A pre-embryo at a stage reached about 5 days after fertilization, and close to the time of implantation. The blastocyst is a ball of cells filled with fluid.
A small mass of cells that results after several days of cell division by a fertilized egg.
A mammalian embryo in the stage of development that follows the morula. It consists of an outer layer of trophoblast to which is attached an inner cell mass.