11. It is derived from the epiblast layer, which lies between the hypoblast layer and the amnion. 12. Within birds and mammals, epiblast cells converge at the midline and ingress at the primitive streak. 13. The epiblast is surrounded by the visceral endoderm, the equivalent of the hypoblast of humans and chicks. 14. Multiple processes control cell lineage specification in the blastocyst to produce the trophoblast, epiblast , and primitive endoderm. 15. During approximately the same time, WNT3 starts to be expressed in the posterior visceral endoderm of the epiblast . 16. PGCs come from the epiblast and move to the hypoblast to form the germinal crescent ( anterior extraembryonic structure ). 17. Primary mesenchyme is the first embryonic mesenchymal tissue to emerge, and it is produced from EMT in epiblast cells. 18. This bilaminar blastocyst also defines the primitive dorsal ventral axis . the epiblast is dorsal and the hypoblast is ventral. 19. PGCs come from the epiblast and migrate subsequently into the mesoderm, the endoderm and the posterior of the yolk sac. 20. Soon amniogenic ( amnion forming cells ) amnioblasts separate from the epiblast and line the amnion, which encloses the amniotic cavity.
