The process of formation of gastrula is called gastrulation. In the beginning, the gastrula is a two layered stage with a cavity called archenteron. This archenteron opens to the exterior by means of an opening called blastopore.
As a result of gastrulation, three primary layers are formed. These are the ectoderm, the endoderm and the mesoderm. First to develop are the ectoderm and the endoderm and eventually the mesoderm is also formed. These three layers are called the primary germ layers from which arise all the organs of the frog.
Gastrulation is a very significant and important stage in the embryogeny of frog. Because it is here that all the future organ forming areas are differentiated.
The early gastrula consists of the formation of the archenteron. During the process of gastrulation there are mass movement of sub cells which are necessary for rearrangement of the presumptive organ forming areas of the blastula. These movements are called formative movements. The process of gastrulation takes place at the following stages.
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Epiboly:
In the late blastula, the anterior half consists of micromeres which constitute the ectoderm while the posterior megameres constitute the endoderm. The germ ring forms the mesoderm. During epiboly the ectoderm overgrows backwards on the endoderm; ultimately the entire embryo (except for the small area called the yolk plug) is covered by the ectoderm. In other words the pigmented micromeres (animal half) grow over the megameres (vegetative half). The reason for overgrowth is the rapid rate of division of micromeres.
Invagination:
A small depression is formed in the region occupied by the grey crescent area. This depression grows inwards and forms the archenteron or gastrocoel or secondary body cavity. The outer opening of the gastrocoel is called the gastropole. As the gastrocoel increases in size the blastocoel gets reduced. Ultimately only a slit like semicircular cavity indicates the remnants of the blastocoel. The blastopore meanwhile becomes expanded and becomes ring shaped.
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Involution:
During this process the cells which have grown backward during emboli now roll inside at the margin of the blastopore. The endoderm is the first to roll inside. The cells of the notochord and mesoderm which were formed outside now migrate over the lip of blastopore and become internal and arrange themselves on the roof, sides and the floor of the archenteron. The notochord cells are found on the roof along the midline. While the endoderm forms the anterior, lateral and ventral walls, the mesoderm forms wing like extensions in the archenter.
Closure of the blastopore:
The archenter at its beginning is marked by a groove. The anterior margin of this groove is called dorsal lip of the blastopore. This groove also extends laterally to form the lateral and ventral lips. At this stage, the blastopore is completely ring like, surrounding the endoderm. The circular area of the endoderm is represented by a mass of cells called the yolk plug. The margins of the lips of the blastopore contract making the opening very small. At this stage the yolk plug has an oval outline and eventually becomes a small speck and is completely covered over by the ectoderm cells.
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Rotation:
At the end of gastrulation the yolk filled cells get rearranged. This rearrangement shifts the centre of gravity of the gastrula and results in a rotation of eighty to ninty degrees. The blastopore moves backwards and is finally located slightly above its original location at the beginning of the gastrula.
Convergence:
During this process there are two kinds of cell movements. First, there is shifting of the cells of the notochord and mesoderm towards the dorsal lips. Secondly, there is a dorsal convergence of notochord and mesodermal cells towards the middorsal area of the blastopore.