Development
Introduction
Early Stages of Development
Quiz part 1
Suites of Developmental
Characters
Quiz part 2
In this tutorial, you will learn:
 The
basics of early development from
fertilization through gastrulation.
 Major differences in early development
between taxonomic groups.
Credits:
Figures and images by N. Wheat unless otherwise noted.
Starfish embryo photos courtesy of K. Wynne.
Sea urchin gastrulation, and frog cleavage & gastrulation videos courtesy of TheDeepSci’s YouTube channel.
Chick gastrulation video courtesy of elishanathan’s YouTube channel.
Blastocyst image from Wikipedia.
Funded by Title V-STEM grant P031S090007.
Introduction
 Development
describes the
changes in an organism from its
earliest beginnings through
maturity.
Fertilization
 Fertilization
is the initial event in
development in sexual reproduction.



Union of male and female gametes.
Recombination of paternal and maternal
genes.
Restoration of the diploid number (two sets
of chromosomes).
Zygote
 The
diploid cell
resulting from
fertilization is now
called a zygote.
Photo courtesy of K. Wynne

Photos in the following
slides illustrate
development in the
starfish (Phylum
Echinodermata).
Cleavage
 Cleavage
– rapid cell divisions following
fertilization.


Very little growth occurs while the cells are
dividing.
Each cell called a blastomere.
2 cell stage
4 cell stage
Photos courtesy of K. Wynne
Cleavage
 This
video shows cleavage in a frog
embryo:
Can’t view the video? Link To YouTube:
http://www.youtube.com/user/TheDeepSci#p/u/5/UPJJwVNzaHw
Morula
 Morula
– the name given to the solid
ball of cells that results from cleavage.
 First
5-7 divisions.
Photo courtesy of K. Wynne
Blastula
 As
divisions continue, a fluid filled cavity,
the blastocoel, forms within the embryo.
 The resulting hollow ball of cells is now
called a blastula.
early blastula
late blastula
Photos courtesy of K. Wynne
Gastrulation
 The
morphogenetic process called
gastrulation rearranges the cells of a
blastula into a three-layered (triploblastic)
embryo, called a gastrula, that has a
primitive gut (archenteron).
early gastrula
late gastrula
Photos courtesy of K. Wynne
The Blastopore
 The
blastopore is the first opening in the
embryo – the point of invagination during
gastrulation.

The blastopore will eventually become
either the mouth or the anus.
 One
end of the gut-tube or the other.
 The
space that forms during this time is the
primitive gut, the archenteron.
Blastopore
Archenteron
Photo courtesy of K. Wynne
Gastrulation
 The
three tissue layers produced by
gastrulation are called embryonic germ
layers.

The ectoderm forms the outer layer of the
gastrula.
 Outer


surfaces, neural tissue
The endoderm lines the embryonic
digestive tract.
The mesoderm partly fills the space
between the endoderm and ectoderm.
 Muscles,
reproductive system
Gastrulation – Sea Urchin
 Gastrulation
in a sea urchin produces an
embryo with a primitive gut (archenteron)
and three germ layers.
Can’t view the video? Link To YouTube:
http://www.youtube.com/user/TheDeepSci#p/u/3/Lgb4wMsZwZA
Gastrulation - Frog
 Result
is again an embryo with gut & 3 germ
layers.
 Gastrulation in the frog is more complicated
due to the moderate amount of yolk
present in the egg.

Blastula wall more than one cell thick.
Can’t view the video? Link To YouTube:
http://www.youtube.com/user/TheDeepSci#p/u/0/WSouNeWnZ_0
Gastrulation - Chick
 Gastrulation
in the
chick is affected by
the large amounts of
yolk in the egg.

Embryo essentially sits
on top of large mass
of yolk.
Can’t view the video? Link To YouTube:
http://www.youtube.com/watch?v=u52pTXtg75s
 Primitive
streak – a groove on the surface
along the future anterior-posterior axis.

Functionally equivalent to blastopore lip in frog.
Gastrulation - Mammal
 In
mammals the blastula is called a
blastocyst.

Inner cell mass will become the embryo while
trophoblast becomes part of the placenta.
Inner cell mass
Trophoblast
Gastrulation - Mammal
 Gastrulation
in mammals involves the
inner cell mass and is similar to that of the
chick due to the fact that mammalian
ancestors and early mammals laid eggs.

