In amphioxus cleavage is holoblastic and almost equal type. Embryonic development is indeterminative type.
First set cleavage is meridional that cut the egg along it median axis.
Both the blastomeres are equal in size.
Second cleavage is again meridiona right angle to the first result four celled structure.
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The rhird cleavage planes involve four blastomeres and are transverse dividing each blastomere into upper four smaller micromeres and lower megameres to the vegetable.
Fourth cleavage is meridional and all eight micromeres and 8 macromeres, divided to form sixteen called stage.
Fifth cleavage is latitudinal (horizontal), and synchronous. They result in thirty two celled stages. The cells are arranged in four tiers. Here the blastula distinguish as larger yolk laden, basal macromeres to smaller less yolky, apical micromeres.
Sixth cleavage is approximately meridional forming 64 celled blastomeres but to seventh cleavage synchronous character lost. Hence forth the progression in the number of blastomere is arithmetic rather than geometric due to irregular division. At this stage the embryo attains a stage called morula. Here blastomeres are loosely arranged. The cells arranged in a single layer leaving a central fluid filled cavity called blasto coel. The layer of blastomere is called blastoderm. Thus the structure is called blastula .
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Blastulation upto the formation of blastula the number of cleavage is called and embryonic development is called blastulation.
Fully developed blastula of Amphioxus is called coeloblastula and it contains a large blastocoels and a blastoderm consisting of a single layer of columnar cells. The cells at the vegetal pole are larger than the ells at animal pole. Mesodermal crescent containing similar sized blastomeres with intercellular junctions responsible for characteristic pear shaped structure to the blastula.