Amphioxus

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Blastulae of various types
Amphioxus
Amphioxus
Amphibian
Bird development
Shark/bird
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Amphioxus (Gastrulation)

• Infolding of presumptive endoderm at blastopore.
• Formation of archenteron
• Chordamesoderm inrolls at dorsal lip of
  blastopore.
• General mesoderm inrolls and ? lateral to
  chordamesoderm

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Frog Gastrulation
1.Yolk prevents simple infolding 2. Involution of
presumptive endoderm at dorsal lip 3. Presumptive
mesoderm inrolls ? chordamesoderm to lie
internal to neurectoderm ? lateral to lie beside
chordamesoderm
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Shark-Macrolecithal

• Presumptive endoderm
• inrolls

2. Chorda- and lateral mesoderm follow (chorda
to underlie neurectoderm)
3. Mesoderm separates from Endoderm gut cavity
formed
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Birds (Macrolecithal)
1. No presumptive endoderm on surface
Endoderm formed by delamination
blastopore
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Cross section of the primitive streak
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archenteron
Mammalian blastula
Primitive streak as in birds. Cells roll
inward to form mesoderm
Inner cell mass migrates to equator
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Neurulation in Amphioxus
Body ectoderm grows dorsally
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Amphioxus
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Neural tube
Chorda rolls up
Lateral mesoderm forms as outpocketing of
archenteron
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Endoderm grows dorsomedially
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Neural tube
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Amphioxus with neural tube formed
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Neurulation- Higher vertebrates
?mesoderm already positioned at end of
gastrulation
-? chordamesoderm positioned -? lateral mesoderm
proliferates
Induction of spinal cord
Chordamesoderm
Body ectoderm grows dorsally
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Becomes mesectoderm (visceral skeleton, etc.)
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Cyclostome or Teleost
Amphibian
Yolk within endocermal cells yolk sac part of
gut
Acellular yolk mass
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Bird- extraembryonic membranes
Birds and Reptiles
Mammals (primates)
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Formation of Extra-embryonic Membranes
1.Embryo sinks into collapsing yolk 2. Allantois
forms off rear of gut 3. Mesoderm splits and
folds to form the amnion and chorion 4. Complete
enclosure of embryo by amnion
Split of mesoderm
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Pig (Chorioallantoic Placenta)
1. Yolk sac remains rudimentary 2. Allantois
furnishes chorionic circulation 3. Broad fusion
chorioallantoic membrane 4. Chorioallantoic
membrane enlarges, develops villi
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Mature extraembryonic membranes of a fetal pig
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Marsupial (Chorio-vitelline Placenta)
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