Polyinvagination or ingression, Biology


  • This is found in discoblastula and periblastula.
  • In it the blastomeres of the blastoderm divide and form new blastomeres which fall down and form a floor at the base of sub-germinal cavity. This is termed as delamination.
  • The layer which is formed at the base of sub-germinal cavity is termed as hypoblast.
  • Hypoblast directly forms the endoderm.
  • Blastomeres formed at the roof of sub germinal cavity are termed as epiblast.
  • Delamination gives rise only to endoderm. Remaining layers develop through polyinvagination.
  • The blastomeres which fall inside are termed as chorda-mesoderm.
  • After polyinvagination, epiblast gives rise to the ectoderm.


  • In discoblastula or periblastula the archenteron cavity is absent.
  • In eutherian mammals, the embryonal-knob broadens to forms a flat plate.
  • Now it gets arranged in a loose group.
  • The lower most layer of this plate forms the endoderm.
  • Remaining blastomeres divide laterally.
  • These blastomeres making space amongst the other blastomeres involute inside, and these form the mesoderm.
  • This type of formation of mesoderm is termed as ingression, the upper blastomeres of the plate form ectoderm.


Posted Date: 10/4/2012 5:38:26 AM | Location : United States

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