What kind of car are you?
“In Nature, Nothing Is Superfluous,
Nothing Is Wasted”
-Leonardo DaVinci
Animal Body Plans
• Symmetry
• Number of Tissue
Layers
• Gastric Design
Asymmetry
Bilateral Symmetry
Radial Symmetry
• Generally found only
in aquatic animals
Number of Tissue Layers
• Acoelomatic = generally two tissue layers (i.e.
flatworms)
• Pseudocoelomatic = three tissue layers with fluid
filled body cavity (i.e. nematodes)
• Coelomatic = three or more tissue layers with
lined body cavity
Gastric Design
• PROTOSTOMES
• Mouth forms first
• Simplest animals (i.e.
Porifera, Nematoda)
up through Annelida
• May give rise to a
two-way feeding
system
• DEUTEROSTOMES
• Anus forms first
• Insects Through
Mammals
• Ensures a one-way
feeding system
How Do Organisms Develop From
A Zygote Into Their Final Forms?
Determinative Embryology
Human Fertilization
• Fusion of ovum and sperm
• Generally in the ampullary
region of the fallopian tube
• Sperm bind to corona
radiata, and penetrate the
zona pellucida by using
acrosomal enzymes
• Once sperm has
penetrated and delivered
DNA, egg hardens
outside, preventing further
sperm from fertilizing
(cortical reaction)
Zygote
• Fertilized ovum
• Formed by union of
two haploid gametes
(ovum & sperm)
Early Animal Development
• Cleavage = rapid
mitotic divisions
– Holoblastic
– Meroblastic
• Rate of cleavage
depends upon the
amount of yolk in the
zygote
(more = slower)
Why is cleavage necessary?
Reasons For Cell Division
• Provide raw materials
for other cells
• Improve their surface
area to volume ratios
• Create an open space
for cells to move into
MORULA
• Latin for “mulberry”
• Earliest stage of
embryonic
development
• Ball of blastomeres
• Post 16 cell
stage/Days 2-4
BLASTULA / BLASTOCYST
• The resulting ball of
cells produced by
meroblastic
cleavage is called a
BLASTULA or
BLASTOCYST
• DAY 5
• Center of blastula is
fluid-filled
BLASTOCOEL
Differences In Blastula Formation
• In radially
symmetrical animals,
the blastocoel is large
• In bilaterally
symmetrical animals,
there are two,
unequally sized poles
– Animal
– Vegetal
Morphogenesis
What is the most important organ in
the human body?
GASTRULATION
• Blastula undergoes
meroblastic cleavage
(unequal divisions) at
one pole
• As a result of these
unequal divisions, one
pole of the blastula
begins to invaginate
(‘cave in’)
• The opening to the
invagination is called
the archenteron and the
cavity is called the
GASTRULA
Organogenesis
• Some of the cells of the
original blastula are
now pushed inside,
lining the gastrula
• Other cells migrate into
the space previously
occupied by blastocoel
• As a result, three tissue
layers are produced
– ECTODERM
– ENDODERM
– MESODERM
DERMAL LAYERS
• ECTODERM (outside): brain, spinal cord,
nerves, nose, ears, eye, skin
• ENDODERM (inside): lining of digestive
tract, pancreas, liver, lungs
• MESODERM (middle): skeleton, muscle,
gonads, kidneys
DIFFERENTIATION
• Changes that result in
the formation of
specialized organs or
structures
• Example: A second
invagination of the
ectoderm creates the
neural tube (i.e. spinal
cord and column)