ORGANOGENY IN AMPHIOXUS.
The development of organ system is called as Organogeny. In Amphioxus the gastrula elongates in the future anteroposterior direction and the blastopore comes nearer to the posterior end of the dorsal surface and starts the development of organ system.
1 . Formation of Neural Tube IN AMPHIOXUS: Neural ectoderm cells of the middorsal line became thickened and elongate. It forms a flattend neuralplate. The neural plate sinks and the ectoderm at the sides rises up to form neural folds. This is from the blastoporal end and extend forward. The neural folds are fused and form the neural tube. It opens at the anterior end by a small opening called neuropore.
The cavity of neural tube gets separated from the cavity of archenteron. The neural tube forms the nerve cord of the adult.
2. FORMATION OF NOTOCHORD IN AMPHIOXUS: Along with the development of neural tube the Notochord development starts. The presumptive Notochordal cells are present mid dorsally in the roof of archenteron and below the neural tube. They evaginate from the endoderm and constrict off to form solid mass of cells. This is started at the anterior end and continues back- ward towards the hind end of elongated gastrula. This row of cells is known as notochórd. The notochordal sheath will develop around the notochord from the sclerotome of mesoderm and development is completed.
3. DEVELOPMENT OF MESODERMI AND COELOM IN AMPHIOXUS : The presumptive mesoderm is present in the dorso-lateral walls of the archenteron. It surrounds the notochord on both the sides. From the wall of archenteron two folds arise in the form of grooves. They divided into a series of segments by several transverse lInes. Each segment is called coelomic pouch. These coelomic pouches separate from the archenteron and form a series of coelomic sacs in between the ectoderm and endoderm. The (vail of the sacs forms the mesoderm. The coelomic sacs will grow in size and extend both up wards and down words and occupy the space between ectoderm and endoderm.
Each coelomic sac divides into two parts.
1. Myoteme : It contains a median muscular splanchnic portion and a laterally placed thin walled parietal portion. They surround the coelomic space or myocoel. The muscular part give dermatome which forms a thin sheet below the ectoderm of the skin.
Scierotome gives rise to a sheath of notochord, and nerve cord. Muscle plate enlarges and forms the muscle bundles.
2. Splanchnotome : It is formed of two sheets separated by space, called splanchnocoel. It forms the coelome of the embryo.
The outer wall of splanchnocoel present in contact with the ectoderm is called somatic layer of mesoderm. The inner wall of splanchnocoel which is in contact with. The endoderm of archenteron is known as splanchnic mesoderm.
The origin of coelom in Amphioxus is called enterocoelic type,brcause it is derived from the archenteron.
HATCHING IN AMPHIOXUS:
About 12 hours after fertilization the embryo hatches out It is elongated and laterally compressed. It is called larva.
The young larva of Amphioxus is without mouth and anus. It swims actively by means of cilia present on the epidermis. The larva gradually elongates. The notochord grows into the anterior end. The anterior end is pointed. The posterior end develops a caudal fin.