develops from surrounded by inner cell mass https://upload.wikimedia.org/wikipedia/commons/7/72/Blastocyst_English.svg This structure forms in the earliest steps of development, before implantation into the endometrium of the uterus has occurred.The ICM lies within the blastocyst cavity and is entirely surrounded by the single layer of cells called trophoblast. [Wikipedia:Inner_cell_mass] trophoblast blastocyst embryonic structure