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*The endoderm and mesoderm develop by proliferation and migration of cells of the epiblast.  
 
*The endoderm and mesoderm develop by proliferation and migration of cells of the epiblast.  
 
**What remains of the epiblast forms the ectoderm.
 
**What remains of the epiblast forms the ectoderm.
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*The epiblast is a circular plate of cells. In gastrulation, cells ingress ventrally from the epiblast to form the three germ layers.
 
*Gastrulation occurs at three embryonic sites:
 
*Gastrulation occurs at three embryonic sites:
 
::'''Primitive Streak'''
 
::'''Primitive Streak'''
 
::'''Node'''
 
::'''Node'''
 
::'''Tail Bud'''
 
::'''Tail Bud'''
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==Gastrulation at the Primitive Streak==
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*The epiblast is a simple circular plate of cells.
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*Cells proliferate and through migration, converge at one pole of the epiblast. These cells become columnar to compensate for the decreased amount of space available.
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**This defines the posterior of the embryo.
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*Eventually, convergence causes cells to be displaced anteriorly. As more cells converge, the primitive streak elongates. 
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*Cells in the primitive groove change shape from columnar to wedge shaped, which produces two elevations either side of a groove.
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*Epiblastic cells of the primitive streak begin to ingress ventrally to form the new germ layers.
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