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.