Blastocyst formation

Cleavage goes on and the embryo comes to have 16 cells when it looks like a mulberry and is thus called morula. The morula is organised into an outer single layer of cells which later form the trophoblast. This layer is known as the outer cell mass. The remaining cells of the morula form the inner cell mass which gives rise to the embryo proper. Fluid enters the morula from the uterine cavity and is placed between the inner and outer cell mass. It forms a cavity such that the cells of the outer cell mass get flattened. This cavity is known as blastocele. The inner cell mass gets attached to one side of the trophoblast. This the blastocyst is formed.