CHAPTER 8
Principles of
Development
8-1
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Organizing cells during development
8-2
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Original thought:
Sperm contained a
living organism
8-3
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Development

Development
Begins when a fertilized egg divides
mitotically
 Specialization/Division of cells occurs
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8-4
Cells become specific cell types (ectoderm,
endoderm, mesoderm)
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8-5
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Fertilization

Contact and Recognition Between Egg
and Sperm
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8-6
Marine organisms
 release enormous numbers of sperm in
the ocean to fertilize eggs
 Many eggs release a chemical molecule
 Attract sperm of the same species
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Fertilization

Sea urchin sperm
 Penetrate a jelly layer surrounding egg
 Next, contacts the vitelline envelope
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Egg-recognition proteins bind to species-specific
sperm receptors on vitelline envelope
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Ensures an egg recognizes only sperm of the same species
In the marine environment
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8-7
Thin membrane above the egg plasma membrane
Many species may be spawning at the same time
Similar recognition proteins are found on sperm of
vertebrate species
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Fertilization

Prevention of Polyspermy (entry of more than one
sperm)
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Sperm head drawn in past vitelline membrane and
fuses with egg plasma membrane
Important changes in the egg surface block
entrance to any additional sperm
In the sea urchin, an electrical potential rapidly
spreads across the membrane
Other animals create an osmotic gradient from
enzyme reactions
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8-8
Water (osmosis) rushes into space
Elevates the envelope
Lifts away all bound sperm except the one sperm that
has successfully fused with the egg plasma
membrane
Known as a cortical reaction
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8-9
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8-10
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Binding Sperm to Sea Urchin Egg
8-11
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Sea Urchin
Time Frame
8-12
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Fertilization
After sperm and egg membranes fuse
 Sperm
loses its flagellum
 Enlarged sperm nucleus migrates inward to
contact the female nucleus - once they meet fertilized egg is now a ZYGOTE (diploid)
 Zygote now enters cleavage
8-13
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Cleavage and Early Development

Cleavage
 Embryo divides repeatedly
 No cell growth occurs, only subdivision
until cells reach regular somatic cell size
 At the end of cleavage
Zygote has been divided into many hundreds
or thousands of cells
 Blastula is formed

8-14
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Types of
Cleavage is
Determined
by Yolk
8-15
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Cleavage Types

Holoblastic
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Meroblastic
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Cleavage extends entire length of egg
Egg does not contain a lot of yolk, so cleavage
occurs throughout egg
Example: mammals, sea stars, worms
Cells divide sitting on top of yolk
Too much yolk and yolk can’t divide
Examples: birds, reptiles, fish
Both determined by amount of Yolk present
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Development of Sea Urchin
8-17
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An Overview of Development Following Cleavage

Blastulation - division of zygote to create a hollow
ball of cells
 Cluster of cells called the blastula
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few hundred to several thousand cells
Forms first germ layer (ectoderm)
Cavity called the blastocoel
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8-18
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An Overview of Development Following Cleavage

Gastrulation (Forms 2nd germ layer - endoderm)
 Involves an invagination of one side of
blastula

Forms a new internal cavity
 gastrocoel
 Opening into the cavity: Blastopore
(becomes opening into animal - mouth/anus)
 Gastrula
has an outer layer of ectoderm
and an inner layer of endoderm
8-19
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Generalized Development showing germ layers
Incomplete/
Blind Gut
Blastopore
(Opening)
8-20
Complete
Gut
Gastrocoel
(Cavity)
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An Overview of Development Following Cleavage
 The
only opening into embryonic gut is
the blastopore

Blind or incomplete gut
 Blind
gut - the opening does not fully
extend to other side (sea anemones)
 Complete gut - in which the opening
extends and produces a second
opening, the anus
8-21
Blind
Complete
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Developmental Characteristics
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Protostomes versus deuterostomes
Fate of Blastopore - opening to gut
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Deuterostome embryos
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Protostome embryos
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8-22
Blastopore becomes the anus
Second opening becomes the mouth
Blastopore becomes the mouth
Anus forms from a second opening
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Protostomes and Deuterostomes Blastopore
Deuterostome
Protostome
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Generalized Development showing germ layers
Incomplete/
Blind Gut
Complete
Gut
Blue = Ectoderm
Yellow = Endoderm
Red = Mesoderm
8-24
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An Overview of Development Following Cleavage

Formation of Mesoderm
 Animals with two germ layers
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Most animals add a 3rd germ layer
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Diploblastic (Endoderm and Ectoderm)
Triploblastic (Endoderm, Ectoderm,
Mesoderm)
Mesoderm
3rd germ layer
 Forms between the endoderm and the
ectoderm
 Mesoderm arises from endoderm

8-25
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Developmental Characteristics
Germ Layer Outcomes:
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Ectoderm
 Epithelium and nervous system
Endoderm
 Lining of the digestive and respiratory tract,
liver, pancreas,
Mesoderm
 Muscular system, reproductive system, bone,
kidneys, blood
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Germ Layer Outcome in mammals
8-28
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An Overview of Development Following Cleavage

Formation of the Coelom
 Coelom
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Upon completion of coelom formation
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Body cavity surrounded by mesoderm
Body has 3 tissue layers and 2 cavities (coelom
and blastocoel)
Animals Without a Coelom are called
Acoelomates (Ex. flatworms)
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8-29
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Coelom Types

Types of organisms based on Coelom
Acoelomate - has mesoderm, but not
cavity or coelom
 Pseudocoelomate - has mesoderm, but
coelom is NOT completely lined with
mesoderm. (Pseudo = False)
 Coelomates - internal cavity completely
lined by mesoderm

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Blastula and Gastrula
Of Embryos
8-32
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8-33
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Vertebrate Development

The Amniotic Egg
Reptiles, birds, and mammals
 Embryos develop within the amnion
 Fluid-filled sac that encloses the embryo
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Provides an aqueous environment to protect
from mechanical shock
 Amniotic
egg contains 4 extraembryonic
membranes including the amnion
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8-34
Yolk, Chorion, Allantois, Amnion
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Vertebrate Development

In the shelled amniotic egg:
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8-35
Yolk sac
 Stores yolk - nutrients
Allantois
 Storage of metabolic wastes during
development
 Respiratory surface for gas exchange
 Helps produce umbilical cord in mammals
Chorion
 Fuses with allantois to aid in increased
respiratory needs
 In mammals will develop into placenta
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Chick Embryo
8-36
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A. Fish Larvae - 1 day old, has large yolk sac
B. 10 day old fish larva, developed mouth, yolk sac smaller
8-37
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Extraembryonic membranes of a mammal
8-38
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Early Development of the human embryo
8-39