some of them combine
with the repressor to trigger the shut off of the
operator gene
 When substrate is high enough it
deactivates the repressor and allows the
operator gene to turn on the structural genes
to function and make enzymes
 Regulatory gene has indirect effect on the
operator gene