part of part_of develops from morula inner cell mass ICM Mass of cells inside the primordial embryo that will eventually give rise to the definitive structures of the fetus. This structure forms in the earliest steps of development, before implantation into the endometrium of the uterus has occurred. The ICM lies within the blastocoele (more correctly termed 'blastocyst cavity', as it is not strictly homologous to the blastocoele of anamniote vertebrates) and is entirely surrounded by the single layer of cells called trophoblast. embryoblast embryoblastus; massa cellularis interna; pluriblastus senior pluriblast blastocyst embryo embryonic structure