The large mass of yolk may be gone, but
the developmental pattern remains.
Question 1
Shortly after fertilization, the zygote
undergoes a series of rapid cell
divisions. This is called
 Meiosis
 Gastrulation
 Organogenesis
 Cleavage
Question 1
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 That
is incorrect.
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Question 1
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 You
are correct!
Question 2
During cleavage, each cell in
the embryo is called a
 Blastula
 Morula
 Gastrula
 Blastomere
Question 2
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is incorrect.
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Question 2
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are correct!
Question 3
The solid ball of cells that
results from cleavage is called
 Blastula
 Morula
 Gastrula
 Blastomere
Question 3
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is incorrect.
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Question 3
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are correct!
Question 4
A fluid filled space called the
blastocoel forms inside the embryo at this point it is a
 Blastula
 Morula
 Gastrula
 Blastomere
Question 4
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Question 4
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are correct!
Question 5
The stage that results in three germ
layers and a primitive gut is the
 Blastula
 Morula
 Gastrula
 Blastomere
Question 5
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Question 5
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are correct!
Question 6
Which of the following is not one of
the three primary germ layers
formed during gastrulation?
 Endoderm
 Myoderm
 Mesoderm
 Ectoderm
Question 6
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Question 6
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are correct!
Question 7
The difference in gastrulation found
in different types of animals is due
primarily to
 The
amount of yolk present
 The size of the blastomeres
 Development in different
species is completely
unrelated
 None of the above
Question 7
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Question 7
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are correct!
Suites of Developmental
Characters
 Two
major groups of triploblastic
animals:
 Protostomes
include flatworms, annelids
and molluscs.
 Deuterostomes include echinoderms
and chordates.
Protostomes & Deuterostomes
 Protostomes
& deuterostomes are
differentiated by:




Spiral vs. radial cleavage
Mosaic vs. regulative cleavage
Blastopore becomes mouth vs. anus
Schizocoelous vs. enterocoelous coelom
formation.
Spiral vs. Radial Cleavage

Spiral cleavage –
occurs in most
protostomes.
 Some
ecdysozoans
show radial or
superficial (insects)
cleavage.

Radial cleavage – is
found in most
deuterostomes.
 Tunicates
and
mammals have
specialized cleavage
patterns.
Mosaic vs. Regulative
Development
 Mosaic
development – cell
fate is determined
by the components
of the cytoplasm
found in each
blastomere.


An isolated
blastomere can’t
develop.
Protostomes
 Regulative
development – the
fate of a cell depends
on its interactions with
neighbors, not what
piece of cytoplasm it
has.


A blastomere isolated
early in cleavage is
able to from a whole
individual (e.g. twins).
Deuterostomes
Fate of the Blastopore
 Protostome
means
“first mouth”.
 Blastopore
becomes the
mouth.

The second
opening will
become the anus.
 Deuterostome
means “second
mouth”.
 The blastopore
becomes the anus
and the mouth
develops as the
second opening.
Blastopore
Photo courtesy of K. Wynne
Coelom Formation
 The
coelom is a body cavity found in
many triploblastic organisms that is
completely surrounded by mesoderm.
 Not all protostomes have a true coelom.


Pseudocoelomates have a body cavity
between mesoderm and endoderm.
Acoelomates have no body cavity at all
other than the gut.
Coelom Formation
 In
protostomes that
have a coelom, a
mesodermal band
of tissue forms
before the coelom is
formed.
 In the process of
coelom formation
called schizocoely,
this mesoderm splits
to form a coelom.
 In
enterocoely, the
coelom forms as
outpocketing of the
gut.
 Typical
deuterostomes have
coeloms that develop
by enterocoely.

Vertebrates use a
modified version of
schizocoely.
Question 8
Radial cleavage is found
primarily in which group?
 Protostomes
 Deuterostomes
 Both
groups show radial
cleavage
 Neither group shows radial
cleavage
Question 8
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Question 8
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are correct!
Question 9
This figure shows
 Radial
cleavage
 Spiral cleavage
 A four-cell stage embryo
 A blastula
Question 9
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is incorrect.
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Question 9
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are correct!
Question 10
Early in cleavage, a blastomere becomes
isolated. It grows into an entire separate
individual. This is an example of __________
development found in ____________.
 Mosaic,
protostomes
 Mosaic, deuterostomes
 Regulative, protostomes
 Regulative, deuterostomes
Question 10
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Question 10
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Question 11
The blastopore will become
the mouth in
 Protostomes
 Deuterostomes
 All
animals
 Neither – it forms something else
Question 11
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is incorrect.
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Question 11
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are correct!
Question 12
“Coel” is a word-part that we see frequently. The
blastocoel is the space inside the blastula, the coelom
is a mesoderm-lined body cavity. What will you find
associated with “coel” words?
 An
organ like the heart
 A proliferation of cells
 A cavity or space
 It indicates a developmental
stage
Question 12
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Question 12
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are correct!
Question 13
Most protostomes show this type of
coelom formation where a band of
mesoderm splits to form the coelom.
 Enterocoely
A
modified version of
enterocoely
 Schizocoely
 A true coelom is never found
in protostomes
Question 13
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is incorrect.
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Question 13
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 You
are correct!