The process of neurulation is the formation of a neural tube. Gradually the gastrula undergoes the process of tubulation or neurulation to become a neurula. The process of gastrulation converts the blastula into a spherical, bi-laterally symmetrical, triploblastic gastrula. Gradually the yolk-plug withdraws to the interior and the blastopore gets reduced into a narrow slit. The closing cells of the blastopore constitute the yolk-plug. The blastopore is gradually covered by certain endoderm cells. Epiboly causes overlapping or ' the roofing over' of the gastrula by the ectoderm. The expansion of the ectoderm is due to epiboly. While the exterior chorda-mesodermal cell invo-lute inside, their place is taken up by the ectoderm. The mesodermal cells occupy the region between inner endodermal and outer ectodermal cells. The involution results in the positioning of chor-damesodermal cells and pharyngeal endodermal cells. The inward movement of the exterior cells through the blastoporal region is called involution. The mesodermal and endodermal cells gradually occupy their positions. The interior region of the archenteron gradually trans-forms into the pharyngeal region. The gastrocoel later be-comes the archenteron. A new cavity among the involuted cells results. Thus the blastocoelic cavity gets reduced. The inwandering cells gradually occupy the region of the blas-tocoele. Initially, the first pharyngeal endodermal cells undergo invagination over the dorsal blastoporal lip. These inwandering of cells is termed as involution. The surface cells representing several prospective zones of the em-bryo begin to wander inside through the blastopore. The lower edge may be called the ' ventral lip'. The region dorsal to the blastoporal opening is called the ' dor-sal lip'. The blastopore gradually assumes a crescentic shape. A deepening of the invagination results in a cavity called the archenteron or gastrocoel.The opening of the archenteron on the surface of the blastula is called the blastopore. As the cells move further inside, an invagination happens. They move toward the interior of the blastula. During this movement at one region on the blastula, the cells wander inside and occupy the blastocoelic cavity.Īt a specific region below the equator the blastoderm cells assume an elongated bottle like shape. They wander and occupy their prospective organ forming zones. During this process the blastodermal cells begin to move. The process of gastrulation is a continuous activity succeeding, cleav-age.
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