In early
embryogenesis of most
eutherian mammals, the
inner cell mass (abbreviated
ICM and also known as the
embryoblast or
pluriblast, the latter term being applicable to all mammals) is the 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.