By Levi Clancy for Student Reader on
At the 8-cell stage, the mammalian embryo undergoes compaction where the spherical and loose blastomeres nestle tight together to establish new cell junctions and form a blastocyst. The blastocyst has an outer trophoblast layer (responsible for implantation into the uterine wall) and an inner cell mass placed eccentrically within the blastocoel. The inner cell mass gives rise to the embryonic epiblast (from which the embryo arises), the overlying amniotic ectoderm (separated from the epiblast by the amniotic cavity) and the underlying hypoblast. The mammalian blastocyst is analogous in structure to the yolky bird embryo, without the yolk.