extraembryonic ectoderm

It stems from the extraembryonic somatic mesoderm on the outer side and the extraembryonic ectoderm on the inner side.

BMP (Bone Morphogenetic Protein) signals from the extraembryonic ectoderm activate expression of fragilis and bias the cells towards PGC.

The chorion and amnion are composed of extraembryonic ectoderm and mesoderm, where as the yolk sac is made of extraembryonic endoderm and mesoderm.