POLYINVAGINATION OR INGRESSION -

• 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.