The inner cell mass is surrounded by the single cell layer of cells called trophoblast. These cells then become stem cells. This mass at the embryonic pole of the blastocyst concerned with the formation of the body of the embryo, and as such is sometimes called the embryoblast.

The inner cell mass differentiates in the ectoderm and endoderm, separated by a layer of mesoderm. The layers will then curve to form two compartiments, one of endoderm and one of ectoderm, both with an external layer of mesoderm. these compartiments will migrate towards the centre of the blastocyst. The external mesoderm will form a stalk that is to form the latter umbilical cord.

Developmental biology