They are also called as germinal layers because entire organs and body are derived from these layer. The three layers are ectoderm, mesoderm and endoderm are known as primary germ layer.ii) roof of archenteron is chordamesoderm.But formation of archenteron causes the embryo to rotate within the vitelline membrane so that blastopore comes near the vegetal pole. In the blastula stage, embryo floats with animal pole upward. Rotation of gastrula: gastrulation causes shift in the center of gravity of the embryo.This movement of the presumptive areas is known as involution. due to increase in size of archenteron as well as formation of yolk plug, there is rapid migration of presumptive areas within the embryo occurs.The blastopore is guided by anterior margin called dorsal lip and backward projecting lateral lip. This groove is the beginning of archenteron and its anterior opening is called blastopore.As invagination progresses archenteron increases in size and the blastocoel become reduced and finally obliterated.Contraction of lips of blastopore: contraction of lips from all side occurs so that blastopore become smaller and narrower.The narrowing of blastopore exerts pressure on underlying yolk laden megameres, result in protruding of some megameres cells as yolk plug.This stage is also known as Yolk plug stage.The invagination gradually grows inward causing migration of cells. In this step, small groove appears due to invagination of megameres near grey crescent region.This overgrowth or spreading of micromere cells is known as Epibloy. In this step, micromeres at animal pole dives more repeatedly and rapidly enclosing the megameres except in the region of yolk plug.Gastrulation completes in following steps.Gastrulation involves some critical changes in the blastula such as- differentiation of cells, transformation from monoblastic to diploblastic layer, formation of three primary germ layers.The process of formation of gastrula is called gastrulation. Gastrula is the two layered embryo stage formed by migration and rearrangement of cells of blastula.The remaining vegetal region is presumptive endoderm.Close to presumptive norocord there is a grey crescent region which is the presumptive mesoderm.
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