Blastocoel bearing stage is called Blastula.The micromeres dives more rapidly than megameres which results in formation of small fluid filled cavity known as Blastocoel or segmentation cavity.One hemisphere of morula is composed of large number of small black and yolkless cells known as micromeres and other hemisphere is composed of fewer number of large white and yolk laden cells known as megameres.As the result of repeated and irregular cleavage, ball of cells is formed known as morula stage.These division is followed by two horizontal cleacage, one toward animal pole and other toward vegetal pole, resulting in 32 celled stage. Fourth and fifth cleavage are also vertical forming 16 celled zygote.The lower 4 large yolk laden cells are known as megameres or hypoblat. The upper 4 cells toward animal pole are small and pigmented known as micromeres or epiblast. Third cleavage is horizontal but above the equatorial line forming unequal size cells.Second cleavage is also vertical but right angle to the first one forms 4 celled stage.First cleavage is vertical two celled stage.The divisions are holoblastic and complete.The cleavage begins as a small depression at animal pole and gradually extends surrounding the zygote, dividing into two cell.The repeated division in the successive fashion is known as cleavage or segmentation. 2-3 hours after fertilization, the zygote begins to divide.
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