What are the changes that take place during formation of primitive streak?

(a) Presumptive mesoderm i.e. the lateral plate mesoderm begins to converge on the midline resulting aggregation of ells.

(b) It creates a relatively broad thickened area on the inner side of blastoderm called as short primitive streak.

(c) It undergoes involution and it spreads in an anterolateral direction as a separate sheet of mesoderm, between the hypoblast and the surface layer.

(d) Most of the mesodermal cells entering the streak from one side and makes again in same side but some cross over to emigrate on the opposite side.

(e) Elongation of short primitive streak is followed by a change of the area pellucid from a circular shape to pear shaped in outline. This signifies that the un-displaced prospective mesoderm and the adjusted area of ectoderm are swimming around the long axis and coming to flank the streak.

(f) Most of the lateral plate mesoderm shortly moved inward and spread lateral, so that the streak is bordered by somatic mesoderm.

(g) Meanwhile the pre-chordal mesoderm also passes inward leaving the anterior end of streak by prospected or presumptive notochord.

(h) At the anterior end of the streak, the notochordal materials thickened to form a node called Hansen’s node where as in conjugation with primitive streak proper, the pattern of morphogenic movement of the various mesodermal components are towards it forms either side, through it and move out again to enter laterally.

(i) The primitive node represents a type of movement so that prospective notochordal tissue converges on the node forming a head process.

The completion of primitive streak takes place with the formation of head process.