It is observed that the egg of amphioxus after fertilization is having a well marked polarity, bilateral, symmetry as well as establishing cyhtoplasmic organization.
The cytoplasm of egg of amphioxus is having well marked differences that lead Conklin (1932) to construct a fate map.
At the very beginning of cleavage Conklin recognized three regions in fertilized egg.
(a) Near vegetal pole there is found a mass of cytoplasm that contains the greatest amount of yok.
(b) The animal hemisphere of the egg consists of cytoplasm that has less yolk and transparent.
(c) Towards the posterior side, there is the presence of mesodermal crescent cytoplasm.
Thus in the cytoplasm of cleaving egg of amphioxus; four distinct regions are well marked, which upon tracing a fate map shows that.
i) The clear cytoplasm which released from germinal vesicle during the maturation division and occupied animal hemisphere develops mainly into epidermal ectoderm (skin – epiderms).
ii) The granular yolky cytoplasm which takes up the region around the vegetal pole develops into the lining of endoderm (lining of elimentary canal).
iii) The crescent of basophilic cytoplasm of posterior sides develops in to muscles and lining of body cavity and thus represents a mesodermal area.
iv) On the opposite side of the mesodermal crescent occurs a notochord crescent and above the notochord crescent occurs a crescent area which develops into the nervous system.
Gastrulation in Amphioxus:
Several changes which take place in the gastrula are called gastrullation.
It starts at the vegetal plate by the flattening of the endodermal plate. It becomes pushed into the blastocoels by the process of invagination. The endodermal plate becomes concave and gradually sinks in words in to the blastocoel which becomes reduced in size.
Blastocoels gives rise to a new cup shaped cavity, called the archenteron. The wide opening of the archenteron is called the blastopore and its rim is called the lip of blastopore. The dorsal lip which is anterior in position possesses the choda cells, while lateral lip and ventral lip are formed by the cells of mesodermal crescent. The chorda cells, situated along the mid dorsal edge of the blastopore, roll so as to form the middle part of the roof of the archenteron, while the roof of the archenteron is formed by the cells of mesodermal crescent around the ventral lip of the blastopore.
While these changes are going on the smaller cells of the ectoderm situated at the animal half spread out by a process, called epiboly. By the invagination of endodermal plates, chorda cells and the cells of the mesodermal crescent, the embryo is elongated in the antero-posterior direction. Active multiplication of ectoderrmal cells shifts the blastopore from antero-dorsal position to a posterior position.
Thus at the end of the gastrulatioin period, there is an auter layer of flagellated endoderm, which helps to rotate the embryo. Above the ectoderm, dorsally, are the cells of neural plate. The inner cells of the gastrula possess a mid-dorsal strip of notochord, on the two sides of which are mesodermal cells. The lateral and ventral inner cells are of endoderm.