Epiblast cells which leave the primitive streak caudal to the primitive node, differentiate into all embryonic cell lines: embryonic endoderm, mesoderm that develops from the mesoblast, primordial sex cells, and, from the caudalmost part, extra-embryonic mesoderm. The cells remaining in the epiblast layer become the ectoderm. This structure differentiates further: cranially to the primitive node and in the region of the chorda dorsalis it forms neural ectoderm (nervous system); in the remaining regions it forms surface ectoderm.
In the two-layered embryonic disc, the hypoblast lies ventrally and the epiblast dorsally (dorso-ventral axis). The development of the caudal proliferation zone establishes the cranio-caudal axis. Recent studies indicate that this axis is pre-formed through polarity of the zygote. The primitive streak, immediately following the caudal proliferation zone, and the later formed axial structures (chorda dorsalis, neural groove) lie in the median plane. They define the bilateral symmetry.