single cell zygotes into hollow, multicellular embryonic balls called blastocysts—an early sign of a healthy, viable embryo. The blastocyst is made of two parts. Its outer shell gives rise to the placenta and the umbilical cord, the developing baby’s support system, while the inner mass of cells, hanging on to the wall of the fluid-filled cavity, becomes the embryo. Both the outer shell and the inner mass form out of the first fertilized cell, through the rapid division of cells, mitosis upon mitosis.
