Blastocyst are embryos that has undergone development five or six days post fertilization period. The number of cells that a human blastocyst may contain ranges between 70 and 120. Blastocyst occur after the morula enters the females uterine cavity and develops a fluid-filled cavity. The morula then becomes what appears to be a hollow ball of cells that is also known as a blastocyst.
Blastocyst Development Video
Within the blastocyst certain cells group together to form the inner cell mass that later goes on to becomes the embryo.