Nodal for epiblast and visceral endoderm development may be unique to eutherians.
PME is the initial molecular landmark of axis formation in reptiles.
Ontogenetically, PME may be the precursor of the blastoporal plate.
Avian Koller’s sickle and posterior marginal zone may have been derived from PME.
AVE gene expression may have been independently acquired in avians and eutherians.