BACKGROUND:The extraembryonic endoderm (ExEn) defines the yolk sac. a set of membranes that provide essential support for mammalian embryos. Recent findings suggest that the committed ExEn precursor is present already in the embryonic Inner Cell Mass (ICM) as a group of cells that intermingles with the closely related epiblast precursor. All ICM cells contain Oct4. https://www.chiggate.com/eeboo-memory-matching-game-animal-pre-school-supply/