32-3: fertilization + development occurs in series of specific, sequential stages
TRANSCRIPT
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32-3: Fertilization + Development
Occurs in series of specific, sequential stages
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Fertilization + Early Development Fertilization
Fusion of two gametes – sperm + egg
Combines 2 haploid cells = 1 diploid cell Called a zygote
Once a zygote is formed, DNA is replicated and cell division starts
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Fertilization + Early Development Rapid cell division is known as cleavage
Cell increases the number of cells (2, 4, 8, etc.) Becomes a giant hollow ball of cells (blastula)
with an empty central cavity (blastocoel)
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Gastrulation The blastula starts to collapse inward – called
a blastopore
Gastrulation Process of changing blastula to gastrula
Creating a multilayered embryo Marked by the changes in cell shapes
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Let’s look at Gastrulation
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Gastrulation (cont.) As the infolding continues, a deep cup-shaped
cavity is formed This is called the archenteron
This cavity will give rise to the gut
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Gastrulation (cont.) 3 germ layers emerge
Ectoderm – outer layer of gastrula
EndodermEndoderm – inner layer
Mesoderm – middle layer; between ectoderm + endoderm (in triploblast)
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Organogenesis Each of the germ layers formed during
gastrulation develops into certain organs ENDODERMENDODERM
Lining of urinary system, reproductive system, + most of the digestive tract, pancreas, liver, lungs, + gills
ECTODERMECTODERM Outer layer of skin, hair, nails, + nervous system
MESODERMMESODERM Skeleton, muscles, inner layer of skin, circulatory
system, + lining of body cavities
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What do the 3 germ layers form?
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Patterns of Development Coelom
A body cavity completely lined by mesoderm Can be used to help classify organisms
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3 types of Body Cavities Acoelomates
Body cavity is absent Mesoderm connects endodermic gut to outer
covering
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3 types of Body Cavities Pseudocoelomates
Means “false” body cavity Mesoderm lines fluid-filled coelom + gut is
suspended in the fluid of the coelom
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3 types of Body Cavities Coelomates
Means a “true” body cavity
Mesoderm lines body cavity and surrounds + supports endodermic gut
Forms attachments for organs
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2 patterns of developmentProtostomeProtostome DeuterostomeDeuterostome
Blastopore becomes the mouth (“1st mouth”)
Blastopore becomes the anus (“2nd mouth”)
Spiral cleavage Radial cleavage
Determinate cleavage Indeterminate cleavage
Schizocoely – coelom up Enterocoely – coelom down
Annelids, Arthropods, Mollusks
Echinoderms, Chordates
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