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. At this stage the whole embryo acquires a characteristic appearance reminiscent of a mulbery and so it is called morula.Ībout fourth and fifth cleavage stages a small space, the blastocoel appears between the blastomeres of morula.
WHEN DOES THE BLASTOPORE BECOME EVIDENT FREE
The blastomeres’ mutual pressure flattens their surfaces in contact with each other but free surfaces of each blastomere remain spherical. The cells formed by cleavage are blastomeres, the upper black blastomeres are called micromeres, and lower white ones are macromeres.įurther cleavages divide the micromeres more rapidly than the lower macromeres whose division is hindered by yolk. The third cleavage is horizontal and above the equator which segments the zygote into upper four smaller, black-coloured cells, and lower four larger, white-coloured cells. A second vertical furrow at right angles to the first divides the zygote into four cells. A vertical furrow from the animal to the vegetal pole divides the zygote completely into two equal-sized cells. Cleavage and Blastulation :Ĭleavage or segmentation is holoblastic and unequal. 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.
WHEN DOES THE BLASTOPORE BECOME EVIDENT FULL
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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