Group of cells that differentiates within (but to one side of) the developing embryo when it is a blastocyst and which will, if all goes well after implantation, form the embryo-proper, or fetus. Cells from the inner cell mass can be used to produce embryonic stem cells.

Stage of development of the early embryo that has undergone blastulation, in which a fluid-filled cavity forms in the formerly solid ball of cells (the morula), about 5 days after fertilisation. For the first time, a distinction can be made between a sheet of cells to one side, which will form the embryo proper, termed the inner cell mass, and the remaining, peripheral cells that constitute the trophectoderm, which -- after the blastocyst "hatches" through the zona pellucida and undergoes implantation -- will form the trophoblast. An old-fashioned term for an inevitable miscarriage, meaning that the ovum (in its classical sense for professional embryologists) has not developed normally after fertilisation, there being present just the supporting tissues and no embryo. The term is descriptive, it has no diagnostic value as to the cause of the miscarriage.

The "upper" layer of the embryo as the inner cell mass forms a circular disk two cells thick, giving rise with further embryonic development to the skin, brain and spinal cord. Synonym: epiblast.

The "lower" layer of the embryo, as the inner cell mass forms a circular disk two cells thick; with further embryonic development it gives rise to the digestive tract and its glands. Synonym: hypoblast.

A state in an embryo, fetus, animal or person in which a small number of cells occur that have a different genome. Arises naturally because cells (rarely, stem cells) can cross between a mother and her fetus, or experimentally by introducing foreign stem cells into an embryo, usually at the stage of blastocyst, when the introduced cells become part of the inner cell mass.

A term sometimes used for the embryo or ovum from the stage of fertilised egg (or zygote) up to the stage of the morula. During this time any of the cells of the fertilised ovum can develop into a whole new embryo -- they are totipotent. As it becomes a blastocyst a proportion of the cells (the inner cell mass) commit to producing the embryo proper, while the remaining majority are destined to form trophoblast. Distinguishing pre-embryo from loose use of the term embryo has some value when debating the morality of embryo research, but it is not universal practice.