FATEMAP OF FERTILISED EGG OF AMPHIOXUS: Conklin in 1932 studied the fate map of Amphioxus. In 1962 Tunguntung described the egg of amphioxus 4 regions are clearly seen.
- Clear cytoplasm occupying the animal pole will mainly become epiderm& ectoderm.
- The granular yolky cytoplasm near the vegetal pole will become endoderm.
- The crescent shaped cytoplasm of the posterior side will become mesoderm.
- Opposite to this mesodermal crescent another crescent is present which becomes presumptive nervous system. The lower part will develop into notochord.
First Cleavage: After 1 1/2 hours* of fertilisation meridional cleavage will take place. It is from anterior end to posterior end (The median axis of the egg is from Animal pole to vegetal pole). The result of this cleavageis two equal blastomeres are formed.
Second Cleavage: It is also meridional but it is at right angles to thç first one. Because of this cleavage 4 equal blastomeres are formed.
Third & Fourth Cleavage: It is in double plane. Both are, in meridional way extending from animal pole to vegetal pole. 8 micromeres and 8 macromeres are resulted.
Fifth Cleavage: It is latitudinal. It is in double plane. 32 celled stage is reached.
Sixth Cleavage: These divisions are approximately meridional. 64 blastomere are formed.
After the cleavage it is difficult to follow the cleavage pattern.
Cleavage: The fertilised egg will become a multicellular structure only because of cleavage. The cleavage is initiated by the holoblastic division and approximately equal further.
BLASTULATION IN AMPHIOXUS: From the 7th cleavage onwards irregular cleavages will occur. The cells of the vegetal pole will divide slowly where as the cells of the animal pole will divide in a quick way. Because of this blastomeres of the animal pole will be small and blastomeres towards vegetal pole will be bigger. The cells are loosely packed. It is called Morula. Then in the centre of the cell mess a jelly substance appear. It will push the blastomeres outside.Hence a single layered blastula is formed. It includes a fluid filled cavity celled blastocoel. This is called blastula stage. The blastocoel appears at the 4 celled stage as the cells are round and then do not unite.
GASTRULATION IN AMPHIOXUS: In amphioxus blastula is converted into gastrula by the following ways.
As gastrulation begins the mitotic activity in the primitive ectoderm cells will increase. Where as the endodermal blastomeres will not show mitotic activity. Because of this the endodermal cells of the vegetal pole will show invagination. In the beginning of the gastrulation the endodermal cells become plate like . This will show an in yard or invagination, in the blastocoel. It gives way for the development of archénteron. When the invagination is carried on the blastocoel becomes oblifrated. The embryo becomes cup shaped structures with two layer of cells. The opening is called blastopore. On the dorsal lip of the archenteron notochordal cells CARE are present.
INVOLUTION IN AMPHIOXUS: The notochordal cells should roll to the middle of the root of the archenteron. In the same way the mesodermal cells present on the ventral lip of the blastopore should rollinside. Themovement of ce& inside is called involution.
EPIBOLY IN AMPHIOXUS: As the changes are taking place, the ectodermal cells of the animaI pole will divide rapidly and they grow over the blastopore. This process is called epiboly. When the mesodermal crescent cells and notochordal crescent cells are totally shifted inside the lips of the biastopore will contract and the blastopore becomes very small. The two layered gastrula is formed. The outer ectoderm and inner endoderm with mesodermal notochordal crescent (cells) and central archenteron.