The blastocyst consists of two cell types: the outer epithelial layer of trophoblast and the undifferentiated inner cell mass (ICM) cells. The ICM later differentiates into two layers: the inner epiblast/embryonic ectoderm and the outer primitive/extraembryonic visceral endoderm. These layers are clearly demarcated by a junction and cells do not mix between these two tissues [1].
The ICM derived epiblast cells that delaminate early give rise to extraembryonic mesoderm, while cells that drop our later give rise to embryonic mesoderm in the primitive streak [1].
Pic:
Hogan