blastocyst: the blastula of a placental mammal in which some differentiation of cells has occurred
blastula: early stage of an embryo produced by cleavage of an ovum; a liquid-filled sphere whose wall is composed of a single layer of cells; during this stage (about eight days after fertilization) implantation in the wall of the uterus occurs
unapproachability: a disposition to be unapproachable; unfriendly and inaccessible