Because of this the endodermal cells of the vegetal pole will show invagination.

Where as the endodermal blastomeres will not show mitotic activity. GASTRULATION IN AMPHIOXUS : In amphioxus blastula is converted into gastrula by the following ways.ġ) Invagination, 2) Involution, and3) Epiboly.Īs gastrulation begins the mitotic activity in the primitive ectoderm cells will increase.

The blastocoel appears at the 4 celled stage as the cells are round and then do not unite. It includes a fluid filled cavity celled blastocoel. It will push the blastomeres outside.Hence a single layered blastula is formed. Then in the centre of the cell mess a jelly substance appear. Because of this blastomeres of the animal pole will be small and blastomeres towards vegetal pole will be bigger. The cells of the vegetal pole will divide slowly where as the cells of the animal pole will divide in a quick way. The cleavage is initiated by the holoblastic division and approximately equal further.īLASTULATION IN AMPHIOXUS : From the 7th cleavage onwards irregular cleavages will occur. 64 blastomere are formed.Īfter the cleavage it is difficult to follow the cleavage patternĬleavage’: The fertilised egg will become a multicellular structure only because of cleavage. Sixth Cleavage: These divisions are approximately meridional. 8 micromeres and 8 macromeres are resulted. Both are, in meridional way extending from animal pole to vegetal pole. Third & Fourth Cleavage: It is in double plane. Because of this cleavage 4 equal blastomeres are formed. Second Cleavage : It is also meridional but it is at right angles to thç first one. The result of this cleavageis two equal blastomeres are formed. It is from anterior end to posterior end (The median axis of the egg is from Animal pole to vegetal pole).

The lower part will develop into notochord ,įirst Cleavage : After 1 1/2 hours* of fertilisation meridional cleavage will take place. Opposite to this mesodermal crescent another crescent is present which becomes presumptive nervous system. The granular yolky cytoplasm near the vegetal pole will become endodermģ The crescent shaped cytoplasm of the posterior side will become mesoderm. Clear cytoplasm occupying the animal pole will mainly become epiderm& ectoderm. In 1962 Tunguntung described the egg of amphioxus 4 regions are clearly seen.ġ. FATEMAP OF FERTILISED EGG OF AMPHIOXUS : Conklin in 1932 studied the fate map of Amphioxus.

Blastopore formation of scypha