Human Development

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Human Development
Ch 28b
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Gastrulation rearranges the blastula to form a
three-layered embryo with a primitive gut

• Gastrulation rearranges the embryo into a
  triploblastic gastrula.

Primary Germ Layers Ectoderm Endoderm Mesoderm
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Fates of the Primary Germ Layers

• Ectoderm
• hair, nails, epidermis, brain, nerves
• Mesoderm
• notochord (in chordates), dermis, blood vessels,
  heart, bones, cartilage, muscle
• Endoderm
• internal lining of the gut and respiratory
  pathways, liver, pancreas

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Body Cavities
coelom
Mesoderm
Eucoelomate- body cavity completely lined with
mesoderm
gut
Ectoderm
Endoderm
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The Formation of Primary Germ Layers
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Gastrulation
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Extraembryonic Membranes

• Amnion epiblast cells form a transparent sac
  filled with amniotic fluid
• Provides a buoyant environment that protects the
  embryo
• Helps maintain a constant homeostatic temperature
• Allows freedom of movement and prevents parts
  from fusing together
• Amniotic fluid comes from maternal blood, and
  later, fetal urine

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Extraembryonic Membranes

• Yolk sac a sac that hangs from the ventral
  surface of the embryo
• Forms part of the digestive tube
• Source of the earliest blood cells and blood
  vessels

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Extraembryonic Membranes

• Allantois a small outpocketing at the caudal end
  of the yolk sac
• Structural base for the umbilical cord
• Becomes part of the urinary bladder
• Chorion helps form the placenta
• Encloses the embryonic body and all other
  membranes

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Neurulation
A Embryonic disc accomplished gastrulation -
ectoderm thickens B Neural plate forms neural
folds and neural groove
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Neurulation
C Neural folds close D Neural tube detached
from surface ectoderm
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Embryonic Folding