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Polyinvagination or ingression, Biology
N 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.
Posted Date: 10/4/2012 5:38:26 AM | Location : United States
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