(i) Prospective ectoderm area is present on and around the animal pole and it is pigmented black. In the blastula, the blastomeres which have to form different germinal layers and different organs of the adult frog can be observed by artificial-vital staining methods of Vogt (1925) and prospective organ region maps or fate maps have been prepared.Īccording to the fate map studies, the whole surface of blastula can be divided into the following three areas: Thus, the resulting embryo having fluid-filled blastocoel is called blastula. It is two-cell thick towards animal pole of the egg and forms the roof of blastocoel, while the sides and floor of the blastocoel is occupied by multilayered blastoderm of large yolky blastomeres. As cleavage proceeds, the blastomeres arrange themselves into a true epithelium called blastoderm. The blastocoel also enlarges due to uptake of more water. The blastocoel shifts more and more towards the animal pole due to more rapid multiplication of the micromeres, and infiltrated by water and albuminous fluid secreted by the surrounding cells. In the beginning it is like narrow crevices between blastomeres of morula, which gradually increases as the cleavage goes on. Cleavage and Blastulation :Ībout fourth and fifth cleavage stages a small space, the blastocoel appears between the blastomeres of morula. It coincides with the embryonic axis and is the only plane which separates the egg into two equivalent parts, each containing half the crescent material. The plane passing through the centre of grey crescent and the animal pole defines the median plane of bilateral symmetry. At this region cortex becomes thin and this area is crescent-shaped. On one side between the black and white areas is a gray crescent region which marks the future dorsal side. The upper half of the zygote or animal hemisphere is pigmented black and it contains the cytoplasm and a nucleus, the lower vegetal hemisphere is white and full of yolk. The fertilised egg or zygote is about 1.6 mm in diameter it rotates within the vitelline membrane so that the animal pole becomes dorsal. The fusion of both male and female pronuclei is called amphimixis. In the fertilisation process, vesicular sperm nucleus and vesicular female nucleus (or pronuclei) fuse together to form zygote nucleus. If fertilisation is delayed, the albumen layers around the ovum become too thick for the sperm to pass through them and the ovum also starts to show degeneration. The entire sperm penetrates the ovum anywhere around the animal hemisphere.
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