Gastrulation refers to a series of changes that takes place during embryonic development converting the single layered blastula into a two-layered embryo called gastrula.

During gastrulation mass migration of formative movements and rearrangement of call takes place with formation of presumptive areas of blastula. Several changes or processes taking place simultaneously during gastrulation.

Gastrulation in Chick:

Mode of cleavage meroblastic, complete and unequal.

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Blastula:

It is pseudoblastula, true blastocoels is not formed and is called sub-germinal cavity.

Presumptive areas: presumptive area confined to area pellucid of blastoderm. Small disc of endoderm lateral mesoderm, prechordal mesoderm, neural plate, embryonic ectoderm are presumptive.

Gastrulation:

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Gastrulation takes place much earlier before the egg is laid

Epibolic out growth not produced

No such blast pore is formed

Yolk plug not formed.

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There is no invagination of future of future endoderm through blast pore takes place.

Near the posterior end of embryo, prospective endodermal cells migrate down (called involution) into the subgerminal cavity.

Ingression of individual yolk laden ectodermic cells takes place into the sub germinal cavity takes place adding cells to thickness of blastoderm.

But the blastoderm splits apart forming outer epiblast (ectoderm) and inner endoderm (hypoblast). No rolling of mesoderm cells happen.

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But here in case of chick embryonic endoderm formed by migration of cells from area pellucid into sub germinal cavity.

The original sub germinal cavity is divided by hypoblast into a narrow outer space or secondary blastocoels and inner archenteron or primitive gut.

The complete gastrula of chick shows:

The embryo becomes diploblastic outer ectoderm and inner endoderm. Respectively called epiblast and hyaloblast.

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The original subgerminal cavity divided into secondary blastocoels and archenteron.

Thickening of blastoderm into mound if tissue is found to indicate starting of primitive steak.

Accumulations of lateral plate mesoderm due to movement of cells initiate primitive steak formation.

After gastrulation formation of three germinal layers takes place in both frog and chick.