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.
- 06 Jul 2017
Amphioxus is a Cephalochordate animal. Its Life history in the early stages shows resemblance with ascidians. HATSCHE’K WILSON. CONKLIN’ worked on the part of embryogenesis of Amphioxus. ‘CONKLINS’ work in more accurate and is recent.
Amphioxus is a unisexual animal. But sexual dimorphism is absent. Amphioxus shows 26 pairs of Gonads. They are present from 25th myotomal segments to 51. These Gonads have no ducts. When mature the overlapping tissue of the Gonad will rupture and the gametes are Iibereted into atrium of Amphioxus. They will come out into the water through the atriopore.
AMPHIOXUS - ORGANISATION OF THE GAMETES
SPERMATOZOAN IN AMPHIOXUS: The mature male sex cell called Sermatozoan. It is 15 to 20 in length. It shows three regions.
1. Head, 2. Middle piece and 3. Tail .
On the head acrosome is present. Head shows a big nucleus. Around the nucleus thin sheet of Cytoplasm is present. It is called Manchetty. The middle piece is small with mitochondrial matrix called nebenkeron. The tail is long and shows movements.
EGG OR OVUM IN AMPHIOXUS: The mature female sex cell is ovum. It is small and 0.12 mm in diameter. It is a microlecithal egg. The cytoplasm around the nucleus will show yolk. In the peripheral cytoplasm yolk is absent. It is granular and is called carticoplasm. The plasma membrane surrounds the cytoplasm. Around this is a mucopolysaccharide membrane is present. It is called vitelline membrance. In between these two layers perivitelline space is present. The nucleus is present towards the animal pole, where as the opposite pole is called vegetal pole. The vegetal pole becomes posterio dorsal side of the embryo. The Animal pole becomes antero-ventral side of the embryo. Hence a gradient polarity is established in the egg.
FERTILISATION IN AMPHIOXUS: As soon as the egg comes in contact with water the vitelline membrane wilI separate from the plasma membrane.
The egg is surrounded by a number of sperms. One sperm will make its entry through the contents of the egg from the vegetal pole. At this time a number of changes take place in the corticoplasm. Now membranes are formed which unite with vitelline membrane It is called Fertilisation
So that no other sperm can enter into the egg. The head and middle piece of the sperm will enter into ooplasm of the egg. The egg nucleus undergoes second maturation division. Second polar body is pushed into the pervitilline space . The sperm nucleus and middle piece will show 180° twist and move towards the egg nucleus. Both the nuclei will unite. Thus a zygote nucleus is formed.